z (DG),
y actual
Chinese
es to the
CASM)
uctivity.
ina was
1:4,000
on were
ophotos
inology.
100,000
netry in
scussed.
Yo
Yo
"China
| in the
China.
y since
n days.
ay find
ed. It is
imunity
Ithough
ountry.
ologies
; in the
of the
ping is
digital
igitized
duction
:edures
rs, and
lready.
ency of
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
production and intensive human work is still required, e.g.
photographs have to be digitized, cartographic features should be
extracted, and ground control points (GCP) must be collected and
measured in the field. Image digitization can be accelerated
considerably by adding some automatic mechanisms on the film
scanner, or ignored completely when digital aerial cameras are
applied. Feature extraction can profit a lot [rom DPWS automated
functions being improved gradually with the current technical
progress. Now, the task of GCP collection and measurement
remains the inevitable labor work, which slows down the
production directly on the one hand, and makes mapping of e.g.
desert and inaccessible regions in west China very hard or even
impossible on the other.
Thus, aerial photogrammetry without any ground control or with
a limited number of GCP is the prerequisite for speeding up the
map production so as to meet the demands of rapid development
in China and closing the map gap in the country's western regions,
in particular.
2. DGPS/IMU-BASED PHOTOGRAMMETRY
Recent developments in Differential Global Positioning System
(DGPS) and Inertial Measurement Unit (IMU) technologies make
direct measurement of sensor Exterior Orientation parameters
(EO) possible, so as that the image orientation can be without or
just with a limited number of GCP determinable. In this kind of
DGPS/IMU-based photogrammetry the traditionally dominating
Aerial Triangulation (AT) becomes now a supporting role in the
image orientation. Depending on how AT is used in the whole
procedure, we can distinguish a Direct Georeferencing (DG) from
an Integrated Sensor Orientation (ISO) conceptually.
2.1 Direct Georeferencing
Use of the image EO determined based on direct DGPS/IMU
measurements for photogrammetric data processing without
conducting an AT over the entire image block presents the
principle of Direct Georeferencing. Advantages herewith are
obvious: No tie point determination (either manual or automatic)
is necessary, no GCP is required and, as a matter of fact, no AT
has to be performed for the entire image block anyway.
Nevertheless, for purpose of a so-called boresight alignment
highly accurate AT results of a small calibration field are still
needed to determine the attitude differences between the image
sensor coordinate system and the IMU one (e.g. Cramer, 2002).
Some simple geometrical considerations (Kremer, 2002) show
that DG is quite suitable for small and medium scale mapping,
especially for orthoprojection, and for large scale mapping,
however, there may be certain limitations using this approach.
2.2 Integrated Sensor Orientation
Simultaneously processing DGPS/IMU data and image
information in a combined AT over the entire image block can be
referred to as Integrated Sensor Orientation. Obviously, this
approach combines advantages of conventional AT and modern
DG, and compensates for their weaknesses. For instance, based
on tie points well-distributed over image areas and GCP over the
block AT is capable of eliminating systematic errors by means of
a self-calibrating adjustment, and thus reaches the highest
accuracy and stability of image orientation. With the direct EO
871
measurements provided by the DGPS/IMU approach the best
initial values are available for both the bundle block adjustment
and the automatic tie point determination in an automatic AT (e.g.
Tang, 1999). Fully profited from the stable geometry supported
by the direct EO measurements the number of GCP required for
AT can be reduced to the minimum, and the performance of the
entire image orientation procedure is tremendously improved
after all.
What a kind of approach to be chosen for a specific project
depends on many factors, e.g. the accuracy required, the image
scale, the availability of ground control information and GPS base
stations and accessibility of the project area. For large scale
mapping projects it would be better to use the integrated sensor
orientation, in general.
3. FIRST PROJECT
With the successful import of a CCNS/AEROcontrol system
(Grimm, 2003) in November 2002, Chinese Academy of
Surveying and Mapping (CASM) organized the first
DGPS/IMU-based photogrammetric project in China in
cooperation with the Germany-based companies IGI and
Techedge. Anyang, a city of Province Henan, Located about 500
km south of Beijing, was chosen for the test flights. CASM was
responsible for all administrative as well as operational issues of
the project. The German team, consisting of the authors, was in
charge of workflow design, system installation, personal training
and technical consulting within the project.
3.1 Project Realization
Two missions of aerial photography were planned and conducted
for a photo scale of 1:4,000 (for 1:1,000 scale mapping) and
1:20,000 (for 1:5,000 scale mapping), respectively. Concerning
many special cases in China (e.g. inaccessible regions)
investigating effects of GPS base station location was also one of
the objectives. Thus, several GPS base stations of different
distances to Anyang were simultaneously used for the missions.
Table 2 shows the major mission parameters.
Mission A Mission B
Photo scale 1:4,000 1:20,000
Map scale 1:1,000 1:5,000
Terra Flat Flathilly
type
Area 5 km x 2.5 km 14 km x 22 km
Block size | 5 strips x18 photos 7 strips x16 photos
Calibration [Inside block, additional outside block, 2 strips of 11
field 11 photos photos each
GSP base ebur erit ; A Em
m Anyang| Hebei Beijing Anyang|Zhengzhou| Beijing
station
Distance 10 | _ |212kmlésikm| - | 150km |451 km
block
Flight date | 18/11/02 | 19/11/02 03/01/03
Table 2. Major mission parameters of the Anyang project.
3.2 Mission A
To prove the applicability of the DGPS/IMU technology for
