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UAV photogrammetry and Structure from Motion to assess calving dynamics at Store Glacier, a large outlet draining the Greenland ice sheet

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This study presents the application of a cost-effective, unmanned aerial vehicle (UAV) to investigate calving dynamics at a major marine-terminating outlet glacier draining the western sector of the Greenland ice sheet. The UAV was flown over Store Glacier on three sorties during summer 2013 and acquired over 2000 overlapping, geo-tagged images of the calving front at an similar to 40 cm ground sampling distance. Stereo-photogrammetry applied to these images enabled the extraction of high-resolution digital elevation models (DEMs) with vertical accuracies of +/- 1.9m which were used to quantify glaciological processes from early July to late August 2013. The central zone of the calving front advanced by similar to 500 m, whilst the lateral margins remained stable. The orientation of crevasses and the surface velocity field derived from feature tracking indicates that lateral drag is the primary resistive force and that ice flow varies across the calving front from 2.5md(-1) at the margins to in excess of 16md(-1) at the centreline. Ice flux through the calving front is 3.8 x 10(7) m(3) d(-1), equivalent to 13.9 Gt a(-1) and comparable to flux-gate estimates of Store Glacier's annual discharge. Water-filled crevasses were present throughout the observation period but covered a limited area of between 0.025 and 0.24% of the terminus and did not appear to exert any significant control over fracture or calving. We conclude that the use of repeat UAV surveys coupled with the processing techniques outlined in this paper have great potential for elucidating the complex frontal dynamics that characterise large calving outlet glaciers.
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The Cryosphere, 9, 1–11, 2015
www.the-cryosphere.net/9/1/2015/
doi:10.5194/tc-9-1-2015
© Author(s) 2015. CC Attribution 3.0 License.
UAV photogrammetry and structure from motion to assess calving
dynamics at Store Glacier, a large outlet draining the Greenland ice
sheet
J. C. Ryan1, A. L. Hubbard2, J. E. Box3, J. Todd4, P. Christoffersen4, J. R. Carr1, T. O. Holt1, and N. Snooke5
1Centre for Glaciology, Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, SY23 3DB, UK
2Department of Geology, University of Tromsø, 9037 Tromsø, Norway
3Geological Survey of Denmark and Greenland, Copenhagen, Denmark
4Scott Polar Research Institute, University of Cambridge, Cambridge, UK
5Department of Computer Science, Aberystwyth University, Aberystwyth, SY23 3DB, UK
Correspondence to: J. C. Ryan (jor44@aber.ac.uk)
Received: 29 March 2014 – Published in The Cryosphere Discuss.: 28 April 2014
Revised: 25 August 2014 – Accepted: 25 November 2014 – Published: 6 January 2015
Abstract. This study presents the application of a cost-
effective, unmanned aerial vehicle (UAV) to investigate calv-
ing dynamics at a major marine-terminating outlet glacier
draining the western sector of the Greenland ice sheet. The
UAV was flown over Store Glacier on three sorties dur-
ing summer 2013 and acquired over 2000 overlapping, geo-
tagged images of the calving front at an 40cm ground
sampling distance. Stereo-photogrammetry applied to these
images enabled the extraction of high-resolution digital ele-
vation models (DEMs) with vertical accuracies of ±1.9m
which were used to quantify glaciological processes from
early July to late August 2013. The central zone of the calv-
ing front advanced by 500m, whilst the lateral margins re-
mained stable. The orientation of crevasses and the surface
velocity field derived from feature tracking indicates that lat-
eral drag is the primary resistive force and that ice flow varies
across the calving front from 2.5md1at the margins to in
excess of 16md1at the centreline. Ice flux through the
calving front is 3.8×107m3d1, equivalent to 13.9Gta1
and comparable to flux-gate estimates of Store Glacier’s an-
nual discharge. Water-filled crevasses were present through-
out the observation period but covered a limited area of be-
tween 0.025 and 0.24% of the terminus and did not appear
to exert any significant control over fracture or calving. We
conclude that the use of repeat UAV surveys coupled with
the processing techniques outlined in this paper have great
potential for elucidating the complex frontal dynamics that
characterise large calving outlet glaciers.
1 Introduction
Observational and modelling studies have demonstrated that
Greenland’s marine outlet glaciers have a complex and po-
tentially non-linear response to both environmental forcing
(e.g. Vieli et al., 2000; Benn et al., 2007; Holland et al., 2008;
Howat et al., 2010; Hubbard, 2011; Joughin et al., 2012; Wal-
ter et al., 2012; Carr et al., 2013) and to changes in front po-
sition (Howat et al., 2007; Luckman et al., 2006; Joughin et
al., 2008). To quantify these processes and feedbacks, regular
and accurate high-resolution measurements are required to
capture the key spatio-temporal linkages between rates of ice
calving, flow, surface lowering and frontal advance/retreat.
Despite significant advances in satellite remote sensing, lim-
itations of spatial resolution (e.g. MODIS) and/or frequency
of repeat imagery (e.g. Landsat or TerraSar-X) render de-
tailed, day-to-day analysis of calving-front dynamics un-
feasible. On the other hand, acquisition of digital imagery
from unmanned aerial vehicles (UAVs) combined with the
development of stereo-photogrammetry software has en-
abled the provision of high-resolution 3-D georeferenced
data on demand for geoscience applications (e.g. d’Oleire-
Oltmanns et al., 2012; Hugenholtz et al., 2012, 2013; White-
head et al., 2013; Lucieer et al., 2014). This represents a
cost-effective technique technique for acquiring aerial data
in remote, hazardous and/or inaccessible regions and re-
cent applications for emerging snow and ice investigation
abound the web (e.g. see the highly informative site of Matt
Published by Copernicus Publications on behalf of the European Geosciences Union.
2 J. C. Ryan et al.: UAV photogrammetry and structure from motion
Table 1. Attributes of the flight surveys and image acquisition of
the UAV.
Interval Glacier Resolution
Flight between No. coverage of DEM
no. Date pictures (s) images (km2) (cm/pixel)
1 1 July 1.55 611 3.17 40
2 2 July 1.51 1051 4.95 38
3 23 August 2.36 567 5.02 39
Nolan; http://www.drmattnolan.org/photography/2013/). To
date, published (peer-reviewed) application appears to be
limited to the investigation of inter-annual changes of a land-
terminating glacier on Bylot Island, Canadian Arctic (White-
head et al., 2013).
Between July and August 2013, an off-the-shelf, fixed
wing UAV equipped with a compact digital camera flew three
sorties over the calving front of Store Glacier, West Green-
land. The aerial photographs obtained during these flights
were used to produce high-resolution (40 cm; Table 1) dig-
ital elevation models (DEMs) and orthophotos of the glacier
terminus. These data allowed for the investigation of the spa-
tially complex and time-varying glaciological processes op-
erating at the glacier’s calving front. The aim of this paper is
to
1. detail the UAV, in terms of its payload and camera set-
tings, and its specific deployment to Store Glacier;
2. describe the techniques used for processing the aerial
images and quantifying glaciological processes;
3. discuss the significance of the data we obtained which
includes calving events, the character, orientation and
morphology of crevasses, surface velocities, ice dis-
charge and changes in thickness and position of the
calving front.
2 Data and methods
2.1 Study site
Store Glacier is a large marine-terminating (tidewater) outlet
glacier located in the Uummannaq district of West Greenland
(Fig. 1). The calving front has a width of 5.3 km and an aerial
calving front (freeboard) of up to 110ma.s.l. (Ahn and Box,
2010). Aerial photography from 1948 onwards reveals that
Store Glacier’s frontal position has remained stable over the
last 65 years (Weidick, 1995). Seasonally, the calving front
exhibits advance and retreat of up to 400m (Howat et al.,
2010). The study here focuses specifically on glacier dynam-
ics during the melt season under open-water, tidal modula-
tion of ice flow.
Figure 1. (a) A typical UAV sortie over Store Glacier. The back-
ground map is a Landsat 8 true colour image from 12 June 2013.
The red line shows the UAV flight path on the 2 July 2013. (b) Lo-
cation of Store Glacier in the Uummannaq region, West Greenland
on a MODIS mosaic image of Greenland (Kargel et al., 2012).
2.2 UAV platform
The UAV airframe is an off-the-self Skywalker X8 (www.
hobbyking.com) which has a wingspan of 2.12m and is
made from expanded polypropylene (EPP) foam (Fig. 2). For
this deployment, the X8 was powered by two 5Ah four-cell
(14.8 V) lithium polymer batteries driving a 910 W brushless
electric motor turning an 11×7 foldable propeller. In this
configuration, the X8 has a flying mass of 3kg (including
1kg payload), which allows for a cruising speed of around
55–70 km per hour with a maximum range of 60km in be-
nign conditions at constant altitude. A small propeller/high-
revolution motor combination was chosen to provide max-
imum instantaneous thrust to ensure a clean launch (for
novice operators) and to handle the potentially strong kata-
batic winds encountered during its 40km sortie.
The autopilot is an open-source project called Ardupilot
(http://ardupilot.com/) based on an Atmel 2560 8 bit micro-
controller and standard radio control parts including 2.4 GHz
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J. C. Ryan et al.: UAV photogrammetry and structure from motion 3
Figure 2. Flowchart of the control set-up and picture of the UAV at
base camp with the relative novices.
radio control and pulse-width modulation (PWM) controlled
servos for aileron and elevon control (Fig. 2). Ardupilot im-
plements a dual-level proportional-integral-derivative (PID)
controller architecture. The lower level controls flight stabil-
isation and the higher level controls based navigation. Tuning
of the PID parameters is necessary to suit the mass and dy-
namics of the airframe to ensure accurate stabilisation with-
out pitch/roll oscillation (lower-level controller) or flight-
path weaving (higher-level controller). The autopilot allows
the UAV to fly autonomously according to a pre-programmed
flight path defined by a series of 3-D waypoints chosen by
the user. The autopilot utilises a GPS for navigation, a triple
axis accelerometer and gyro for stabilisation and a baromet-
ric pressure sensor for altitude control. These parameters are
logged to memory at 10Hz throughout the flight (Fig. 2).
The advantage of this package is that it can be assembled
within a day from off-the-shelf parts and is cost effective
at less than USD2000. The X8 is also relatively straight-
forward to fly, robust, easily repairable and floats, all added
bonuses when being deployed in remote areas by potential
novices. Furthermore, the Ardupilot firmware is open source
and hence can be programmed for specific requirements, for
example camera triggering (see below).
Two lightweight digital cameras were tested at the field
site: a Panasonic Lumix DMC-LX5 10.1 megapixel (MP)
camera with a 24mm wide-angle zoom lens and a 16.1 MP
Sony NEX-5N with a 16mm fixed focal length lens though
results presented here are limited to the former. A SPOT GPS
tracking device was also included in the payload to facilitate
recovery should a mission fail (which it did). The focal length
of the Lumix lens was adjusted to 5.1 mm (35 mm equivalent)
to allow the widest possible coverage which gave the camera
a 73.7horizontal and 53.1vertical field of view. A short
exposure time of 1/1600 and a focal ratio of 8 were chosen
to prevent overexposure and blurring of the ice surface. The
Ardupilot open-source code was amended to trigger the cam-
era automatically at user defined time or distance intervals at
or between certain waypoints. The cameras were mounted
pointing downwards within the airframe using neoprene and
velcro straps to dampen vibration in a custom recessed aper-
ture cut in the bottom with a UV filter to protect the lens and
seal it.
2.3 Flight planning
The open-source software APM Mission Planner (http://
plane.ardupilot.com/) was used for flight waypoint manip-
ulation and planning in conjunction with the 30 m Greenland
Mapping Project (GIMP) DEM (Howat et al., 2014). To op-
timise spatial coverage against required resolution, flight en-
durance and stability, the UAV was programmed to fly at a
constant altitude of 500ma.s.l. (Fig. 1). Based on the cam-
era’s focal length and field of view (53.1by 73.7), the
ground (sea-level) footprint at 500ma.s.l. for each photo was
450×750m. To ensure coverage of the entire glacier ter-
minus and overlap for successful photogrammetric process-
ing, the four transects broadly parallel to the calving front
were flown with 250m separation yielding a side overlap
between photos of 70% (Fig. 1). The mean ground speed
of the UAV was 70 km h1and the camera trigger inter-
val was adjusted between surveys. On flights 1 and 2, the
interval between camera triggers was 1.5s corresponding to
a forward overlap of 94% and over 1000 geotagged images
acquired. Flight 3 had a 2.4 s interval yielding a 90% forward
overlap and 581 images (Table 1).
UAV operations were based out of a field camp with the
advantage of a 50m area of flat alluvial terrace with rela-
tively boulder and bedrock free ground for manual remote-
control take-off and landing. This location did, however, re-
quire a 10 km transit to the calving front over a 450 m high
peninsula which significantly reduced the useful endurance
over the target. Of the six sorties flown over outlet glaciers in
the region during July and August 2013, the three over Store
Glacier were most successful. Each sortie was 40km long
and 35 min in duration after the UAV had attained its oper-
ating altitude at the start of the mission and was passed from
manual remote-control mode into autopilot mode (Fig. 1).
Visual and remote-control contact is lost within a few kilo-
metres of the UAV being placed in autopilot mode; hence,
validation of the mission plan is essential.
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4 J. C. Ryan et al.: UAV photogrammetry and structure from motion
Figure 3. (a) Surface elevation difference between two DEMs col-
lected on 1 July and 2 July. Red areas show elevation loss whilst
blue areas show elevation gain. White circles highlight the calving
events that occurred between the two UAV surveys. (b) The position
of the calving front of Store Glacier during the summer of 2013. (c)
Calving-front retreat observed between 12 June and 1 July. Inset
is an orthorectified image of the water-filled crevasses observed on
1 July with a pixel resolution of 30cm. (d) Calving-front advance
observed between 1 July and 23 August. Inset is an orthorectified
image showing water-filled crevasses observed on 23 August. The
coverage and size of water-filled crevasses is smaller.
2.4 Three-dimensional model generation
Three-dimensional data were extracted from the aerial pho-
tos using Agisoft PhotoScan Pro software (Agisoft LLC,
2013). This software’s strength lies in its ability to fully auto-
mate workflow and enables non-specialists to process aerial
images and produce 3-D models which can be exported as
georeferenced orthophotos and DEMs (e.g. Figs. 3 and 6).
The first stage of processing is image alignment using the
structure-from-motion (SfM) technique. SfM allows for the
reconstruction of 3-D geometry and camera position from a
sequence of two-dimensional images captured from multi-
ple viewpoints (Ullman, 1979). PhotoScan implements SfM
algorithms to monitor the movement of features through a
sequence of multiple images and is used to estimate the loca-
tion of high-contrast features (e.g. edges), obtain the relative
location of the acquisition positions and produce a sparse 3-
D point cloud of those features. The Ardupilot flight logs of
the onboard navigation sensors allow the camera positions
and the 3-D point cloud to be georeferenced within instru-
ment precision. SfM also enables the camera calibration pa-
rameters (e.g. focal length and distortion coefficients) to be
automatically refined; hence, there is no need to pre-calibrate
the cameras and lens optics (Verhoeven, 2011).
Once the photos have been aligned, a multi-view recon-
struction algorithm is applied to produce a 3-D polygon mesh
which operates on pixel values rather than features and en-
ables the fine details of the 3-D geometry to be constructed
(Verhoeven, 2011). The user determines the precision of the
final 3-D model based on image resolution and pixel foot-
print. A medium quality setting was chosen yielding DEMs
with between 38 and 40cm/pixel ground sampling resolu-
tion (GSD), which were resampled to a Cartesian 50cm
grid to enable intercomparison (Table 1). Higher resolutions
(<30cm GSD) are attainable but the increase in computa-
tional time and the accuracy of georeferencing limits the ben-
efits of such apparent precision.
Two problems of accuracy were encountered in DEM pro-
duction: (1) PhotoScan failed to reconstruct a flat sea level
of constant elevation, and (2) relative positional errors be-
tween the DEMs constructed from different sorties were up
to 17.12m horizontally and 11.38m vertically. Positional er-
rors were due to the specified limits of the onboard L1 GPS
of ±5.0m horizontally and, when combined with the baro-
metric sensor, to a similar accuracy vertically. These were
compounded by the time lag between the camera trigger-
ing and actual photograph acquisition. Hence, a secondary
stage of processing was carried out which involved 3-D co-
registration of the DEMs. To do this, the horizontal and verti-
cal coordinates of common control points (CPs) based on dis-
tinct features such as cliff bases, large boulders and promon-
tories were extracted from the georeferenced orthoimages.
The CPs that were at sea level were nominally given ele-
vation values of zero, re-imported into PhotoScan and sub-
sequently reprocessed along with a geodetic GPS ground
CP located at 70.401N, 50.665E and 335.85m altitude
on the bedrock headland overlooking the glacier’s north-
ern flank. During this secondary stage of processing, Pho-
toScan’s optimisation procedure was run to correct for possi-
ble distortions. After processing with the CPs, a flat sea level
across the glacier front was produced and the relative errors
between the three DEMs were reduced to ±1.41m horizon-
tally and ±1.90m vertically. The georeferenced 3-D DEMs
and orthophotos were then exported at 50cm pixel size for
further analysis in ArcGIS and ENVI software packages.
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J. C. Ryan et al.: UAV photogrammetry and structure from motion 5
2.5 Analysis
Changes in calving-front positions were obtained from these
data combined with a Landsat 8 panchromatic image ob-
tained on 12 June (Fig. 3b). Each calving-front position was
digitised according to the procedure outlined by Moon and
Joughin (2008), whereby a polygon of the calving-front re-
treat or advance is digitised and divided by the width of the
glacier. This method has been used in previous studies (e.g.
Howat et al., 2010; Schild and Hamilton, 2013) and enables
intercomparison of results. Surface elevation change was cal-
culated from the residual difference of the DEMs (Fig. 3a).
Ice flow across the terminus region was calculated by
feature tracking performed on successive DEMs using the
ENVI Cosi-CORR software module (Fig. 4b). These veloc-
ities were then used to estimate ice flux through the calving
front for the same period under the assumption of plug flow
(uniform velocity profile with depth) and using a calving-
front cross section obtained from Xu et al. (2013) and mod-
ified by single and multi-beam echo sounder bathymetry ob-
tained by S/V Gambo in 2010 and 2012 (Chauché, unpub-
lished). The frontal cross section was divided into 10m ver-
tical strips and under the plug-flow assumption, each was as-
signed its corresponding horizontal velocity (Fig. 4a). The
floatation depth and buoyancy ratio across the calving front
was calculated using the ice surface (freeboard) elevation
and total ice thickness with a value for the density of ice of
917kgm3and for sea water of 1028kgm3(Fig. 5a).
To investigate the distribution and patterns of crevassing,
each DEM was Gaussian filtered at 200 pixels (100m) in
ArcGIS and subtracted from the original DEM to yield the
pattern of negative surface anomalies. These anomalies were
converted into polygons to map and hence quantify crevasse
distribution and character (Fig. 6a). The resulting polygons
were enclosed by a minimum bounding rectangle, which al-
lowed the orientation, width, length and depth of crevasses
to be extracted (Fig. 6a, Table 2). Water-filled crevasses were
automatically located in the ENVI package using the su-
pervised maximum likelihood classification (MLC) method.
Representative training samples for water-filled areas were
chosen from the colour composite orthophoto (Fig. 6b). The
trained tool then classifies pixels that are interpreted as wa-
ter into the desired class. The resulting raster image was con-
verted into a shapefile and used to mask and define the area of
the water-filled crevasses across the terminus. These proce-
dures allow thousands of crevasses in multiple orthoimages
and DEMs to be quantified easily without the difficulties and
dangers associated with direct field measurements.
2.6 Uncertainties and limitations
The relative horizontal uncertainties between the DEMs were
investigated by feature tracking the stationary bedrock at the
sides of the glacier. The root mean square (rms) horizon-
tal displacement was ±1.41m which provides us with an
Figure 4. (a) Surface elevation changes between 2 July and 23 Au-
gust. An average thinning of 0.12md1was estimated for the sur-
veyed area. (b) The ice-flow speed structure of the terminus of Store
Glacier between 1 and 2 July 2013. The centre of the glacier flows at
approximately 16 md1whilst the margins flow less than 5m d1.
Dotted white lines show the lateral margins of the glacier. The black
line represents the locations of the horizontal velocity and surface
elevation values that were used to estimate ice flux. The white line
represents the location of the depth values used to estimate ice flux.
The cross section of the calving front derived from these profiles is
displayed in (a).
approximate error estimate. The relative vertical uncertain-
ties between the DEMs were estimated by calculating ele-
vation differences between bedrock areas, which revealed an
error estimate of ±1.9m. The two-stage procedure outlined
in Sect. 2.4 therefore enabled us to improve the relative po-
sitional uncertainties from nearly 20m to less than 2m. For
future studies, it is thought that several CPs on the bedrock
either side of the glacier front would further reduce these un-
certainties. A telemetric differential GPS deployed on or near
the calving front, which is sufficiently large/bright to identify
within the aerial imagery would allow further ground control
in the centre of DEMs, away from bedrock CPs.
Due to the lack of reflected light from deep crevasse re-
cesses, the DEM generation process cannot quantify the nar-
rowest sections of all fractures and resultant crevasse depths
are therefore a minimum estimate. The technique is also
clearly limited to line of sight precluding narrow fractures
which extend for tens of centimetres horizontally and poten-
tially up to a few metres vertically (Hambrey and Lawson,
2000; Mottram and Benn, 2009).
Finally, there are a number of practical difficulties when
operating an autonomous aircraft in remote and inaccessible
environments. Mission planning is critical; knowledge of the
local weather conditions, as well as up-to-date satellite im-
agery and DEMs are a prerequisite.
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6 J. C. Ryan et al.: UAV photogrammetry and structure from motion
Table 2. Attributes of mean crevasse width, length and orientation
in each zone labelled in Fig. 5. Orientations are measured along
the long axis of each crevasse with respect to the direction of flow
which is 0.
Mean Mean Mean
Zone width (m) length (m) orientation ()
Zone 1 3.6 9.4 9.2
Zone 2 4.8 14.0 36.7
Zone 3 10.5 32.6 85.1
Zone 4 6.5 17.8 60.4
Zone 5 3.5 8.5 10.8
3 Results
Three successful UAV sorties were flown over Store Glacier
calving front providing imagery, orthophotos and DEMs on
1 and 2 July and the 23 August, herein referred to as flights
and associated products 1 to 3, respectively (Table 1). The
interval between flights 1 and 2 was 19 hours and compari-
son between these outputs enables identification of processes
operating over a daily (short) timescale, be it a very specific
snapshot. The third sortie was flown 52 days later and com-
parison between these outputs enables investigation of late-
seasonal change. The footprint of the four cross-glacier tran-
sects flown extends just over 1km upstream from the calving
front and herein is referred to as the terminus.
3.1 Short timescale calving and surface elevation
change
Residual elevation change between 1 and 2 July (Fig. 3a) re-
veals that the front retreated in two sections by up to 50 and
80m, respectively. The more northerly calving event (A) re-
sulted in a 450m wide section of the terminus retreating by
between 20 and 50m, whilst event B produced between 20
and 80m of retreat across a 400m section (Fig. 3a). In addi-
tion to these two calving events (which are discussed in sec-
tion 3.6), the central 4.5 km frontal section advanced between
12 to 16m (Fig. 3a). At its lateral margins, the calving front
shows no discernible systematic change though there are iso-
lated, small calving events, for example, within 50m of the
southern flank (Fig. 3a). Upstream of the calving front, there
is no net change in mean surface elevation away from the
front and the dappled pattern of residual elevation change is
a result of the advection of crevasses and seracs. Successive
long profiles of the terminus between the 1 and 2 July reveal
specific down-glacier crevasse advection with flow (Fig. 6)
at a rate of 5 and 16md1on profile 1 and 2, respectively.
These results provide corroboration for the surface velocities
derived by feature tracking in Sect. 3.4.
3.2 Seasonal timescale calving-front position and
surface elevation change
Over the entire melt season, larger fluctuations in calving-
front position are observed (Fig. 3b). Over the 19-day pe-
riod from 12 June to 1 July, mean frontal retreat was 160m
(Fig. 3c) and between 2 July and 23 August, the calving
front advanced by an average of 110m to a position simi-
lar to that in 12 June (Fig. 3d). These mean values, however,
do not convey the full extent and detail of the changes ob-
served in the calving front. For example, the central section
of the calving front retreated by up to 525m between the
12 June and 1 July and advanced by up to 450m between
2 July and 23 August (Fig. 3b). Furthermore, the lateral mar-
gins of Store Glacier (the southern 850m and the northern
1.5km) are relatively stable with <50m change in posi-
tion. Over the 52-day period between 2 July (Flight 2) and
23 August (Flight 3) widespread surface lowering of 6.1m
(or 0.12md1)was observed across Store Glacier terminus
(Fig. 4a), which is significantly larger than the estimated ver-
tical uncertainties of the DEMs (±1.9m). Despite the same
dappled patterns caused by local advection of crevasses and
seracs, we infer this to be associated with dynamic thinning
1km upstream of the calving front, which is discussed in
Sect. 4.2.
3.3 Bathymetry
The deepest sector of the calving front is located 1km south
of the centreline and exceeds 540 m below sea level (Fig. 5a).
This 200 m wide sector also corresponds to the greatest thick-
ness of 600m. To the south of this deepest point, the bot-
tom rises rapidly to a 200m deep shelf located 500m from
the flank. To the north of the deepest point, the bottom shal-
lows more gently to within 400m where it becomes steeper
towards the fjord wall.
3.4 Surface velocities
Maximum surface-flow velocities of 16m d1between 1
and 2 July are consistent with results obtained in previous
studies using other techniques, such as feature tracking im-
ages from a land-based time-lapse camera (between 11 and
15md1)(Ahn and Box, 2010; Walter et al., 2012). The
spatial pattern of surface flow from feature tracking of im-
ages between the 1 and 2 July varies considerably across
the terminus of Store Glacier (Fig. 4b) attaining velocities
of 16md1(5.8kma1)near the centre of the glacier down
to 2.5md1at the lateral flanks. Surface velocities are re-
lated to slope, depth, thickness and distance from the lateral
margins (Fig. 5c, d). As would be expected, maximum veloc-
ities correlate with maximum depth and towards the northern
flank are linearly correlated (R2=0.90) with frontal depth
(Fig. 5c). Towards the southern flank the relationship is less
apparent especially between 200 and 350m depths. There
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J. C. Ryan et al.: UAV photogrammetry and structure from motion 7
Figure 5. (a) Profiles showing the sea floor bathymetry and ice sur-
face elevation at the calving front. These data were combined with
surface velocities to estimate the ice flux of Store Glacier. Where the
floatation percentage is over 100%, it is assumed that the ice is not
thick enough to be fully grounded in hydrostatic equilibrium. (b)
The relationship between effective basal shear stress and velocity.
(c) The relationship between depth and velocity. At depths deeper
than 400m, velocities are fairly constant. The two differing rela-
tionships between 150 and 350 m represent velocities from different
sides of the glacier. (d) Relationship between velocity and distance
from the lateral margins. The positive correlation demonstrates the
importance of the resistance provided by the fjord walls.
is a strong correlation between velocities and distance from
the lateral margins which can be approximated by a power
function (R2=0.90) (Fig. 5d). Although application of the
floatation criteria reveals parts of calving front to be buoyant
(Fig. 5a), side-scan sonar observations reveal that the glacier
toe was resting on the fjord bed (Chauché, unpublished).
When the surface flow pattern is combined with frontal
bathymetric data we estimated that the mass flux through the
calving front of Store Glacier was 3.8×107m3d1, equiva-
lent to 13.9Gta1.
Seasonal flow patterns were not obtainable between 2 July
and 23 August as the majority of any matching features
within the study area required for tracking had already calved
into the ocean. Furthermore, it is likely that the morphology
of many crevasses and seracs will have changed significantly
through melt and deformation and would not be recognised
by the cross-correlation procedure.
3.5 Crevassing
The morphology and orientation of crevasses varies
markedly across the terminus (Fig. 6). The largest crevasses
occur in a sector south of the glacier centre line in zone 4
(Fig. 6, Table 2). Here, crevasses have mean minimum depths
of 18m, lengths of 68m and widths of 31m. The largest
crevasses are up to 30m deep, over 500m long and nearly
200m wide but no crevasses that penetrated below sea level
were identified. Most crevasses in this region are arcuate with
limbs pointing towards the calving front and are orientated
obliquely to the direction of ice flow (Fig. 6). This arcuate
morphology of crevasses continues across the central 3km of
the terminus in zone 3 (Fig. 6). Here, crevasses have mean a
depth of 10.5 m, length of 50m and widths of 18 m (Table 2).
In zone 2, 300 to 500 m from the northern flank, crevasses are
aligned obliquely to the direction of ice flow (30–45). Up to
the fjord walls in zones 1 and 5, crevasses are generally ori-
entated parallel to the ice flow (>15) (Fig. 6, Table 2) and
are much smaller with a mean lengths of 22m and width of
8m (Table 2). No discernible difference in average crevasse
depths, lengths or widths was observed between early July
and late August and the pattern and character of crevassing
was also similar.
Water-filled crevasses were clustered in zone 4, coincid-
ing with the sector of larger crevasses (Fig. 6b). Water-filled
crevasses covered 12 000m2or 0.24 % of the survey area (to
1km from the calving front) on 2 July (Table 1). Some
42 individual water-filled crevasses were identified with the
largest having an area of 1200 m2. By 23 August, the number,
size and total area of water-filled crevasses were lower: only
10 water-filled crevasses could be identified, the largest of
which was 400m2and with a total area of 1230m2(0.025%
of the survey area). We were not able to ascertain the depth of
water in the crevasses as no common crevasses could be iden-
tified which drained or filled between flights but this would
be a specific aim of future studies which, with regular sor-
ties, could potentially determine the depth of a crevasse be-
fore filling or after drainage or otherwise exploit the light
reflectance relationship with water depth (e.g. Fitzpatrick et
al., 2014).
Successive profiles of the terminus from 1 and 2 July
demonstrate how the UAV surveys are capable of captur-
ing the displacement of crevasses, which advect downstream
at a rate of 5 and 16md1in profiles 1 and 2, respectively
(Fig. 6). The techniques used in this study are therefore capa-
ble of identifying changes in crevasses geometry, particularly
width and depth through time.
3.6 Calving events
The two calving events identified between 1 and 2 July ap-
pear to take place under contrasting conditions. Event A con-
sisted of the calving of multiple, relatively small ice blocks
with the glacier failing along two main crevasses located 30
and 50m behind the calving front. These crevasses were be-
tween 8 and 10m deep, respectively, and in this instance,
the crevasses located closest to the front were the ones that
failed. Event B appears to be a single large event caused by
the fracturing of a series of parallel crevasses which were
up to 14m deep and 60m behind the calving front. Unlike,
calving event A, the crevasses that failed in event B were
not the closest to the calving front. Indeed, there were other
www.the-cryosphere.net/9/1/2015/ The Cryosphere, 9, 1–11, 2015
8 J. C. Ryan et al.: UAV photogrammetry and structure from motion
crevasses that were deeper and located nearer to the front, yet
did not calve. Water was not observed in any of the crevasses
along which calving took place.
4 Discussion
4.1 Changes occurring over a daily timescale
The orientation of crevasses suggests that lateral drag is an
important resistive stress on Store Glacier. The lateral mar-
gins of Store Glacier are characterised by crevasses that are
orientated parallel to the direction of flow which suggests that
they have formed in response to simple shear stresses associ-
ated with the drag of the fjord walls (Fig. 6) (Benn and Evans,
2010). The importance of lateral drag is further demonstrated
by the morphology of crevasses found near the glacier flow
line (Fig. 6). Their arcuate nature indicates that the princi-
pal tensile stresses operating on the ice have been rotated by
lateral gradients in ice velocity. These gradients are caused
by the simple shear stress between the fjord walls and the
margins of the glacier which cause the ice to flow slower
(Fig. 4b) (Benn and Evans, 2010).
The simple shearing caused by velocity gradients is fur-
ther demonstrated by the differing relationship between ve-
locity and depths between the north and south side of the
glacier (Fig. 5c, d). On the north side, the velocity increases
gradually from the fjord wall to the centre of the glacier,
reflecting the gradual deepening of bathymetry and the re-
sulting decrease of basal and lateral drag. On the south side,
the velocities are higher than the north side for given depths
and distances from the lateral margins (Fig. 5c, d). We hy-
pothesise that, because the deepest part of the glacier is sit-
uated 1km south of the centreline, the ice on south side is
more influenced by faster flowing ice which exerts a simple
shear stress on the shallower, adjacent ice (250–400m thick).
This causes the shallow ice to flow faster than ice with sim-
ilar thicknesses and distance from the lateral margins on the
north side (Fig. 5c).
The mass flux through the calving front was calculated at
3.8×107m3d1which needs to be balanced by three main
frontal processes: calving, submarine melting and advective
advance. Both calving and advance were observed in this
study but it is likely that submarine melting also has a large
role in ice output at a daily timescale. For example, Xu et
al. (2013) used oceanographic data to calculate a melt water
flux of between 0.5 and 1.1 ×107m3d1from Store Glacier
in August 2010 equivalent to 13–29% of the mass flux cal-
culated by our study. For comparison, Rink glacier has an
ice flux of 3.0×107m3d1of which 27% is estimated to
be lost through submarine melting each day (Enderlin and
Howat, 2013).
Figure6.(a)Distribution and patterns of crevasses on Store Glacier.
Dry crevasses, which are large structural features, are shown in or-
ange. Narrower crevasses that are observed in the orthorectified im-
ages but whose 3-D geometry is not constructed are shown in black.
The areas of water-filled crevasses are shown in blue and occur al-
most exclusively in zone 4. The regions of the terminus discussed
are designated by the dotted red lines and are referred to as the
black numbers. Transects 1 and 2 shown in the inset demonstrate
how crevasses adverted downstream between 1 and 2 July. In T1,
a series of calving events occurred which are discussed as calving
event A. In T2, the calving front advanced 16m. (b) Illustration of
the terminus of Store Glacier with ellipsoids proportional to the av-
erage length, width and orientation of crevasses shown in (a) for the
respective zones. The colour of the ellipsoids represents the propor-
tion of crevasses that are water filled in each zone where WF refers
to water filled in the legend. The italicised numbers denote the den-
sity of crevasses per 10m2in each zone. Arrows illustrate inferred
direction of principal strain.
4.2 Changes occurring over a seasonal timescale
The lack of variation in the position of the lateral margins
of the glacier shows that a balance is maintained between
the ice flux input and submarine melting and calving out-
put in this zone throughout the melt season. The balance
could be explained by the mechanism of calving events. At
the lateral margins calving is characterised by small, regu-
The Cryosphere, 9, 1–11, 2015 www.the-cryosphere.net/9/1/2015/
J. C. Ryan et al.: UAV photogrammetry and structure from motion 9
lar events such as calving event A (Fig. 3a). The regularity
of these small events means that any small advance or re-
treat is regulated almost instantly by changes in calving rate
which returns the lateral margins of the glacier to the same
position. Calving rate could also be moderated by changes in
the bathymetry. When the lateral margins advance, calving
rates increase due to the abrupt deepening of the bathymetry
seaward of the lateral margins of the glacier which cause
basal drag to be reduced. Ice-flow acceleration can lead
to increased longitudinal stretching and deeper crevassing,
thereby increasing calving rate and leading to retreat to its
original, bathymetrically pinned position.
The centre of the calving front is much more active with
calving and submarine melt rates that vary on a seasonal
timescale. We propose that the main cause of variability is
due to calving rates which are highly irregular throughout
the melt season (Jung et al., 2010). Our observations also
support the suggestion that calving rates are dominated by
major calving events which have a time interval of around
28 days (e.g. Jung et al., 2010). If the calving front advances
for 28 days at 16md1, it will advance 448m. A large,
single calving event can therefore yield a retreat of 448m
and would explain the variation in the position of the calving
front during the melt season (Fig. 3b). On 25 August 2013,
a tabular iceberg with a length of 500m was observed to
calve from the central zone of Store Glacier.
Towards the end of the melt season (23 August), a
widespread surface deflation of 0.12md1was observed
(Fig. 4a). Application of a simple degree-day model reveals
that part of this lowering can be attributed to ablation. Av-
erage daily air temperatures were recorded at an automated
weather station (AWS) located near the UAV launch site
(Fig. 1) and, using a melt factor of 6–10mm per degree
per day (Hock et al., 2005), surface lowering due to abla-
tion is estimated between 0.038 and 0.064md1. It follows
that ablation alone cannot account for the entire lowering
rate observed and, hence, we infer an additional component
of dynamic thinning due to relative strain extension across
this zone, related to reduced upstream delivery of flux and/or
frontal kinematics associated with enhanced late-season sub-
marine melting and/or calving rates. GPS measurements by
Ahlstrom et al. (2013) tentatively support the former inter-
pretation and reveal that surface velocities 8km upstream of
Store’s calving front tend to decrease between July and Au-
gust. However, this raises questions regarding the timescales
over which dynamic thinning and surface melt occur and
whether or not the flow regime across the terminus is, to
some extent, isolated or operating independent from pro-
cesses upstream supplying mass. Either way, these questions
are beyond the scope of the data sets presented here and re-
quire a study of greater areal extent and temporal coverage.
Another important observation is the order of magnitude
reduction of the area of water-filled crevasses between early
July and late August (Fig. 6). Surface air temperatures di-
rectly influence the extent of water-filled crevasses. AWS
data reveal that mean daily air temperature was 6C dur-
ing the 4 days prior to the UAV sortie on the 2 July. In con-
trast, mean temperature was 3.5C on the 4 days prior to
the UAV sortie on 23 August. Water-filled crevasses have
been hypothesised to penetrate deeper than crevasses with-
out water (Weertman, 1973; van der Veen, 1998) and hence
act as mechanism for calving (Benn et al., 2007). The calving
events observed in this study did not specifically fail at water-
filled crevasses and hence our limited results show no support
for this mechanism. However, studies of greater scope with
daily coverage will be required to determine definitively if
water-filled crevasses have any appreciable impact on calv-
ing dynamics at Store Glacier or elsewhere.
5 Conclusions and future directions
A UAV equipped with a commercial digital camera enabled
us to obtain high-resolution DEMs and orthophotos of the
calving front of a major tidewater glacier at an affordable
price. Airborne lidar currently presents the only alternative
method for acquiring DEMs with comparable accuracy and
precision. However, to fly consecutive sorties in a remote en-
vironment is likely to be prohibitively expensive and with
sufficient ground control points the digital photogrammetry
approach may also exceed the accuracy of this technique.
The three sorties flown enabled key glaciological param-
eters to be quantified at sufficient detail to reveal that the
terminus of Store Glacier is a complex system with large
variations in crevasse patterns surface velocities, calving pro-
cesses, surface elevations and front positions at a daily and
seasonal timescale. Surface velocities vary across the termi-
nus and are influenced by both basal and lateral drag (Figs. 4b
and 5c, d). The oblique orientation and arcuate nature of
crevasses suggests that the principal extending strain rate is
orientated obliquely to the direction of flow and we there-
fore propose that resistive stresses at the terminus of Store
Glacier are dominated by lateral drag (Fig. 6). With this in
mind, the retreat of Store Glacier into a wider trough could
significantly increase the ice discharge. We estimated that
the ice flux through the calving front of Store Glacier was
13.9Gta1and we observed a small terminus advance be-
tween 1 and 2 July (Figs. 3a and 5a). This advance reveals
that, during this period, the sum of calving and submarine
melt rates are less than the ice flux. Calving is an irregular
process and that the position of the calving front returned
to its 12 June position by 23 August suggests that over this
timescale calving and submarine melting balance ice flux
(Fig. 3b). Water-filled crevasses covered 0.24% of the survey
area on 2 July but this fell to 0.025% on 23 August (Fig. 6).
It remains to be seen whether water-filled crevasses are more
likely to initiate calving events but our tentative results here
indicate no support for this mechanism.
Future studies, with more frequent sorties could be used to
compare and investigate further glaciological changes over
www.the-cryosphere.net/9/1/2015/ The Cryosphere, 9, 1–11, 2015
10 J. C. Ryan et al.: UAV photogrammetry and structure from motion
a more continuous timespan. There is also the possibility of
more sophisticated payloads with radiation, albedo and other
multi-band sensors as well as radar and laser altimetry. There
are many potential cryospheric applications for investigation,
such as sea ice, marine and terrestrial-terminating glaciers
and, with increased range, ice sheets, that can be achieved
with the use of repeat UAV surveys. We have demonstrated
that for calving outlet glaciers, a UAV carrying a high-
resolution digital camera would be sufficient to investigate
the following projects:
analysis of the thickness and back stress exerted by the
ice mélange during the winter and the effect of its break
out on glacier flow, calving rate and character;
seasonal changes in the depth, density, orientation and
nature of crevassing and their impact on calving rate and
character;
the influence of daily to seasonal melt and supraglacial
lake drainage on downstream dynamics and calving;
analysis of daily to seasonal fluctuations in calving flux,
terminus position and impact on upstream dynamics and
thinning.
Acknowledgements. We thank Matt Nolan, Doug Benn and
Mauri Pelto for their thorough and insightful reviews, and Anders
Damsgaard for his short comments: all of which greatly improved
the manuscript. Funding for the fieldwork was made possible
by the UK Natural Environmental Research Council (NERC)
grant NE/K005871/1 (Subglacial Access and Fast Ice Research
Experiment (SAFIRE): Resolving the Basal Control on Ice Flow
and Calving in Greenland). NERC also funded The Cryosphere
page charges. J. C. Ryan is funded by an Aberystwyth University
Doctoral Career Development Scholarship (DCDS). We are also
indebted to the crew of S/V Gambo who worked tirelessly to pro-
vide logistical support, and to the Uummannaq Polar Institute and
Children’s Home who provided accommodation in Uummannaq.
Edited by: A. Kääb
References
AgiSoft LLC: AgiSoft PhotoScan, available at: http://www.agisoft.
ru/products/photoscan/ (last access: 14 February 2014), 2013.
Ahlstrøm, A. P., Andersen, S. B., Andersen, M. L., Machguth, H.,
Nick, F. M., Joughin, I., Reijmer, C. H., van de Wal, R. S. W.,
Merryman Boncori, J. P., Box, J. E., Citterio, M., van As, D.,
Fausto, R. S., and Hubbard, A.: Seasonal velocities of eight ma-
jor marine-terminating outlet glaciers of the Greenland ice sheet
from continuous in situ GPS instruments, Earth Syst. Sci. Data,
5, 277-287, doi:10.5194/essd-5-277-2013, 2013.
Ahn, Y. and Box, J. E.: Glacier velocities from time-lapse pho-
tos: technique development and first results from the Ex-
treme Ice Survey (EIS) in Greenland, J. Glaciol., 56, 723–734,
doi:10.3189/002214310793146313, 2010.
Benn, D. I., Warren, C. R., and Mottram, R. H.: Calving processes
and the dynamics of calving glaciers, Earth-Sci. Rev., 82, 143–
179, doi:10.1016/j.earscirev.2007.02.002, 2007.
Benn, D. I. and Evans, D. J. A.: Glaciers and glaciation, London:
Hodder Education, 2010.
Carr, J. R., Vieli, A., and Stokes, C. R.: Climatic, oceanic and to-
pographic controls on marine-terminating outlet glacier behavior
in north-west Greenland at seasonal to interannual timescales, J.
Geophys. Res., 118, 1210–1226, 2013.
d’Oleire-Oltmanns, S., Marzolff, I., Peter, K. D., and Ries, J. B.:
Unmanned Aerial Vehicle (UAV) for monitoring soil erosion in
Morocco, Remote Sens., 4, 3390–3416, 2012.
Enderlin, E. M. and Howat, I. M.: Submarine melt rate estimates
for floating termini of Greenland outlet glaciers (2000–2010), J.
Glaciol., 59, 67–75, doi:10.3189/2013jog12j049, 2013.
Fitzpatrick, A. A. W., Hubbard, A. L., Box, J. E., Quincey, D. J., van
As, D., Mikkelsen, A. P. B., Doyle, S. H., Dow, C. F., Hasholt,
B., and Jones, G. A.: A decade (2002–2012) of supraglacial lake
volume estimates across Russell Glacier, West Greenland, The
Cryosphere, 8, 107–121, doi:10.5194/tc-8-107-2014, 2014.
Hambrey, M. J. and Lawson, W.: Structural styles and deformation
fields in glaciers: A review, Deform. Glacial Mater., 176, 59–83,
doi:10.1144/Gsl.Sp.2000.176.01.06, 2000.
Hock, R.: Glacier melt: a review on processes and their modelling,
Progr. Phys. Geogr., 29, 362–391, 2005.
Holland, D. M., Thomas, R. H., De Young, B., Ribergaard, M. H.,
and Lyberth, B.: Acceleration of Jakobshavn Isbrae triggered
by warm subsurface ocean waters, Nat. Geosci., 1, 659–664,
doi:10.1038/Ngeo316, 2008.
Howat, I. M., Joughin, I., and Scambos, T. A.: Rapid changes in ice
discharge from Greenland outlet glaciers, Science, 315, 1559–
1561, doi:10.1126/science.1138478, 2007.
Howat, I. M., Box, J. E., Ahn, Y., Herrington, A., and McFadden, E.
M.: Seasonal variability in the dynamics of marine-terminating
outlet glaciers in Greenland, J. Glaciol., 56, 601–613, 2010.
Howat, I. M., Negrete, A., and Smith, B. E.: The Greenland Ice
Mapping Project (GIMP) land classification and surface eleva-
tion data sets, The Cryosphere, 8, 1509–1518, doi:10.5194/tc-8-
1509-2014, 2014.
Hubbard, A.: The Times Atlas and actual Greenland ice loss, Geol.
Today, 27, 214–217, 2011.
Hugenholtz, C. H., Levin, N., Barchyn, T. E., and Baddock,
M. C.: Remote sensing and spatial analysis of aeolian sand
dunes: A review and outlook, Earth-Sci. Rev., 111, 319–334,
doi:10.1016/j.earscirev.2011.11.006, 2012.
Hugenholtz, C. H., Whitehead, K., Brown, O. W., Barchyn, T. E.,
Moorman, B. J., LeClair, A., Riddell, K., and Hamilton, T.: Ge-
omorphological mapping with a small unmanned aircraft sys-
tem (sUAS): Feature detection and accuracy assessment of a
photogrammetrically-derived digital terrain model, Geomorphol-
ogy, 194, 16–24, doi:10.1016/j.geomorph.2013.03.023, 2013.
Joughin, I., Das, S. B., King, M. A., Smith, B. E., Howat,
I. M., and Moon, T.: Seasonal speedup along the western
flank of the Greenland ice sheet, Science, 320, 781–783,
doi:10.1126/science.1153288, 2008.
Joughin, I., Smith, B. E., Howat, I. M., Floricioiu, D., Alley, R. B.,
Truffer, M., and Fahnestock, M.: Seasonal to decadal scale vari-
ations in the surface velocity of Jakobshavn Isbrae, Greenland:
The Cryosphere, 9, 1–11, 2015 www.the-cryosphere.net/9/1/2015/
J. C. Ryan et al.: UAV photogrammetry and structure from motion 11
Observation and model-based analysis, J. Geophys. Res.-Earth
Surf., 117, F02030, doi:10.1029/2011jf002110, 2012.
Jung, J., Box, J. E., Balog, J. D., Ahn, Y., Decker, D. T., and Haw-
becker, P.: Greenland glacier calving rates from Extreme Ice Sur-
vey (EIS) time lapse photogrammetry, C23B-0628, American
Geophysical Union, San Francisco, 2010.
Kargel, J. S., Ahlstrøm, A. P., Alley, R. B., Bamber, J. L., Benham,
T. J., Box, J. E., Chen, C., Christoffersen, P., Citterio, M., Cogley,
J. G., Jiskoot, H., Leonard, G. J., Morin, P., Scambos, T., Shel-
don, T., and Willis, I.: Brief communication Greenland’s shrink-
ing ice cover: “fast times” but not that fast, The Cryosphere, 6,
533–537, doi:10.5194/tc-6-533-2012, 2012.
Lucieer, A., Turner, D., King, D. H., and Robinson, S. A.: Using an
Unmanned Aerial Vehicle (UAV) to capture micro-topography of
Antarctic moss beds, Int. J. Appl. Earth Observ.Geoinform., 27,
53–62, 2014.
Luckman, A., Murray, T., de Lange, R., and Hanna, E.: Rapid and
synchronous ice-dynamic changes in East Greenland, Geophys.
Res. Lett., 33, L03503, doi:10.1029/2005GL025428, 2006.
Moon, T. and Joughin, I.: Changes in ice front position on Green-
land’s outlet glaciers from 1992 to 2007, J. Geophys. Res.-Earth
Surf., 113, F02022, doi:10.1029/2007jf000927, 2008.
Mottram, R. H. and Benn, D. I.: Testing crevasse-depth models: A
field study at Breioamerkurjokull, Iceland, J. Glaciol., 55, 746–
752, doi:10.3189/002214309789470905, 2009.
Schild, K. M. and Hamilton, G. S.: Seasonal variations of outlet
glacier terminus position in Greenland, J. Glaciol., 59, 759–770,
doi:10.3189/2013jog12j238, 2013.
Ullman, S.: The interpretation of structure from motion, Proc. Roy.
Soc. London, B203, 405–426, 1979.
van der Veen, C.: Fracture mechanics approach to penetration of
bottom crevasses on glaciers, Cold Reg. Sci. Technol., 27, 213–
223, 1998.
Verhoeven, G.: Taking computer vision aloft – archaeological three-
dimensional reconstructions from aerial photographs with Photo-
scan, Archaeol. Prospection, 18, 67–73, 2011.
Vieli, A., Funk, M., and Blatter, H.: Tidewater glaciers: Frontal
flow acceleration and basal sliding, Ann. Glaciol., 31, 217–221,
doi:10.3189/172756400781820417, 2000.
Xu, Y., Rignot, E., Fenty, I., Menemenlis, D., and Flexas, M.
M.: Subaqueous melting of Store Glacier, West Greenland
from three-dimensional, high-resolution numerical modeling
and ocean observations, Geophys. Res. Lett., 40, 4648–4653,
doi:10.1002/Grl.50825, 2013.
Walter, J. I., Box, J. E., Tulaczyk, S., Brodsky, E. E., Howat, I.
M., Ahn, Y., and Brown, A.: Oceanic mechanical forcing of a
marine-terminating Greenland glacier, Ann. Glaciol., 53, 181–
192, doi:10.3189/2012aog60a083, 2012.
Weertman, J.: Can a water-filled crevasse reach the bottom surface
of a glacier?, IAHS Publ., 95, 139–145, 1973.
Whitehead, K., Moorman, B. J., and Hugenholtz, C. H.: Brief
Communication: Low-cost, on-demand aerial photogrammetry
for glaciological measurement, The Cryosphere, 7, 1879–1884,
doi:10.5194/tc-7-1879-2013, 2013.
Weidick, A.: Greenland, with a section on Landsat imagesof Green-
land, in, Satellite image atlas of glaciers of the world, edited by:
Williams, R. S. and Ferrigno, J. G., US Geological Survey, Wash-
ington, DC, C1–C105 (USGS Professional Paper 1386-C), 1995.
www.the-cryosphere.net/9/1/2015/ The Cryosphere, 9, 1–11, 2015
... Repeated UAV surveys have been performed to derive glacier surface velocities (e.g., Immerzeel et al., 2014;Kraaijenbrink et al., 2016;Wigmore and Mark, 2017;Benoit et al., 2019), to map surface temperatures of debris-covered glaciers (Kraaijenbrink et al., 2018), and to determine glacier albedo (Ryan et al., 2017) and surface roughness (Rossini et al., 2018). Multi-temporal UAV orthophotos and digital surface models (DSMs) have also been used to assess calving dynamics of outlet glaciers (Ryan et al., 2015;Jouvet et al., 2017), to investigate seasonal glacier surface changes (e.g., Groos et al., 2019;Yang et al., 2020), and to estimate surface mass balance patterns (Van Tricht et al., 2021). Moreover, UAV-based aerial surveys have been used for manual and automatic mapping of periglacial landforms (e.g., Dąbski et al., 2017;Mather et al., 2019;Glasser et al., 2020) and proglacial river geometries (Avian et al., 2020). ...
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Unoccupied Aerial Vehicles (UAVs) equipped with optical instruments are increasingly deployed in high mountain environments to investigate and monitor glacial and periglacial processes. The comparison and fusion of UAV data with airborne and terrestrial data offers the opportunity to analyse spatio-temporal changes in the mountains and to upscale findings from local UAV surveys to larger areas. However, due to the lack of gridded high-resolution data in alpine terrain, the specific challenges and uncertainties associated with the comparison and fusion of multi-temporal data from different platforms in this environment are not well known. Here we make use of UAV, airborne, and terrestrial data from four (peri)glacial alpine study sites with different topographic settings. The aim is to assess the accuracy of UAV photogrammetric products in complex terrain, to point out differences to other products, and to discuss best practices regarding the fusion of multi-temporal data. The surface geometry and characteristic geomorphological features of the four alpine sites are well captured by the UAV data, but the positional accuracies vary greatly. They range from 15 cm (root-mean-square error) for the smallest survey area (0.2 km²) with a high ground control point (GCP) density (40 GCPs km⁻²) to 135 cm for the largest survey area ( > 2.5 km²) with a lower GCP density (<10 GCPs km⁻²). Besides a small number and uneven distribution of GCPs, a low contrast, and insufficient lateral image overlap ( < 50–70%) seem to be the main causes for the distortions and artefacts found in the UAV data. Deficiencies both in the UAV and airborne data are the reason for horizontal deviations observed between the datasets. In steep terrain, horizontal deviations of a few decimetres may result in surface elevation change errors of several metres. An accurate co-registration and evaluation of multi-temporal UAV, airborne, and terrestrial data using tie points in stable terrain is therefore of utmost importance when it comes to the investigation of surface displacements and elevation changes in the mountains. To enhance the accuracy and quality of UAV photogrammetry, the use of UAVs equipped with multi-spectral cameras and high-precision positioning systems is recommended, especially in rugged terrain and snow-covered areas.
... In general, SfM algorithms proceed by sequentially (a) extracting a set of distinctive local features in the available images, (b) robustly matching them across images, (c) optimising their 3D positions, (d) determining the camera parameters and (e) adding more images to the reconstruction during each iteration. The outputs of this process are the camera parameters and a sparse 200 point cloud with 3D points consisting of matched 2D features (Ryan et al., 2015). Using fixed camera parameters so-obtained, a dense point cloud can be estimated using a process called Multi-View Stereo (MVS). ...
Article
We present an approach for extracting quantifiable information from archival aerial photographs to extend the temporal record of change over a region of the central eastern Greenland Ice Sheet. The photographs we use were gathered in the 1930s as part of a surveying expedition, and so they were not acquired with photogrammetric analysis in mind. Nevertheless, we are able to make opportunistic use of this imagery, as well as additional, novel data-sets, to explore changes at ice margins well before the advent of conventional satellite technology. The insights that a longer record of ice margin change bring is crucial for improving our understanding of how glaciers are responding to the changing climate. In addition, our work focuses on a series of relatively small and little studied outlet glaciers from the eastern margin of the Ice Sheet. We show that whilst air and sea surface temperatures are important controls on the rates at which these ice masses change, there is also significant heterogeneity in their responses, with non-climatic controls (such as the role of bathymetry in front of calving margins) being extremely important. In general, there is often a tendency to focus either on changes of the Greenland Ice Sheet as a whole, or on regional variations. Here, we suggest that even this approach masks important variability, and full understanding of the behaviour and response of the Ice Sheet requires us to consider changes that are taking place at the scale of individual glaciers.
... In these cases, updated knowledge of the glacier morphology is essential. This datum can be achieved through the production of digital elevation models (DEMs) using structure from motion (SfM) conducted using drones (Ryan et al., 2015;Fugazza et al., 2018), helicopter (Girod et al., 2017), or ground-based manual acquisitions (Piermattei et al., 2015;Piermattei et al., 2016;Fugazza et al., 2018). Furthermore, SfM provides orthoimages too, which help data interpretation. ...
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In mountain glaciers, the influence of bedrock geometry on glacier surface morphology is often assumed; quantitative evidence, however, is rare. In our research, we measured the ice thickness of the Planpincieux Glacier (North-west Italy) and detected the bedrock topography using ground-penetrating radar. Additionally, we investigated the glacier surface morphology using structure from motion and the glacier kinematics using digital image correlation of terrestrial images. A digital terrain analysis showed evidence of recurrent crevasses whose position corresponded to bedrock steps. On average, since 2014, their positions varied between 6 and 14 m and were 40 ± 8 m downstream of the bedrock steps. Bedrock and glacier topography presented out-of-phase correlated undulations that approximately fit a sinusoidal function of different amplitude. Moreover, we show the morphological evolution of an unstable sector whose thickness at the end of the ablation seasons has remained approximately constant since 2014. Contrarily, the ice melting during the 2020 ablation season caused a volume loss of >30%. Since, in general, the damages provoked by a potential ice avalanche depend primarily on the involved volume, this finding demonstrates that frequent morphology monitoring is essential for correct glacial hazard assessment.
... A key strength of UAV remote sensing is the ability to easily repeat surveys to model changes in forest dynamics (Anderson and Gaston, 2013;Gao et al., 2020;Guimarães et al., 2020;Toth and Jóźków, 2016). Repeat UAV surveys are becoming more common, with applications in tracking coastal erosion, river banks and glacier dynamics, but studies in forests are limited (Bagaram et al., 2018;Gonçalves and Henriques, 2015;Guerra-Hernández et al., 2017;Hemmelder et al., 2018;Larrinaga and Brotons, 2019;Ryan et al., 2015). Aligning UAV data is a difficult problem, with errors likely to cause artefacts and bias in tracking changes, and work is needed to find the best way to resolve these issues (Fu et al., 2021;Messinger et al., 2016;Swinfield et al., 2019). ...
Thesis
Tropical rain forests are important carbon stores and harbours of biodiversity but are being cleared at an unprecedented rate. There is an estimated 2 billion hectares of degraded forest globally, which retains a large proportion of its biodiversity. Restoration of these lands is needed to meet global commitments to combat the interlinked climate and biodiversity crises, and effective, scalable and affordable monitoring of the restoration process is essential. High resolution remote sensing technologies offer the best hope for monitoring at scale. In particular, unoccupied aerial vehicles (UAVs) offer a viable option for high spatial and temporal resolution remote sensing, though methods to guide forest restoration with these are still in their infancy. This thesis introduces approaches for the use of remote sensing data to guide tropical forest management, with particular focus on the use of UAV data in the context of restoration, looking at canopy structure, composition and dynamics. First, I introduce the context of tropical forest restoration, discussing the contribution of remote sensing to monitoring and understanding projects, with a focus on the recent developments around the use of UAVs. I also introduce the main study site of this thesis --- an ecosystem restoration concession of nearly 100 km^2 in Sumatra, Indonesia, known as Hutan Harapan. Next, I introduce a method for delineating individual tree crowns in three dimensions from remote sensing data in the form of point clouds, as created by light detection and ranging (LiDAR) and UAV structure from motion (SfM) approaches. This method, MCGC, makes use of graph cut concepts from mathematics combined with understanding of tree crown geometry and allometric scaling to automatically map tree crowns. I validate this approach using data collected in Borneo, comparing forests with three distinctive structures, showing the power of this approach to both map trees and estimate aboveground biomass. In Chapter 3, I develop a pipeline for automatic mapping of key tree species prevalence at Hutan Harapan from photographs taken from a UAV. I show it is possible to break up imagery over management units into superpixels, and through a combination of spectral and textural patterns in the imagery, train an automatic classifier to detect the species of interest from UAV imagery. I then show the power of this approach to map prevalence of key tree species indicative of the successional stage of forest recovery and demonstrate the utility of this approach for guiding management. I find that using an extra camera to take photographs with additional wavebands only slightly improved mapping accuracy. Finally, I use a combination of a LiDAR survey in 2014 and UAV surveys in 2017 and 2018 to track the effects of the strong El Niño event of 2015-16 on the canopy at Hutan Harapan, looking at 3 sites of varying recovery status spanning 100 ha of forest. I find that early-successional forest was less resistant to the drought than taller secondary forest – with canopy height loss and high mortality. However, in the subsequent high-rainfall period, I observe that early-successional forests recovered strongly. Together, the analyses demonstrate that early-successional stages lost and then regained canopy height to a greater extent that taller forest, highlighting the power of repeat surveys using LiDAR and UAVs to track canopy dynamics. Finally, I critically evaluate the methods developed, highlighting how the insights they provide can be useful for restoration practitioners, underlining the key role that remote sensing, especially with a UAV, can play whilst also needing further development.
... The availability and affordability of UAV technology has spurred interest in the field of glacier dynamics, with numerous studies applying UAV derived data to assess glacier dynamics at high spatial resolutions. Researchers have employed UAVs as tools to assess glacier calving (Ryan et al., 2015;van Dongen et al., 2021), track glacier motion (Immerzeel et al., 2014;Che et al., 2020), and measure mass loss (Bash et al., 2018;Fugazza et al., 2018; in remote regions. The majority of UAVbased cryosphere studies have been focused in Antarctica (Westoby et al., 2015;Westoby et al., 2016;Florinsky and Bliakharskii, 2019), the polar regions (Ewertowski et al., 2016;Tonkin et al., 2016;Bernard et al., 2017a;Bernard et al., 2017b;Cimoli et al., 2017), the European Alps (Mauro et al., 2015;Boesch et al., 2016;De Michele et al., 2016;Fugazza et al., 2018;Rossini et al., 2018;Vivero and Lambiel, 2019), and High Mountain Asia (Immerzeel et al., 2014;Brun et al., 2016;Kraaijenbrink et al., 2016;Vincent et al., 2016;Brun et al., 2018). ...
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The glaciers of the North Cascades have experienced mass loss and terminus retreat due to climate change. The meltwater from these glaciers provides a flux of cold glacier meltwater into the river systems, which supports salmon spawning during the late summer dry season. The Nooksack Indian Tribe monitors the outlet flow of the Sholes Glacier within the North Cascades range with the goal of understanding the health of the glacier and the ability of the Tribe to continue to harvest sustainable populations of salmon. This study compares the UAV derived glacier ablation with the discharge data collected by the Tribe. We surveyed the Sholes Glacier twice throughout the 2020 melt season and, using Structure-from-Motion technology, generated high resolution multispectral orthomosaics and Digital Elevation Models (DEMs) of the glacier on each of the survey dates. The DEMs were differenced to reveal the surface height change of the glacier. The spectral data of the orthomosaics were used to conduct IsoData unsupervised classification. This process divided the survey area into Snow, Ice, and Rock classes that were then used to attribute the surface height changes of the DEMs to either snow or ice melt. The analysis revealed the glacier lost an average thickness of −0.132 m per day (m d ⁻¹ ) with snow and ice losing thickness at similar rates, −0.130 m d ⁻¹ and −0.132 m d ⁻¹ respectively. DEM differencing reveals that a total of −550,161 ± 45,206 m ³ water equivalent (w.e.) was discharged into Wells Creek between the survey dates whereas the stream gauge station measured a total discharge of 350,023 m ³ . This study demonstrates the ability to spectrally classify the UAV data and derive discharge measurements while evaluating the small-scale spatial variability of glacier melt. Assessing ablation in small alpine glaciers is of great importance to downstream communities, like the Nooksack Indian Tribe who seek to understand the magnitude and timing of glacier melt in order to better protect their salmon populations. With this paper, we provide a baseline for future glacier monitoring and the potential to connect the snow surface properties with the rate of snow melt into a warming future.
... Upscaling from discrete spectroscopic measurements, yielding data at a finer spatial resolution, was achieved using a DJI Phantom Pro 3 drone (hereafter, DPP3) following the uncrewed aerial vehicle (UAV) methodologies described by Ryan et al. [30,35,70]. Between DOY 203 and 230, the DPP3 was flown periodically over the study site at 70 m altitude above the ice surface (Figure 1b), using a pre-defined flight-plan. ...
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Ice surface albedo is a primary modulator of melt and runoff, yet our understanding of how reflectance varies over time across the Greenland Ice Sheet remains poor. This is due to a disconnect between point or transect scale albedo sampling and the coarser spatial, spectral and/or temporal resolutions of available satellite products. Here, we present time-series of bare-ice surface reflectance data that span a range of length scales, from the 500 m for Moderate Resolution Imaging Spectrometer’s MOD10A1 product, to 10 m for Sentinel-2 imagery, 0.1 m spot measurements from ground-based field spectrometry, and 2.5 cm from uncrewed aerial drone imagery. Our results reveal broad similarities in seasonal patterns in bare-ice reflectance, but further analysis identifies short-term dynamics in reflectance distribution that are unique to each dataset. Using these distributions, we demonstrate that areal mean reflectance is the primary control on local ablation rates, and that the spatial distribution of specific ice types and impurities is secondary. Given the rapid changes in mean reflectance observed in the datasets presented, we propose that albedo parameterizations can be improved by (i) quantitative assessment of the representativeness of time-averaged reflectance data products, and, (ii) using temporally-resolved functions to describe the variability in impurity distribution at daily time-scales. We conclude that the regional melt model performance may not be optimally improved by increased spatial resolution and the incorporation of sub-pixel heterogeneity, but instead, should focus on the temporal dynamics of bare-ice albedo.
... The latter prevents the SfM-MVS methods from correctly constructing the deeper parts of the crevasse. Additionally, the lowest points of the crevasses are very narrow and may not be captured accurately (Ryan et al., 2015). However, in an aerodynamic context those narrow crevasses are not likely to have a significant influence on the heat exchange since they lie below the penetration depth of effective turbulent mixing (Nicholson et al., 2016). ...
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The aerodynamic roughness length (z0) is an important parameter in the bulk approach for calculating turbulent fluxes and their contribution to ice melt. However, z0 estimates for heavily crevassed tidewater glaciers are rare or only generalised. This study used uncrewed aerial vehicles (UAVs) to map inaccessible tidewater glacier front areas. The high-resolution images were utilised in a structure-from-motion photogrammetry approach to build digital elevation models (DEMs). These DEMs were applied to five models (split across transect and raster methods) to estimate z0 values of the mapped area. The results point out that the range of z0 values across a crevassed glacier is large, by up to 3 orders of magnitude. The division of the mapped area into sub-grids (50 m × 50 m), each producing one z0 value, accounts for the high spatial variability in z0 across the glacier. The z0 estimates from the transect method are in general greater (up to 1 order of magnitude) than the raster method estimates. Furthermore, wind direction (values parallel to the ice flow direction are greater than perpendicular values) and the chosen sub-grid size turned out to have a large impact on the z0 values, again presenting a range of up to 1 order of magnitude each. On average, z0 values between 0.08 and 0.88 m for a down-glacier wind direction were found. The UAV approach proved to be an ideal tool to provide distributed z0 estimates of crevassed glaciers, which can be incorporated by models to improve the prediction of turbulent heat fluxes and ice melt rates.
Article
Hailuogou (HLG) Glacier, a rapidly receding temperate glacier in the southeastern Tibetan Plateau, has been observed to lose mass partly through ice frontal mechanical ablation (i.e., ice collapse). These events are difficult to monitor and quantify due to their small scale and frequent nature. However, recent developments in Uncrewed Aerial Vehicles (UAV) have provided a possible approach to track their spatiotemporal variation and their impact on the geomorphological evolution of the glacier terminus area. Here, we present analysis from UAV surveys conducted over eight field campaigns to the HLG Glacier, providing evidence of glacier change between October 2017 and November 2020. Structure from Motion with Multi-View Stereo (SfM-MVS) was applied to produce multi-temporal Digital Surface Models (DSMs) and orthophoto mosaics, from which geomorphological maps and DEMs of Difference (DoDs) were derived to quantify glacier changes. These analyses reveal that at the margins of the glacier terminus retreated 132.1 m over the period of analysis, and that in the area specifically affected by collapsing (i.e., the glacier collapsed terminus), it retreated 236.4 m. Overall the volume lost in the terminal area was of the order of 184.61 ± 10.32 × 10⁴ m³, within which the volume change due to observed collapsing events comprises approximately 28%. We show that ice volume changes at the terminus due to a single ice collapse event may exceed the interannual level of volume change, and the daily volume of ice loss due to ice calving exceeds the seasonal and interannual level by a factor of ~2.5 and 4. Our results suggest that the evolution of the HLG Glacier terminus is dominantly controlled by the frontal ice-water interactions. If the future evolution of glaciers such as HLG Glacier is to be robustly predicted, the contribution of mechanical ablation should be accounted for by numerical models.
Article
In a warming Arctic, as glacier snowlines rise, short‐ to medium‐term increases in seasonal bare‐ice extent are forecast for the next few decades. These changes will enhance the importance of turbulent energy fluxes for surface ablation and glacier mass balance. Turbulent energy exchanges at the ice surface are conditioned by its topography, or roughness, which has been hypothesised to be controlled by supraglacial hydrology at the glacier scale. However, current understanding of the dynamics in surface topography, and the role of drainage development, remains incomplete, particularly for the transition between seasonal snow cover and well‐developed, weathered bare‐ice. Using time‐lapse photogrammetry, we report a daily timeseries of fine (millimetre)‐scale supraglacial topography at a 2 m2 plot on the Lower Foxfonna glacier, Svalbard, over two 9‐day periods in 2011. We show traditional kernel‐based morphometric descriptions of roughness were ineffective in describing temporal change, but indicated fine‐scale albedo feedbacks at depths of ~60 mm contributed to conditioning surface topography. We found profile‐based and two‐dimensional estimates of roughness revealed temporal change, and the aerodynamic roughness parameter, z0, showed a 22‐32% decrease from ~1 mm following the exposure of bare‐ice, and a subsequent 72‐77% increase. Using geostatistical techniques, we identified ‘hole effect’ properties in the surface elevation semivariograms, and demonstrated that hydrological drivers control the plot‐scale topography: degradation of superimposed ice reduces roughness while the inception of braided rills initiates a subsequent development and amplification of topography. Our study presents an analytical framework for future studies that interrogate the coupling between ice surface roughness and hydro‐meteorological variables and seek to improve parameterisations of topographically evolving bare‐ice areas.
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Digital photogrammetry is becoming an important tool for remote data acquisition. The use of digital outcrop models (DOMs) for fracture analysis has grown in the field of geosciences. Structure-from-Motion- Multi-view Stereo (SfM-MVS) is a recent technique that allows the creation of DOMs in a simple and inexpensive way. To direct an in-depth study of the application of SfM-MVS in the investigation of fractured media, the thesis sought to answer which parameters related to these systems can be obtained through the 3D models generated by SfM-MVS, and which factors have an influence on the data accuracy. Models of different outcrops (igneous, sedimentary and metamorphic rocks) were generated using sets of images at different scales, captured by different cameras and platforms. The models were analyzed using three approaches: to measure fracture properties; qualitative analysis of the factors that influence its quality and the derived data; and investigation of the quality levels offered by Agisoft Metashape. As a result, a list of best practices for fieldwork for the construction of an DOM was produced. It was concluded that the RPA is the most flexible platform available for different topographies, as it allows flight planning or manual piloting, while offering a good cost-benefit. In fracture analysis, algorithms that calculate orientation, spacing and persistence are easily implemented. These three parameters depend on the ability to identify fractures in the generated 3D model. This, in turn, is influenced by the spatial resolution and the presence of deformations in the model. For roughness, the difficulty of quantification lies in the efficient conversion of the Joint Roughness Coefficient (JRC) to three-dimensional space. In investigating the options offered by the Metashape program, the analyzed data suggest the accuracy-quality pairs high-high and highest-high as the best costbenefit ones, the first being the safest for application in field conditions. The results obtained point to the existence of a tripod, composed of three highly correlated elements: scope, time and cost. Changing one of them without impacting the others will sacrifice the final quality of the model, and consequently of the analysis. Another important result was the development of the multiplatform application SurvAid, an image survey planning tool; and the open software CAPI, dedicated to the structural analysis of DOMs. In conclusion, SfM-MVS proved to be a powerful technique for generating MDAs, which can be used in fracture analysis. Still, it should be seen as a complementary tool to field work.
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A map of Greenland in the 13th edition (2011) of the Times Comprehensive Atlas of the World made headlines because the publisher's media release mistakenly stated that the permanent ice cover had shrunk 15% since the previous 10th edition (1999) revision. The claimed shrinkage was immediately challenged by glaciologists, then retracted by the publisher. Here we show: (1) accurate maps of ice extent based on 1978/87 aerial surveys and recent MODIS imagery; and (2) shrinkage at 0.019% a-1 in ~50 000 km2 of ice in a part of east Greenland that is shown as ice-free in the Times Atlas.
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We present 17 velocity records derived from in situ stand-alone single-frequency Global Positioning System (GPS) receivers placed on eight marine-terminating ice sheet outlet glaciers in South, West and North Greenland, covering varying parts of the period summer 2009 to summer 2012. Common to all the observed glacier velocity records is a pronounced seasonal variation, with an early melt season maximum generally followed by a rapid mid-melt season deceleration. The GPS-derived velocities are compared to velocities derived from radar satellite imagery over six of the glaciers to illustrate the potential of the GPS data for validation purposes. Three different velocity map products are evaluated, based on ALOS/PALSAR data, TerraSAR-X/Tandem-X data and an aggregate winter TerraSAR-X data set. The velocity maps derived from TerraSAR-X/Tandem-X data have a mean difference of 1.5% compared to the mean GPS velocity over the corresponding period, while velocity maps derived from ALOS/PALSAR data have a mean difference of 9.7%. The velocity maps derived from the aggregate winter TerraSAR-X data set have a mean difference of 9.5% to the corresponding GPS velocities. The data are available from the GEUS repository at doi:10.5280/GEUS000001.
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In September 2011 two Greenland stories hit the press, by far the bigger of which was the widespread misreporting (and consequent backlash) that Greenland had apparently lost 15 per cent of its ice cover since 1999. The public are used to an annual press bombardment of record temperatures, ever increasing melt and ice retreat in polar regions but a 15 per cent loss in 12 years does seem fanciful.
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Many of Greenland's marine-terminating outlet glaciers have undergone rapid retreat in the past decade, accompanied by accelerated flow and dynamic thinning. Superimposed on this pattern of retreat, these glaciers undergo seasonal variations in terminus position, corresponding roughly to wintertime advance and summertime retreat. We compiled near-daily time series of terminus position for five of Greenland's largest outlet glaciers (Daugaard Jensen, Kangerdlugssuaq and Helheim glaciers in East Greenland, and Jakobshavn Isbrae and Rink Isbrae in West Greenland) using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. There are spatial differences in the timing of the onset of seasonal retreat among all the glaciers in our study, as well as variability in terminus behavior for individual glaciers from year to year. We examine whether this spatial and temporal variability is linked to above-freezing air temperatures or high sea surface temperatures, but find no simple relationship. Instead, we hypothesize that terminus geometry (ice thickness, subglacial topography, fjord bathymetry) exerts an important control on the response of marine-terminating glaciers to climate perturbations. Models for predicting outlet glacier response to climate change need to include this complex interaction between geometry and environmental forcing.
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As part of the Greenland Ice-sheet Mapping Project (GIMP) we have produced three geospatial datasets for the entire ice sheet and periphery. These are (1) a complete, 15 m resolution image mosaic, (2) ice-covered and ice-free terrain classification masks, also posted to 15 m resolution and (3) a complete, altimeter-registered Digital Elevation Model posted at 30 m. The image mosaic was created from a combination of Landsat-7 and RADARSAT-1 imagery acquired between 1999 and 2002. Each pixel in the image is stamped with the acquisition date and geo-registration error to facilitate change detection. This mosaic was then used to manually produce complete ice-covered and ice-free land classification masks. Finally, we used satellite altimetry and stereo-photogrammetric DEMs to enhance an existing DEM for Greenland, substantially improving resolution and accuracy over the ice margin and periphery.
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Remotely sensed glaciological measurements can be expensive, often involving a trade-off between resolution, scale, and frequency. We report on a case study in which two low-cost techniques were used to generate digital elevation models and orthomosaics of an Arctic glacier in consecutive ablation seasons. In the first aerial survey we used an unmanned aerial vehicle and acquired images autonomously. The following year we took advantage of the helicopter used for site access, and were able to acquire images manually, for little additional helicopter time. We present a preliminary assessment of accuracy and apply these data to measure glacier thinning and motion.
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Supraglacial lakes represent an ephemeral storage buffer for meltwater runoff and lead to significant, yet short-lived, episodes of ice-flow acceleration by decanting large meltwater and energy fluxes into the ice sheet's hydrological system. Here, a methodology for calculating lake volume is used to quantify storage and drainage across Russell Glacier, West Greenland, between 2002 and 2012. Using 502 MODIS scenes, water volume at ~200 seasonally occurring lakes was derived using a depth-reflectance relationship, which was independently calibrated and field validated against lake bathymetry. The inland expansion of lakes is strongly correlated with air temperature: during the record melt years of 2010 and 2012, lakes formed and drained earlier, attaining their maximum volume 38 and 20 days earlier than the 11 yr mean, as well as occupying a greater area and forming at higher elevations (> 1800 m) than previously. Despite occupying under 2% of the study area, lakes delay the transmission of up to 7-13% of the bulk meltwater discharged. Although the results are subject to an observational bias caused by periods of cloud cover, we estimate that across Russell Glacier, 28% of supraglacial lakes drain rapidly (< 4 days). Clustering of such events in space and time suggests a synoptic trigger mechanism. Further, we find no evidence to support a unifying critical size or depth-dependent drainage threshold.
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[1] We present three-dimensional, high-resolution simulations of ice melting at the calving face of Store Glacier, a tidewater glacier in West Greenland, using the Massachusetts Institute of Technology general circulation model. We compare the simulated ice melt with an estimate derived from oceanographic data. The simulations show turbulent upwelling and spreading of the freshwater-laden plume along the ice face and the vigorous melting of ice at rates of meters per day. The simulated August 2010 melt rate of 2.0±0.3 m/d is within uncertainties of the melt rate of 3.0±1.0 m/d calculated from oceanographic data. Melting is greatest at depth, above the subglacial channels, causing glacier undercutting. Melt rates increase proportionally to thermal forcing raised to the power of 1.2–1.6 and to subglacial water flux raised to the power of 0.5–0.9. Therefore, in a warmer climate, Store Glacier melting by ocean may increase from both increased ocean temperature and subglacial discharge.
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[1] Discharge from marine-terminating outlet glaciers represents a key component of the Greenland Ice Sheet mass budget and observations suggest that mass loss from northwest Greenland has recently accelerated. Despite this, the factors controlling outlet glacier dynamics within this region have been comparatively poorly studied. Here we use remotely sensed data to investigate the influence of atmospheric, oceanic, and glacier-specific controls on the frontal position of Alison Glacier (AG), northwest Greenland, and nine surrounding outlet glaciers. AG retreated by 9.7 km between 2001 and 2005, following at least 25 years of minimal change. Results suggest that sea ice and air temperatures influence glacier frontal position at seasonal and interannual timescales. However, the response of individual outlet glaciers to forcing was strongly modified by factors specific to each glacier, specifically variations in fjord width and terminus type. Overall, our results underscore the need to consider these factors in order to interpret recent rapid changes and predict the dynamic response of marine-terminating outlet glaciers to atmospheric and oceanic forcing.
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Nye has estimated that the depth L of crevasses is equal to Tjog, where T is the tensile stress causing extending flow, Q is the density of ice, and g is the gravitational acceleration. This expression for L is derived on the assumption that the crevasses are closely spaced and free of water. It is shown in this paper that the depth of an isolated crevasse is a factor sr/2 greater than the depth calculated by Nye for closely spaced crevasses. It is shown further that the presence of water in a crevasse can increase its depth. A crevasse filled with water up to at least 97.4% of its depth can penetrate to the bottom of a glacier. Water-filled cavities can exist directly beneath water filled crevasses. RÉSUMÉ. Une crevasse emplie d'eau peut-elle atteindre la surface basale d'un glacier ? Nye a estimé que la profondeur L des cre-vasses est égale à Tjog, où Test la contrainte d'élongation causant un écoulement d'extension, est la densité de la glace et g est l'accélération de la pesanteur. Cette expression pour L est dérivée en supposant que les crevasses sont proches et vides d'eau. On montre ici que la profondeur d'une crevasse isolée est plus grande d'un facteur de JT/2 que la profondeur calculée par Nye pour les crevasses proches l'une de l'autre. Il est montré de plus que la présence d'eau dans une crevasse peut augmenter sa profondeur. Une crevasse emplie d'eau d'au moins 97,4% de sa profondeur peut pénétrer jusqu'au fond d'un glacier. Des cavités pleines d'eau peuvent exister directement sur des crevasses emplies d'eau.