centre line can be computed from the overlap and sidelap. The following parameters are computed.
Ø Roll angle at the centre pixel s roll at the first
Ž . Ž
pixel of detector 1 q centre pixel y 1 roll at the last pixel of detector 3 y roll at the first pixel of
. detector 1 rtotal number of pixels.
Ø Look angle s mirror step numbermirror step
size q roll at the centre pixel. The adjustment using the orbit attitude modelling
Ž .
approach described by Radhadevi et al. 1998 can now be performed. The effect of small errors in the
look angle will be corrected by updating the attitude angles. To account for the scale variations between
the detectors, the relative focal length values com- puted for different image points are employed.
3. Test details
The accuracy of the model for simultaneous pro- cessing of images from three CCD arrays of the
IRS-1C PAN camera was investigated in two tests. The objectives of the tests were:
Ø to determine the accuracy that can be obtained for
full PAN scenes corrected with the model; Ø
to verify the consistency and robustness of the model; and
Ø to compare the accuracies obtained by using dif-
ferent types of control for modelling. The ground coordinates of points, which can be
used as checkpoints as well as control points, were gathered from three sources. A few survey control
Fig. 3. Planimetric error vectors of test 2 with one surveyed GCP and three check point sources.
points were available, while most of the points were obtained from manual identification on 1:25,000-
and 1:50,000-scale Survey of India maps. The sur- veyed control can have an identification error which
may reach 10 m. A 1:50,000-scale map has an error of approximately 12.5 m in planimetry and
20 m in height. In practice, while digitising a specific feature for control, the error could be 25–
35 m in planimetry. The measurement errors of the pixel coordinates of GCPs, as well as checkpoints,
can vary between 0.5 to 2 pixels due to the manual and monoscopic measurement mode used.
The image data and imaging geometry details used for the tests are given in Table 1.
4. Results
Table 2 presents the accuracies obtained for the Ž
. two tests. Average root-mean-square RMS errors
for both tests of 10.5 m in the latitude direction and 11.4 m in the longitude direction were ob-
tained when a single surveyed GCP was used for modelling. When a GCP from 1:25,000-scale maps
was used for modelling, average RMS errors of
34.3 m in latitude and 39.2 m in longitude were obtained. When the GCP used for adjusting the block
was obtained from a 1:50,000-scale map, the average RMS errors were 50.4 m in latitude and 43.3 m
in longitude. Experiments with an increased number of control points showed that the errors are not
dependent on the number of control points, but rather on the accuracy of the controlrcheck points.
The specified control accuracy figures of each control source were met with a single surveyed GCP.
Figs. 2 and 3 show the error vectors for the two tests. RMS errors shown in the plots are computed from
Ž all checkpoints obtained from the three different
. sources . Unfortunately, these check points do not
have homogeneous accuracy and similar to that of the single GCP. However, their use was dictated in
order to have a uniform distribution of the check points over the whole block and thus be able to
check possible errors caused by the automatic over- lap computations, lack of control in all, but one,
subscenes, etc. Most of the survey points fall in
Ž .
subscene C9 of the test 1 block see Fig. 2 and Ž
. subscene D7 of the test 2 block see Fig. 3 . The
cluster of small error vectors in these two subscenes is due to this reason. The checkpoints are uniformly
distributed in the block and the position of the GCP used in the adjustment is also indicated. Note that in
Ž test 2, subscenes D2 and D3 were not available Fig.
. 3 . The number of overlap lines is approximate,
which will be reflected as a latitude error. The model is not sensitive to the location of the GCP. The
geoidal height of the GCP is used in the adjustment without converting it to the ellipsoidal height,
whereas the satellite position given in the ephemeris is with respect to the ellipsoid. Although this intro-
duces an error, overall the method is appropriate for continuous full PAN scenes acquired during the same
pass.
5. Conclusions
