UAV Systems for Photogrammetric Data Acquisition of Archaeological Sites

Article (PDF Available) · January 2012with146 Reads
DOI: 10.1260/2047-4970.1.0.7
The use of UAV systems for surveying archaeological sites is becoming progressively more common due to the considerable potential in terms of rapidity of survey, costs and accuracy. The paper presents the first results of the photogrammetric survey of the archaeological site of Himera in Sicily (Italy) using by UAV systems. A complete documentation of the site through the production of a DSM and an ortho image were carried out. The research further evaluated two different image processing workflows: a typical photogrammetric approach and a computer vision approach. An ortho image of the archaeological site with a very high resolution was obtained.
M. Lo Brutto
*, A. Borruso
, A. D’Argenio
Dept. of Civil, Environmental, Aerospace and Materials Engineering, University of Palermo, Italy -
Consorzio Ticonzero, Palermo, Italy - (aborruso, adargenio)
KEY WORDS: UAVs, Photogrammetry, Archaeology, Computer Vision, DSM, Ortho image
The use of UAV systems for surveying archaeological sites is becoming progressively more common due to the considerable
potential in terms of rapidity of survey, costs and accuracy. The paper presents the first results of the photogrammetric survey of the
archaeological site of Himera in Sicily (Italy) using by UAV systems. A complete documentation of the site through the production
of a DSM and an ortho image were carried out. The research further evaluated two different image processing workflows: a typical
photogrammetric approach and a computer vision approach. An ortho image of the archaeological site with a very high resolution
was obtained.
* Corresponding author.
The documentation and preservation of archaeological sites
often require the development of fast and easy techniques for
3D data acquisition, also in difficult conditions. Close range
photogrammetry and terrestrial laser scanning are the most
common techniques used. These techniques made it possible to
obtain a high level of detail and accuracy and result to be very
effective, especially for small or medium-extension
archaeological sites (up to tens of hectares). However, for large
archaeological sites close range photogrammetry and terrestrial
laser scanner are not always the most suitable survey
techniques; whereas, the information obtained from aerial or
satellite images provide an overview of the study area, which is
fundamental for the interpretation of archaeological structures.
In fact, images obtained by metric aerial cameras (film and
digital) or by high resolution satellite sensor have been used in
archaeology for long (Cowley, 2011). It should be pointed out
that such images have some limitations linked to the geometric
resolution (typically of some decimetres and inadequate for
detailed studies), to the periods of acquisition (which does not
always correspond to a given particularly useful date for the
purposes of the archaeological work), and ultimately to the cost.
In the last years small UAVs (Unmanned Aerial Vehicles) have
become standard platforms for large-scale aerial mapping of
areas at limited extent. Many tests have been done to verify the
photogrammetric aspects and their potential applications (Haala
et al., 2011; Eisenbeiss & Sauerbier, 2011). In particular, the
performances of these photogrammetric systems are very high
above all for aerial survey of archaeological sites (Eisenbeiss et
al., 2005; Chiabrando et al., 2011; Hendrickx et al., 2011).
Beyond the acquisition phases, the image processing phase
remains an unexplored topic as it should clearly defines the real
capabilities of these systems for photogrammetric data
collection and for archaeological survey.
In order to evaluate the UAVs workflow, some tests were
performed for the study of the archaeological site of Himera
(Sicily, southern Italy) using by close-range aerial
photogrammetry techniques with micro UAVs. The main
purpose was to obtain, besides the photographic documentation,
a Digital Surface Model (DSM) and an ortho image of the site.
The work was used to test different image processing
workflows. The images were processed with both typical
photogrammetric and computer vision approaches in order to
identify the more efficient process. The two different
approaches were compared in relation to the accuracy and
automation of orientation and to the quality of photogrammetric
data production.
The adopted pipeline was composed of different step like
automatic flight planning, image orientation, image matching
and DSM processing, ortho image generation.
The UAVs survey was allowed to acquire the data to extract a
DSM and an ortho image of the site with a very high resolution
and good accuracy.
The studied area corresponds to the higher part of the Himera
archaeological site. The ancient city of Himera was situated on
the coast of northern Sicily, about 30 kilometres far from
Palermo. Himera, along with Selinunte, represents the western
limit achieved by the Greek colonization. The first known
settlement of Himera dates back to the VII Century BC; until its
destruction (409 BC), the city extended to cover a surface of
about 100 hectares.
The ancient urban area was built on a very complex landscape
which included an area at the sea level (Lower Town) and a
higher one (Upper Town); the Upper Town was located in a
upland, named Himera Plane.
The work focused only on the Upper Town, where it is still
possible to recognize two main groups of buildings: the
“Northern Quarters” and the Sacred Area of the Athena
Temenos. The “Northern Quarters” cover a surface of about four
hectares and host the ruins of regularly shaped buildings; the
Sacred Area of the Athena Temenos hosts four temples and is
bounded by a perimeter wall.
The aerial photogrammetric survey on the Himera site was
realized using as carrier a microdrone md4-200 remotely piloted
quadricopter. This carrier is equipped with four independent
brushless electric motors and is able to fly up to 20 minutes at a
maximum height of about 150 m (Table 1).
Load capacity (kg) 0,2
Weight (kg) 0,9
Diameter (cm) 70
Operation range (m) 500
Maximum relative
height (m)
Flight range (min) 20
4 x 250W flatcore
Camera Pentax Optio A40
Auto-pilot Yes
Table 1: Technical specifications of md4-200
The quadricopter is radio controlled by a human operator, so the
operation range is mostly limited by the reach of the radio
control. Md4-200 electronic equipment includes an Inertial
Measurement Unit (IMU), a GPS antenna and a communication
system, which allows a ground based station to receive
telemetry data from the carrier and video signals from the
onboard sensors. Furthermore, an onboard Flight Controller
device permits to autonomously execute automated flights.
Automation includes route and all the issues related to image (or
other kind of data) acquisition (such as: carrier attitude, camera
orientation, zoom level). This feature is of particular interest in
photogrammetric applications since it allows the design of flight
plans with the desired forward overlap and side overlap. Thanks
to the GPS antenna, flight planning can be designed on
geospatial basis. This procedure can be executed in a software
(mdCockpit) provided by the carrier manufacturer or, as in this
case, in an external GIS environment. Owing to the low load
capacity of the quadricopter, the system is equipped with a 12
Mpx compact consumer camera (Pentax Optio A40) slightly
modified to be mounted on the bottom of the carrier.
The data acquisition workflow of the Himera site was composed
by the following steps:
Preliminary study of the area;
• Flight planning;
• Survey execution.
The “Preliminary study of the area” was subdivided in an
identification of the site on cartography (mostly small scale
maps and orthophotos), which was included in a GIS project,
and an in situ inspection for the location of all possible
problems for the flight (obstacles, physiography, electric cables,
For the “Flight planning” the archaeological site was divided in
two parts: a larger area (about 135 m x 285 m), corresponding
to the highest part of the ancient inhabited town (Upper Town)
and a smaller one (about 35 m x 60 m), that includes the
remains of the religious building of Athena Temenos. Owing to
the limited autonomy of the drone, different photogrammetric
flights were planned to cover the entire area. The Upper Town
was covered by three flights at a relative height of 100 m, while
for the Athena Temenos was designed a single flight at 20 m
from the ground surface (Figure 1). The “Flight planning” was
performed in GIS environment and resulted in a code to be
uploaded on the Flight Controller.
Figure 1: Flight planning
The images acquisition was executed setting the camera to its
maximum resolution (12 Mpx) and using a focal length of 7.9
mm (Table 2).
Pentax Optio A40
Focal length
7.9 mm
CCD Width
7.6 mm
CCD Height
5.7 mm
Pixel size
1.9 µm
Shutter time
Table 2: Camera parameters
With these parameters the Ground Sample Distance (GSD) was
about 2.4 cm, for the flights from 100 m, and 0.4 cm, for the
flight from 20 m. Image forward overlap and side overlap were
set to 80% and 60%, respectively (Table 3).
Flight height above ground
100 m 20 m
Forward overlap 80%
Side overlap 60%
GSD 2.4 cm 0.4 cm
Ground coverage
per image
96 m x 72 m 19 m x 14 m
Table 3: Planned flight parameters
Regarding the “Survey execution”, the flights were executed in
autonomous mode, following the predefined flight path. This
mode is the most useful for photogrammetric data acquisition.
Images acquisition was performed in “stop mode”; the UAV
system flies to a predefined way point and stops at its location
to acquire the image (Eisenbeiss & Sauerbier, 2011). For every
way point three images were acquired to assure the adequate
data redundancy.
8 Progress in Cultural Heritage Preservation – EUROMED 2012
Before executing the flights, 30 targets, to be used as Ground
Control Points (GCPs) during the image processing, were
distributed along the area. A topographic survey was carried out
to measure the coordinates of the targets using a Leica TCR
1105 total station. From a single point station, all the targets in a
local reference system were measured. The accuracy of the
target coordinates was estimated about 1÷2 cm. Subsequently,
two targets were measured additionally by a static GPS
surveying with a Topcon Hyper Pro using as master station a
permanent station of the University of Palermo GNSS Network.
These two last points were used to convert the coordinates from
the local reference system to global reference system WGS84-
ETRS2000 datum (UTM projection). The conversion was
performed through a
translation and a rotation of all the targets.
No scale factor was applied during the coordinates computation
to not introduce to the result all deformations of the
cartographic projection. The cartographic height was obtained
adding the difference between the height in the local reference
system and the height in the WGS84-ETRS2000 datum
calculated for one GPS point to all targets.
Flight execution lasted about two hours and produced 165
images at 100 m and 48 images at 20 m.
The flights with micro UAVs are usually planned following the
typical aerial flight where all images are acquired with a nadir
view. This condition may not always be obtained with micro
UAVs flight, because the short weight, the line and the attitude
of flight can be modified by the wind.
The first phase of data processing was dedicated to the
identification of the most suitable images for the work. In
particular, it has been first verified the forward overlap and the
side overlap. The figure 2 shows an example of a sequence of
three images taken from the same waypoints; it can be noted
that, even if the images were taken in a very short interval of
time (few seconds), significant displacements of the camera
may occur.
Besides, using consumer digital camera and in the absence of
any device able to compensate for the movement of the sensor
during the acquisition, the radiometric quality of the images can
be degraded. This can affect both the automatic autocorrelation
process and the manual photogrammetric measurement.
Therefore, the presence of images particularly blurred has been
verified; the figure 3 shows a sequence of three images, taken
by the same way point, where the image quality degrades
considerably due to this effect.
The redundancy of images for each way point, however, has
allowed the selection of a data set with the best characteristics
as regards the block geometry and the radiometric quality.
Overall 58 images were selected for flights from 100 meters and
17 images for the flight from 20 meters. Each flight from 100
meters was consisted of a photogrammetric block of three
strips; two blocks had direction north-south, the third east-west.
The flight from 20 meters had four strips and direction north-
south (Table 4).
Figure 2: Sequence of three images taken from the same
waypoints with significant displacements of the camera
Figure 3: Sequence of three images taken by the same way
point in which image quality degrades considerably.
Images Strips Direction
F179 100 m 23 3 North-South
F180 100 m 20 3 East-West
F182 100 m 15 3 North-South
F183 20 m 17 4 North-South
Table 4: Photogrammetric block configuration
4.1 Camera calibration
A fully automatic self-calibration was carried out in laboratory
using iWitness Pro software and 20 black & white coded targets
to determine the interior orientation parameters of the camera.
The camera was set with the same parameters to be used during
the flights: focal length fixed at minimum zoom (widest angle),
focus fixed to infinite. The coded targets positioned to form a
3D calibration grid (Figure 4).
A network of 20 convergent images was taken from each side
and from the diagonal of the grid. The network included also
images with ±90º roll angles. Through the camera calibration
the principal distance (c), the principal point position (xp, yp)
and the radial distortion coefficients (K1, K2, K3) were
calculated (Table 5).
For more accurate results, the camera calibration parameters
should be obtained under conditions that are similar to the
photogrammetric survey. This approach is not simple for UAV
project because it requires a suitable test field and flight plan.
Moreover, using consumer digital camera the overall accuracy
obtained with lab calibration parameters and field calibration
parameters are similar (Pérez et al. 2011).
Figure 4: 3D calibration grid for camera calibration.
Parameter Value Stand. dev.
c 8.765 mm 0.001 mm
xp -0.054 mm 0.001 mm
yp 0.019 mm 0.001 mm
K1 2.07420e-003 1.9049e-005
K2 1.69550e-006 2.4102e-007
K3 -6.61121e-007 9.1445e-008
Table 5: Camera calibration parameters.
4.2 Images orientation
In UAV projects, the images processing is still a topic of great
interest as shown in several papers; not always the traditional
photogrammetric systems are the most effective. Due to the
various problems that may occur during the acquisition phase
(irregular block geometry, poor radiometric quality) “not all
software packages could be used in all possible applications of
UAV photogrammetry; quite often, only the combination of
several packages enabled us to completely process the data set”
(Eisenbeiss, 2009). Some studies have been conducted to verify
the possibility of using free and low cost software solutions
(Neitzel et Klonowski, 2011), others to develop integrated
procedures of computer vision and photogrammetry for fully
automatic UAV image orientation (Barazzetti et al., 2010). In
some applications, it was used an approach by the computer
vision techniques to obtain orientation data and surface model
to use as input for photogrammetric packages (Haala et al.,
2011; Rosnell et al, 2011).
In this work the Flight F180 (Table 4) was used as dataset to
test the different image orientation realized using two
approaches: photogrammetric and computer vision techniques.
The photogrammetric orientation was carried out with the
software Socet Set by BAE Systems; this package is one of the
most popular software for the processing of aerial images.
For the photogrammetric image orientation the camera
calibration parameters, previously calculated with iWitness Pro,
were used. The approximate microdrone position and
orientation data, provided by the GPS/IMU, were also used; in
this way it was possible to reconstruct the photogrammetric
block configuration.
An automatic aerial triangulation (AAT) was performed by the
routine APM (Automatic Measurement Point) of the software
Socet Set selecting a tie point pattern with 25 points per image
distributed uniformly under a regular grid. Overall 776 image
coordinates were measured obtaining automatically 82 tie
points. The automated process required a phase of manual
editing to correct some false matching. A quality check on some
points showed that the poor stability of the microdrone produces
images with very different perspectives of the same point
(Figure 5).
Figure 5: A tie point in several images
The exterior orientation parameters were computed using 8
GCPs; the control points were measured in semi-automated
mode, identifying manually the target on an image and
automatically by searching in all other through image matching.
As shown in figure 6 the position of the control points was not
suitable, however through a rigorous bundle block adjustment
10 Progress in Cultural Heritage Preservation – EUROMED 2012
was possible to calculate the exterior orientation parameters
obtaining a RMS for images residuals of ±1.4 pixel and for
GCPs residuals of ±4.7 cm, ±3.7 cm and ±4.6 cm in the X, Y
and Z directions respectively.
Figure 6: Block configuration and GCPs
Computer vision techniques was applied using the PhotoScan
software (by Agisoft LLC) which is a low cost image-based
package aimed to obtain high quality 3D model. The software is
based on multi-view 3D reconstruction technology and can
operate with calibrated and un-calibrated images in both
controlled and uncontrolled conditions. The general workflow
includes the fully automatic image orientation and 3D model
reconstruction. All the processes can be performed with
different levels of accuracy and many parameters can be set to
improve the final result.
For lens calibration, PhotoScan software uses a pinhole camera
model in the typical formulation of computer vision and, like all
the computer vision applications, while carrying out photos
alignment estimates both internal and external camera
orientation parameters, including nonlinear radial distortions.
For this reason, the camera calibration parameters previously
calculated were not used.
With PhotoScan the automatic orientation was performed in
various steps, with increasing accuracy. In a first step the
images orientation was calculated with a low accuracy: the
process oriented the 20 images of flight F180 in 4 minutes
computing about 10000 tie points. Subsequently, the orientation
was re-calculated with a medium accuracy obtaining about
54000 tie points in 7 minutes. Finally, the calculation was
performed with the higher accuracy setting; a total of about
100000 tie points were determined in less than 10 minutes. The
processing time, although with a much bigger number of points
and with the higher accuracy setting, was much shorter than
those obtained with the software Socet Set (about 15 minutes).
It is important to note that the computation time depends on the
PC characteristics, thus the previous information was reported
to evidence the increase of computational load. In particular, for
this work a PC with 32-bit Windows XP, a processor with 2.4
GHz and 4 GB of RAM was used. These characteristics,
especially the 32-bit operation system, could affect the
performance of the software.
The image orientation was obtained in a local reference system
and in an arbitrary scale. The software do not provide any
accuracy information about the image orientation. The camera
parameters calculated during the self calibration were different
in comparison with those obtained with lab calibration using
iWitness Pro.
The photogrammetric targets were manually measured to
transform the image orientation in the global reference system
(WGS84-ETRS2000 datum - UTM projection). Assigning the
cartographic coordinates to the targets, the photogrammetric
block was referenced in the global reference system through a
3D transformation (3D translation, 3D rotation and a scale
factor). Nine markers were used to compute the parameters of
the 3D transformation obtaining a RMS of ±2.5 cm, ±1.5 cm
and ±2.4 cm in the X, Y and Z directions respectively.
4.3 DSM and Ortho image generation
A DSM and an ortho image were calculated with the two
software after the images orientation.
With the software Socet Set the module NGATE, that performs
image correlation and edge matching on each image pixel, was
used. The DSM was calculated in a regular grid with a
resolution of 50 cm by selecting the best images in relation to
the radiometric characteristics and to their Base/Height ratio
(Figure 7).
Figure 7: DSM obtained with Socet Set
Using the software PhotoScan it was not possible to define a
geometric resolution of the DSM but the level of detail was
chosen regarding the parameter "Target quality" (Ultra High,
High, Medium, and Low, Lowest) and of the maximum number
of faces in the final mesh. A "Target quality" medium and a
maximum number of 200000 faces for the mesh were set for our
work. The calculation of the DSM has allowed to obtain a
surface with a point’s resolution ranging from 20 cm to 60 cm
(Figure 8).
With both software were made of the ortho images with a
geometric resolution of 5 cm. Only with Socet Set it was
possible to select the best images for ortho image generation. A
qualitative comparison between the two ortho images has
showed no noticeable differences (Figure 9).
For this reason, to obtain the data of the whole archaeological
site, all the images were processed by computer vision approach
with the software PhotoScan and the same workflow used for
the flight F180. In this way processing time was significantly
reduced. The figure 10 shows the ortho image obtained for the
archaeological site of Himera by the flights from 100 meters.
Figure 8: DSM obtained with PhotoScan
Figure 9: Visual check between the ortho images made with
PhotoScan (in B&W) and made with Socet Set (in RGB)
Figure 10: Ortho image of the archaeological site of Himera
The use of UAVs for the aerial survey of the archaeological site
of Himera evidenced the high performance of these systems.
Some problems could occur during image acquisition, however
with a very high redundancy of images a DSM and an ortho
image suitable for large scale mapping with very high resolution
and accuracy can be obtained.
The computer vision technique has proven to be more simple
and more fast than the photogrammetric one, though the last is a
rigorous approach. For this reason the computer vision approach
can be used on the survey of the archaeological sites; and
further tests should to be performed in different locations with
difficult morphological conditions to evaluate the real
performance of UAV systems for rapid and accurate data
References from Journals:
Chiabrando, F., Nex, F., Piatti, D., Rinaudo, F., 2011. UAV
and RPV systems for photogrammetric surveys in
archaeological areas: two tests in the Piedmont region (Italy).
Journal of Archaeological Science, 38(3), pp. 697-710.
Eisenbeiss, H., Sauerbier, M., 2011. Investigation of UAV
systems and flight modes for photogrammetric applications.
The Photogrammetric Record, 26(136), pp. 400-421.
Hendrickx, M., Gheyle, W., Bonne, J., Bourgeois, J., De Wulf,
A., Goossens, R., 2011. The use of stereoscopic images taken
from a microdrone for the documentation of heritage – An
example from the Tuekta burial mounds in the Russian Altay.
Journal of Archaeological Science, 38(11), pp. 2968–2978.
References from Other Literature:
Barazzetti, L., Remondino, F., Scaioni, M., 2010. Fully
automated UAV image-based sensor orientation. International
Archives of the Photogrammetry, Remote Sensing and Spatial
Information Sciences, Vol. XXXVIII-1, p. 6.
Cowley, D.C., 2011. Remote Sensing for Archaeological
Heritage Management. EAC Occasional Paper No. 5;
Occasional Publication of the Aerial Archaeology Research
Group No. 3; Europae Archaeologiae Consilium: Budapest,
Hungary, p. 307.
Eisenbeiss, H., 2009. UAV photogrammetry. Diss. ETH No.
18515, Institute of Geodesy and Photogrammetry, ETH Zurich,
Switzerland, Mitteilungen Nr.105, p. 235.
Eisenbeiss, H., Lambers, K., Sauerbier, M., Zhang, L., 2005.
Photogrammetric documentation of an archaeological site
(Palpa, Peru) using an autonomous model helicopter.
International Archives of the Photogrammetry, Remote Sensing
and Spatial Information Sciences, Vol. XXXIV-5/C34, pp. 238-
Haala, N., Cramer, M., Weimer, F., Trittler, M., 2011.
Performance test on UAV-based photogrammetric data
collection. International Archives of the Photogrammetry,
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XXXVIII-1/C22, p. 6.
Neitzel, F., Klonowski, J., 2011. Mobile 3D mapping with a low
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Photogrammetry, Remote Sensing and Spatial Information
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Pérez, M., Aguera, F., Carvajal, F., 2011. Digital camera
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1/C22, p. 6.
12 Progress in Cultural Heritage Preservation – EUROMED 2012
Rosnell, T., Honkavaara, E., Nurminen, K., 2011. On geometric
processing of multi-temporal image data collected by light UAV
systems. International Archives of the Photogrammetry,
Remote Sensing and Spatial Information Sciences, Vol.
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