The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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either by mere shifts, by shifts and drifts, or by an affine 
transformation, all in image space. It has been shown in the past 
that the effect of the biases is almost constant in the images, so 
that by using one well-defined GCP per image, the bulk of the 
bias can be removed (Fraser and Hanley, 2003). Results can be 
improved if a modest number of additional GCPs are available. 
Third, a generic pushbroom sensor model can be initialized 
from information contained in the metadata files and refined in 
an orientation process by the use of well-distributed GCPs. If 
the metadata of CARTOSAT 1 imagery contained information 
about the orbit path and attitudes, the generic pushbroom 
scanner model could also be used for direct georeferencing, and 
the GCPs could be used for determining biases in the orbit path 
and attitude information. As this is not the case, the orbit path 
and attitudes need to be determined from GCPs alone. The 
number of GCPs required will be slightly higher than would be 
the case if orbit path and attitude information were available. 
In this paper, the authors assess and compare the geopositioning 
accuracy achievable with CARTOSAT 1 imagery via the 3D 
affine, bias-corrected RPC and generic pushbroom sensor 
models. A stereo pair of images covering Hobart, Australia, 
distributed by ISRO as part of the Cartosat-1 Scientific 
Assessment Programme (C-SAP), was processed using Barista. 
Unencrypted RPC files were provided by ISRO for research 
purposes in this case. General information about the satellite 
can be found in CARTOSAT la (2006) and CARTOSAT lb 
(2006). Scene related information is contained in the provided 
metadata files. A set of GPS-surveyed 3D points for the area of 
Hobart was available for the reported analysis. In order to 
assess the georeferencing accuracy that can be achieved for 
CARTOSAT 1 images, bundle adjustment was carried out, 
applying the three sensor models accommodated within Barista 
and using a subset of the available 3D points as GCPs. The 
remaining 3D points served as independent check points. 
2. THE HOBART DATA SET 
2.1 Imagery 
For the assessment of the georeferencing accuracy a stereo pair 
of panchromatic CARTOSAT 1 images was used. The nominal 
flying height of the satellite is 618 km and the two cameras are 
mounted with a tilt of +26 degrees (fore) and -5 degrees (aft). 
Table 1 shows some satellite specifications. Further information 
can be found in CARTOSAT la (2006) and CARTOSAT lb, 
(2006). The provided metadata files specified a slightly 
different value of around 638 km for the flying height. No scene 
specific across-track angle information is given. The forward 
scene covers an area of roughly 30 km x 30 km around the city 
of Hobart. The area covered by the aft view is about 27 km x 30 
km. The elevations in this area range from sea level up to over 
1260 m at the peak of Mt Wellington. The acquisition date of 
the Hobart images was 1 October 2006. 
was then assessed via the remaining 60 points, which served as 
independent checkpoints. For the generic pushbroom sensor 
model an adjustment was also performed with 15 GCPs and 54 
checkpoints in order to investigate the influence of the number 
of GCPs on the results. Figure 1 shows the CARTOSAT 1 
forward-looking scene with the distribution of the 15 GCPs. 
The 9 GCPs covered the same area but with a less dense point 
distribution. 
Orbital altitude 
618 km 
Swath 
Fore 
29.42 km 
Aft 
26.24 km 
Along-track view angle 
Fore 
+26° 
Aft 
-5 0 
Across-track resolution (at 
Nadir) 
Fore 
2.452 m 
Aft 
2.187 m 
Ground sampling distance (along-track) 
2.54 m 
B/H ratio 
0.62 
Image size 
12 000 x 12 000 
Pixel size 
7x7 microns 
Focal length 
1945 mm 
Table 1. CARTOSAT 1 specifications. 
Figure 1. CARTOSAT 1 forward-looking scene of Hobart 
with 15 GCPs. 
3. SENSOR MODELS IN BARISTA 
2.2 3D Points 
In addition to the imagery, a test field of altogether 69 3D 
points was utilised. This test field is described in Fraser and 
Hanley (2005). The points were distributed all over Hobart and 
covered the top left quarter of the scene. They were measured 
with GPS and were mainly centres of road roundabouts, 
determined as the centres of ellipses fitted to points around the 
roundabout perimeter. A selection of 9 points was used to 
provide GCPs for the bundle adjustment. The absolute accuracy 
The processing of the CARTOSAT 1 data set of Hobart was 
carried out with the software system Barista. Barista is 
developed at the CRC-SI and has reached the status of a 
commercially available product. It is a software system for the 
generation of spatial information products from satellite 
imagery. Data sets from high-resolution satellites such as 
QuickBird, WorldView, IKONOS, SPOT 5, and ALOS/PRISM 
have been processed successfully with Barista. Further details 
about the software can be found in Barista (2008).