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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 ∼40 cm ground
sampling distance. Stereo-photogrammetry applied to these
images enabled the extraction of high-resolution digital ele-
vation models (DEMs) with vertical accuracies of ±1.9 m
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 2.5 m d−1at the margins to in
excess of 16 m d−1at the centreline. Ice flux through the
calving front is 3.8×107m3d−1, equivalent to 13.9Gta−1
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 110m a.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 ∼3 kg (including
1 kg 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 40 km 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
The Cryosphere, 9, 1–11, 2015 www.the-cryosphere.net/9/1/2015/
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 10 Hz 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 USD 2000. 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 24 mm 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 73.7◦horizontal and 53.1◦vertical 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 500 m a.s.l. (Fig. 1). Based on the cam-
era’s focal length and field of view (53.1◦by 73.7◦), the
ground (sea-level) footprint at 500m a.s.l. for each photo was
∼450×750 m. 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 h−1and 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 40 cm/pixel ground sampling resolu-
tion (GSD), which were resampled to a Cartesian 50 cm
grid to enable intercomparison (Table 1). Higher resolutions
(<30 cm 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 17.12 m horizontally and 11.38 m vertically. Positional er-
rors were due to the specified limits of the onboard L1 GPS
of ±5.0 m 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 70.401◦N, −50.665◦E and 335.85 m 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 ±1.41 m horizon-
tally and ±1.90 m vertically. The georeferenced 3-D DEMs
and orthophotos were then exported at 50cm pixel size for
further analysis in ArcGIS and ENVI software packages.
The Cryosphere, 9, 1–11, 2015 www.the-cryosphere.net/9/1/2015/
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
917 kg m−3and for sea water of 1028kg m−3(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 0.12md−1was 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 m d−1whilst the margins flow less than 5md−1.
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 ±1.9 m. The two-stage procedure outlined
in Sect. 2.4 therefore enabled us to improve the relative po-
sitional uncertainties from nearly 20 m to less than 2 m. 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
80 m, 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 400 m 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 16m d−1on 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 850 m and the northern
1.5 km) 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 0.12 m d−1)was observed across Store Glacier terminus
(Fig. 4a), which is significantly larger than the estimated ver-
tical uncertainties of the DEMs (±1.9 m). Despite the same
dappled patterns caused by local advection of crevasses and
seracs, we infer this to be associated with dynamic thinning
1 km upstream of the calving front, which is discussed in
Sect. 4.2.
3.3 Bathymetry
The deepest sector of the calving front is located 1 km 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 ∼600 m. To the south of this deepest point, the bot-
tom rises rapidly to a 200m deep shelf located 500 m 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 16md−1between 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
15 m d−1)(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 16 m d−1(5.8 km a−1)near the centre of the glacier down
to 2.5 m d−1at 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 350 m 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 3.8×107m3d−1, equiva-
lent to ∼13.9 Gt a−1.
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 18 m, lengths of 68 m and widths of 31 m. The largest
crevasses are up to 30m deep, over 500m long and nearly
200 m 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 18m (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
8 m (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 12 000 m2or 0.24% of the survey area (to
∼1 km 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 1230 m2(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 16 m d−1in 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 10 m 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 14 m deep and 60 m 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 1 km 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
3.8×107m3d−1which 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 ×107m3d−1from 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 ×107m3d−1of which 27 % is estimated to
be lost through submarine melting each day (Enderlin and
Howat, 2013).
Figure 6. (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 16 m d−1, it will advance ∼448 m. A large,
single calving event can therefore yield a retreat of ∼448 m
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 0.12m d−1was 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 0.064m d−1. 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 ∼6◦C dur-
ing the 4 days prior to the UAV sortie on the 2 July. In con-
trast, mean temperature was ∼3.5◦C 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
13.9 Gt a−1and 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
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