4.0 Description of a software system for Terrain 
Mapping Using IRS-1C Imagery 
The software system developed at Space 
Applications Centre (ISRO) for terrain mapping 
using IRS-1C imagery was used for analysis of 
IRS-1C capabilities. This system consists of the 
following basic elements: 
(i) Ground Control Point (GCP) tools 
(ii) Digital Elevation Model (DEM) generation 
and editing 
(iii) Orthoimage generation 
(iv) Quality Evaluation and 
(v) Mapping using GIS 
All these are additionally supported by image 
processing and graphics libraries. The hardware 
consists of an Indigo-2 R-4000 based softcopy 
photogrammetry workstation with stereo display 
monitor, crystal eye glasses for stereo view, a 
scanner and a plotter. The details of the software 
system are explained in the subsequent sections. 
4.1 GCP Generation 
Basically this consists of a scanner system, wherein 
the desired maps are scanned in an appropriate 
resolution. The digital image is properly 
thresholded, so that all the linear features are 
clearly identifiable. The scanned image is then 
transferred to the main system for GCP 
identification along with the stereo pair images. 
Co-ordinates of a point in terms of latitude and 
longitude is obtained by a projective transformation 
on the surrounding known grid points of the map. 
The height of the point is interpolated from the 
nearest elevation contour lines. The image co- 
ordinates of the control points are identified 
parallelly on the display device. 
4.2 DEM generation and editing 
A DEM is regarded as a numerical description of the 
surface of an object on measured or derived co- 
ordinates of numerous scattered points. With the 
advent of digital phtogrammetry workstations with 
high computation power, DEMs can be derived by 
purely digital approach, from the satellite stereo 
pairs. The digital mode of DEM generation from a 
satellite stereo pair consists of the following steps 
(i) Automatic identification of conjugate points 
(ii) Determination of satellite orientation using 
a model based on orbit and attitude parameters 
(iii) Three dimensional co-ordinate 
determination by the method of space intersection 
(iv) Bundle Adjustment 
(v) Height interpolation to compute heights at 
regularly spaced grid points and 
(vi) Point editing to remove the spurious height 
points and reinterpolation 
4.3 Orthoimage Generation 
Basically a precise ground to image relation along 
with the DEM and the raw data are the requirements 
to generate an Orthoimage. This is a geocoded 
product corrected for all the geometric errors 
including terrain relief and the camera tilt, which can 
be directly used for topographic mapping. The 
Orthoimage can directly go into a. GIS. For an 
output grid of latitude and longitude and height 
(obtained from DEM) time and pixel of the input 
image can be calculated by an iterative way using 
equations (2),(3) using the updated orientation of 
the image. And the gray value for this point is 
generated by resampling the input image. The DEM 
can be the one derived from the same stereo pair or 
it is digitised/derived from map. In case of map 
DEM GCPs are also required additionally, to get 
mapping between ground and image. 
4.4 Quality Evaluation 
One of the most critical component of the mapping 
is the accuracy of the product used in terms of its 
tickmarks and the internal distortion. The system 
contains accuracy checks at every process level. 
i.e. the model accuracy on GCPs and on check 
points, is given immediately after space resection 
and intersection. In addtion to this the Orthoimage 
evaluation with respect to its tickmarks is done on 
checkpoints within the system. The checkpoints are 
identified on the Orthoimage manually, and their 
estimated positions w.rt the tickmarks are 
compared with the actual values. The RMS of these 
errors are quoted as the location accuracy and the 
standard deviation represents the internal distortion. 
Apart from this quantitative approach, Orthoimage 
is evaluated qualitatively by overlaying map features 
either digitally or photographically. The DEM can be 
qualitatively evaluated by (i) draping the 
Orthoimage on the derived DEM and comparing this 
with the draping of Orthoimage over map derived 
DEM and (ii) comparing the contours obtained from 
both image derived DEM and map derived DEM. 
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996