ıl 2004
THE INFLUENCE OF GROUND CONTROL POINTS IN THE
TRIANGULATION OF LEICA ADS40 DATA
S. Alhamlan ? *, J. P. Mills?, A. S. Walker 5 T. Saks*
School of Civil Engineering and Geosciences, University of Newcastle, UK — (s.m.alhamlan, j.p.mills)@ncl.ac.uk
D aim hl e. NM = MQ, . * ~ = : . " ac
Leica Geosystems GIS and Mapping, LLC, USA — stewart.walker@gis.leica-geosystems.com
Leica Geosystems GIS and Mapping GmbH, Switzerland — tauno.saks(@gis.leica-geosystems.com
Commission I, WG 1/6
KEY WORDS: Accuracy, ADS40, Aerial Triangulation, Bundle Adjustment, Digital Sensor, Precision
ABSTRACT:
This paper is concerned with the influence of ground control points (GCPs) on the aerial triangulation of datasets acquired using a
Leica ADS40. The ADS40 comprises seven parallel linear sensors in the focal plane of a single lens system. Three panchromatic
sensors scan the same ground area at different angles to acquire forward, nadir and backward scenes. After the raw imagery and
metadata are downloaded, determination of exterior orientation parameters and subsequent image rectification enable stereo viewing
in three possible geometric combinations. Residual parallax in the rectified data, however, means bundle adjustment is necessary be-
fore the maximum photogrammetric accuracy can be achieved. In order to assess the influence of bundle adjustment, a dataset of
ADS40 imagery was acquired for an 8 x 8 km study area around Waldkirch in Switzerland. The dataset consists of four adjacent
strips of ADS40 imagery plus two cross strips scanned by each of the sensor’s seven channels. The ADS40 imagery of the study area
was pre-processed, rectified and triangulated interactively using the ADS40 ground processing software GPro, the SOCET SET"
photogrammetric software suite and the ORIMA bundle adjustment software. Repeated triangulation of the dataset in different con-
figurations of strips, scenes and GCPs was applied. The number of GCPs was increased gradually in different arrangements to assess
the influences on the triangulation process and analyse the need for GCPs when using GPS/IMU data. Based on the findings, rec-
ommendations are made as to the most suitable block configuration to be selected when using data of this kind.
1. INTRODUCTION
This paper reports on the influence of ground control points
(GCPs) in the aerial triangulation of datasets acquired using
Leica Geosystems' Airborne Digital Sensor, the ADS40. The
Leica ADS40 comprises seven parallel linear sensors (three
panchromatic and four multispectral channels), placed next to
one another in the focal plane of a single lens system. The pan-
chromatic sensors scan the same ground area at different angles
to acquire forward, nadir and backward scenes (Figure 1).
| Figure I. Forward (F ), Nadir (N) and Backward (B) scenes
(image courtesy Leica Geosystems GIS and Mapping)
After the raw imagery and metadata are downloaded, the de-
termination of exterior orientation parameters (based on posi-
tion and attitude data from the integrated GPS/IMU system) and
* Corresponding author.
subsequent image rectification is possible. Consequently, stereo
viewing in three combinations of two panchromatic bands, i.e.
forward/nadir (F/N), forward/backward (F/B) and na-
dir/backward (N/B) is enabled. Nevertheless, at this stage the
rectified data is insufficient for photogrammetric measurement
because of the remaining parallax in the rectified images (pre-
sent, for example, as result of any misalignment between the
IMU and camera axes and the datum between GPS/IMU and
the ground control system), necessitating photogrammetric
bundle adjustment.
In order to assess the influence of bundle adjustment on im-
agery of this type, a dataset of ADS40 imagery was acquired
for an 8 x 8 km study area around Waldkirch in Switzerland.
The dataset consists of four adjacent strips of ADS40 imagery
plus two cross strips scanned by each of the sensor’s seven
channels. The ADS40 imagery of the study area was pre-
processed, rectified and triangulated interactively using the
ADS40 ground processing software GPro, the BAE Systems
SOCET SET® photogrammetric software suite and the Leica
ORIMA bundle adjustment software. Repeated triangulation of
the dataset in different configurations of strips, scenes and
GCPs was applied. The number of GCPs was increased in dif-
ferent arrangements to assess the influences of the triangulation
process and analyse the need for the GCPs in the presence of
GPS/IMU data. To achieve this, three phases of testing were
designed, with measurements taking place as follows:-
Phase I: Block I (four parallel strips), pre-triangulation.
Phase II: Block I, post-triangulation.
