180 
4.1 PROCESSING OF ANTARCTIC IMAGES BY THE ORTHO-MODULE SOFTWARE 
Ortho-Module is a commercial software module which performs the following functions: 
• Camera and scanner calibration (production of corrected 1A image using camera/scanner calibration data), 
• Split-screen point digitizing (digitizing of GCP’s and tie points (tp’s) - points whose ground coordinates are 
not known, but can be visually recognized - using both the images of the stereopair on the same screen), 
• Triangulation (establishment of relationships among two or more images and the ground), 
• Digital Elevation Model (DEM) generation (interpolation of a raster surface trough the reference points), 
• Ortho-image generation (production of an orthographic image using the grid of the DEM and the terrain 
displacement corrected image). 
Ortho-images are produced according to the program flow shown in Fig. 2. Using ephemeris data, the first step of 
the process computes all satellite orientation parameters and a local ground system for use in triangulation, i.e., 
geocentric (satellite) to topocentric (image) coordinates change. 
Figure 2. Ortho-Module Flow Diagram 
The triangulation process, using CCP’s and tp’s, provides a model to iteratively calculate the spatial 
relationship between the SPOT sensor and the ground coordinate system for each line of data in a stereo image. A 
Lambert Conformal Conic projection has been used. Triangulation accuracy is measured by residuals and 
standard deviations (SD) in mm (SD pixe [ = SD mm / pixel_size mm ). Residuals describe how much the measured 
image coordinates differ from their computed location after the adjustments. It is possible to reenter or delete 
GCP’s and tp’s according to the residuals shown during the process. With 2 iterations of the algorithm, the results 
were as in Table 2 (e.g., Del6 is the worst control point). 
Table 2. Residuals 
Iteration 
Standard deviation (SD) 
Maximal residual 
Image 
Point 
1 
0.061157 
0.2035 
10343 
Del6 
2 
0.049084 
0.1651 
10343 
Del6 
Table 3. Check Point Analysis 
Number of points compared: 7 
Point: del8 
Mean X residual: 0.01083 
RMS X residual: 0.05203 
Max X residual: 0.07761 
Point: del6 
Mean Y residual: 0.01692 
RMS Y residual: 0.05845 
Max Y residual: 0.13677 
Point: s4 
Mean Z residual: -0.00883 
RMS Z residual: 0.03910 
Max Z residual: 0.06437