Small drones for geo-archaeology in the steppe: locating and
documenting the archaeological heritage of the Orkhon Valley in
Mongolia
M. Oczipka*a, J. Bemmannb, H. Piezonkab, J. Munkhabayarc, B. Ahrensb, M. Achtelikd, F. Lehmanna
a
German Aerospace Center (DLR), Inst. Robotics and Mechatronics, Rutherford St. 2, 12489 Berlin,
Germany
b
Bonn University, Department of Pre- and Protohistoric Archaeology, Regina-Pacis-Weg 7
53113 Bonn, Germany
c
Mongolian Academy of Sciences, Institute of Archaeology, Jucov street 77, Ulaanbaatar 51,
Mongolia
d
Ascending Technologies GmbH, Konrad-Zuse-Bogen 4, 82152 Krailling, Germany
Keywords: UAV, aerial photography, aerial triangulation, Digital Surface Model, ortho photo mosaic, geo-archaeology
SUMMARY
The international project “Geo-Archaeology in the Steppe – Reconstruction of Cultural Landscapes in the Orkhon valley,
Central Mongolia” was set up in July 2008. It is headed by the Department of Pre- and Protohistoric Archaeology of
Bonn University. The project aims at the study of prehistoric and historic settlement patterns, human impact on the
environment and the relation between towns and their hinterland in the Orkhon valley, Central Mongolia. The
multidisciplinary project is mainly sponsored for three years by the German Federal Ministry of Education and Research
(BMBF) and bridges archaeology, natural sciences and engineering (sponsorship code: 01UA0801C). Archaeologists of
the Mongolian Academy of Sciences and of the Bonn University, geographers of Free University Berlin, geophysics of
the Institute for Photonic Technology Jena and the RWTH Aachen University, and geographers and engineers of the
German Aerospace Centre Berlin collaborate in the development of new technologies and their application in
archaeology1. On the basis of Russian aerial photographs from the 1970s, an initial evaluation regarding potential
archaeological sites was made. Due to the poor geometric and radiometric resolution of these photographs, identification
of archaeological sites in many cases remained preliminary, and detailed information on layout and size could not be
gained. The aim of the flight campaign in September 2008 was therefore the confirmation of these sites as well as their
high resolution survey. A 10 megapixel range finder camera was used for the recording of high resolution aerial
photography. This image data is suited for accurate determination and mapping of selected monuments. The airborne
camera was adapted and mounted on an electrically operated eight propeller small drone. Apart from high resolution
geo-referenced overview pictures, impressive panoramic images and very high resolution overlapping image data was
recorded for photogrammetric stereoscopic processing. Due to the overlap of 85% along and across the track each point
in the image data is recorded in at least four pictures. Although a smaller overlap might be sufficient for generating
digital surface models (DSM), this redundancy increases the reliability of the DSM generation. Within this
photogrammetric processing digital surface models and true ortho photo mosaics with a resolution up to 2,5 cm/pixel in
X, Y, Z are derived.
1.
OCTOCOPTER, CAMERA, FLIGHT PLANNING AND MEASUREMENT FLIGHTS
Over the last years unmanned aerial vehicles (UAV) became more and more attractive platforms for many remote
sensing applications. Next to small drones some UAVs have the same size as an aircraft with a long endurance and high
altitude operation ability. In the beginning of this development, most UAVs were powered by fuel engines. Although
fuel powered engines still allow for much longer endurance time, improvements in battery technology with an increased
*
[email protected]; phone +49 030 67055423; fax 1 222 555-876; www.dlr.de/rm
energy density make operation times up to 25 minutes possible. These drones are less noisy and attract less attention then
fuel powered engines. For the measurement flights in the field campaign in autumn 2008 a completely new designed
drone was used. Referring to the expression helicopter this device is called an Octocopter. In contrast to standard
helicopters, Octocopters only use different rotation speeds and directions at the rotors for steering. Except for the motors
no mechanical parts are used in this UAV. Therefore, they are less sensitive to harsh conditions and need less
maintenance. This device weighs about 3.9 kg including the stabilized camera mount and can be easily taken apart and
reassembled for transport. In addition electrically powered UAV can be easily transported in aircrafts and transferred to
the operating area. Of course local legal limitations such as restrictions on the use of GPS signals or flight regulations
could apply.
Real-time barometric measurements and GPS/INS is used to control the altitude and the position of the Octocopter.
Apart from take off and landing, an optimized flight controlling soft- and hardware allows fully automated photo
acquisition. In case the measurement flight cannot be carried out as expected, e.g. in the case of wind speeds over 12 m/s
the Octocopter tries to return to its take off position. If this is not possible an emergency procedure allows its
autonomous landing.
For the recording of the aerial photographs a LEICA M8 range finder camera and a 21 mm VOIGTLÄNDER lens was
used. The M8 is one of the few cameras without an anti aliasing filter. Thus a very high pixel contrast can be archived.
In a first step, potential archaeological sites were identified using black and white Russian aerial photographs recorded in
the 1970s. Based on these georeferenced images, the coordinates of upper left and lower right corners of the target were
identified. Due to poor radiometric and geometric resolution of the aerial photographs, the initially identified area of
interest was too small and a much larger area had to be flown. The corners for the new flight planning were identified
using a simple handheld GPS receiver. In a fully automated process – using complex but easy to use flight planning
software – the flight lines and trigger points were calculated and transferred to the Octocopter.
With a resolution of 10 megapixels at a flying altitude of 76m above ground a geometric resolution of 2.5 cm is
achieved. In fact most areas were recorded with a 5cm resolution from an altitude of 170m during this survey.
Fig. 1. Octocopter with mounted camera and remote control
Table 1. Camera and image parameter
x across flight direction
y in flight direction
Focal length
21 mm
21 mm
Pixel distance
6,8 µm
6,8 µm
Pixel number
3936
2630
Maximum angle of aperture
65,0°
46,1°
Altitude above ground
76,2 m
76,2 m
Overlap
85%
85%
Geometric resolution
2,5 cm
2,5 cm
Camera parameter (Leica M8 + Voigtländer
21 mm)
Flight parameter
Size of the recorded area / picture
97,1 m
64,9 m
Optical image center
14,6 m
9,7 m
Overlap at maximum ground height
85%
85%
Table 2: UAV parameters (Octocopter “Falcon 12”)
Specifications
Weight of Vehicle (ready to fly incl. battery)
4900 g
Dimensions
125 cm x 120 cm x 20 cm
Battery Type
Lithium Polymer 11.1 V 16-32 Ah
Max. Speed
50 km/h
Max. Wind Speed
40 km/h
Usual Operating Altitude
300 m
Payload
1500 g
GPS
available
Max. Flight Time at Max. Payload
Up to 20 min
2.
CAMERA CALIBRATION AND PHOTOGRAMMETRIC PROCESSING
For the radiometric calibration of the camera the parameters of DSNU (Dark Signal Non-Uniformity) and PRNU (Photo
Response Non-Uniformity) are measured under laboratory conditions2. These Parameters are used to correct the image
data radiometrically. The inner orientation was determined in a geometric camera calibration process. In this case, a
relatively new method using diffractive optical elements and a laser beam was applied3.
Exterior orientation is assigned from the trajectory and ground control points (GCP). GCPs were signalized during the
measurement flights with 20 cm x 20 cm plastic tiles. The position of these tiles was recorded with two Trimble 5700
GPS receivers – one as reference and one as rover – and optimized during dGPS post processing. The position of the
GCPs could be determined with an accuracy of less than 2 cm in X, Y and 4cm in Z. The octocopters trajectory is logged
during the measurement flight. It contains the position measured by a L1 GPS receiver, the height derived from a
calibrated barometer and the rotation angles roll, pitch and yaw recorded using a MEMS (Micro-Electro-Mechanical
system) sensor. This log file also contains the trigger points of each image. Although these measurements are not very
accurate, they can be used as an approximate camera position and orientation for post processing. Overview aerial
photographs allow correcting errors manually.
Inner orientation and GPS/ INS values describing the trajectory of the camera as well as the control points are used to set
up the aerial triangulation (AT). AT is used to calculate the exterior orientation. With doing so all projective distortions
are eliminated. Within this automatic process homologous points in the images are determined. A minimum of 150 tie
points per image is required. In areas of poor contrast or repetitive patterns tie points are added manually. With the
results of the AT a digital surface model (DSM) is derived from the image data. Two different matching algorithms are
applied. One algorithm is a stereo matching process called Semi-Global Matching4 developed by Heiko Hirschmueller at
the German Aerospace Centre, the other is a combination of feature based and least square matching. In contrast to the
combined algorithm, SGM calculates a 3D coordinate for each pixel. The result is a dense grid of height values. The
DSMs are used to generate ortho photos. True ortho photos can only be generated based on the SGM process because all
image pixels have a real 3D coordinate. An example of differences in the derived ortho photos is given in Fig. 2. While
the left image is a true ortho photo without any perspective distortion, the right image shows an ortho photo and the
remaining distortion. Due to the distortion, the extensions of this object cannot be clearly identified in the ortho photo.
Fig. 2. Comparison between true ortho photo and ortho photo. The image data was recorded in the morning and in the
afternoon. The two algorithms chose two different photos for the generation of the ortho photo. This is the reason for
the different shadows.
3.
RESULTS AND VISUALISATION
Within the project ”Geo-Archaeology in the Steppe – Reconstruction of Cultural Landscapes in the Orkhon valley,
Central Mongolia”, the archaeological sites previously located in the Russian photographs were visited during the first
field campaign in Mongolia in autumn 2008. A total of 14 sites were recorded in 14 days (Table 3). In some cases, the
ortho photos showed that settlement structures are more extended than initially estimated on the basis of the black and
white aerial photographs. This and the very high resolution of DSM and image data, as well as panoramic views and
generated 3D models are part of the geographic basis for the detailed archaeological analysis (Fig. 3 and Fig. 5).
Final products are digital surface models and ortho photo mosaics of all 14 sites. Interactive 3D models and oblique
views are also generated. Table 3 shows overviews of all the sites including descriptions and preliminary interpretations.
Fig. 3. Panoramic 360° view
Table 3. Overview of archaeological sites (source: Bemmann et al., modified)
ARKHANGAJ AIMAG
Khotont sum
Site
number
MOR-1
MOR-13
Site type
Description
Period
Settlement
(palace?)
Dojtyn Tsagaannuur.
Complex consisting
of one large and two
small
raised
platforms
and
a
surrounding
horseshoe-shaped
suite of rectangular
buildings,
overall
dimensions c. 230 m
x 150 m (possibly
spring palace of
Ögedei Khan).
Medieval
(Mongol
period)
Walled
enclosure
Square walled enclosure measuring c.
200 m x 200 m with
remains of building
structures inside.
Early
Medieval
Image
MOR-14
Walled
enclosure
Öndör Dov. Rectangular walled enclosure measuring c.
210 m x 160 m.
Early
Medieval
MOR-17
Walled
enclosure
Khöövör.
Rectangular walled enclosure
measuring
c. 110 m x 75 m.
Early
Medieval
MOR-15
Walled
enclosure
Square walled enclosure measuring c.
60 m x 60 m with a
square annex of c. 50
m x 50 m.
Early
Medieval
MOR-16
Walled
enclosure
Rectangular walled
enclosure measuring
c. 180 m x 140 m.
Early
Medieval
MOR-4
Cemetery
Early
Medieval
Not yet processed
MOR-3
Walled
enclosure
Date
unknown
Not yet processed
MOR-2
Walled
enclosure
Kharaat.
Cemetery
with 26 individual
structures
(dense
stone packings and
small settings of few
stones) spread over
an area of c. 200 m x
60 m.
Kharaat. Rectangular
walled
enclosure
measuring c. 240 m x
200 m.
Rectangular walled
enclosure measuring
c. 220 m x 170 m
Date
unknown
MOR-6
Walled
enclosure
Square walled enclose measuring c.
100 m x 100 m
Early
Medieval
MOR-10
Settlement
Khushuutijn
am.
Half-circular
settlement
area
measuring c. 400 m x
250 m with building
structures within and
a further rectangular
structure beyond the
curving boundary.
PostMedieval
(17th-18th
cent. AD)
MOR-18,
MOR-19
Burials?
Bayan gol am. Two
rectangular
enclosures
with
interior
structures
beside one another,
the
smaller
measuring c. 120 m x
100 m.
Early
Medieval
MOR-20
Burial
Rectangular
monument formed by
a low bank and
surrounding
ditch
with round mound
within the enclosure,
dimensions c. 40 m x
35 m.
Early
Medieval?
MOR-21
Site type
unclear
Small
rectangular
structure defined by a
shallow
bank
measuring c. 20 m x
15 m.
Date
unknown
Khashaat sum
Site
number
MOR-9
Site type
Description
Period
Walled
enclosure,
find
scatters
Lungijn dörvölzhin.
Square walled enclosure of c. 120 m x
120 m on the highest
point of a promontory
with
traces
of
building
structures
inside and enclosed
outer
settlement;
Palaeolithic
and
Neolithic
find
scatters.
Late
Upper
Palaeolithic,
Neolithic,
Early
Medieval,
Medieval
Image
N
Fig. 4. View of a walled enclosure from a Russian aerial photograph
Fig. 5. 3D view with 2x height exaggeration from the same archaeological site in the middle Orkhon valley
3.1 Outlook
Octocopters are reliable platforms for airborne remote sensing and the recorded image data is an excellent source for
large scale mapping. In September 2009 a second field campaign will take place in the Orkhon valley. This campaign
aims at the survey of other, newly discovered sites. In order to increase productivity and geometric resolution a new 22
megapixel camera is implemented. The Octocopter was further developed both, technically and electronically.
Additionally the flight control software as well as logging options were enhanced to ease photo measurement flights and
the following photogrammetric processing steps.
REFERENCES
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