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To give a more detailed description of the calculation a sketch is 
drawn in figure l. The figure shows how a pixel on the ortho photo line on 
the distance A from the satellite track is registred in Spot on the distance 
C. The line is for both the ortho photo line and the simulated Spot line. In 
the computer the following calculations for the pixel are done. 
- As a line is an ortho photo the distance A will correspond to the same 
distance on the circular earth surface, B. B and the earth radius will give 
the angle o 
- The ground coordinates in x and y for the pixel will by interpolation in 
the DTM give the height h. 
- The side earth radius + the satellite altitude, the side earth radius + h 
and the angle o will give the angle & 
- 8 and the satellite altitude will give the distance C. C is the distance 
from the track at which Spot will register the pixel. C will then be 
transformed to the column on the simulated Spot line where the pixel is to 
be stored. 
Produced Spot-images and alternative products 
Three images have been simulated with the method, all three with 
different tracks of the satellite. The central image is simulated with the 
track on the middle across the ortho photo. The eastern image has the track 
274 km to the east of the central track and the western image the track 
108 km to the west. 
In the scanner the images have been plotted at a scale of 1:200 000. 
The central image has been magnified to the scales of 1:50 000, 1:20 000 and 
1:10 000. In figure 2 a detail of the image at a scale of 1:50 000 can be 
seen, to be compared with the ortho photo of the same area and at the same 
scale in figure 3. Forestry maps have been copied together with the simulated 
image to illustrate the possible use of Spot in forestry. In figure 4 it is 
done at a scale of 1:20 000 and in figure 5 at a scale of 1:10 000. 
Conclusion 
In the simulation three images have been produced. In the images 
the stereo effects of Spot can be studied as the images are produced as both 
nadir and off-nadir registration of the same area. Furthermore the images 
will be used in developing methods and computer programmes for processing of 
Spot-data. 
When the images are studied it can be found that the small pixel 
size of Spot gives a very detailed information of the landscape. The 
boundaries between forest and cut areas are easily found. Even the small 
roads can be found. The different kinds of forest though is harder to find. 
For that purpose the other detectors of different wavelength in Spot will be 
needed. 
When the images are studied it must be kept in mind that the simu- 
lation is done from ortho photos that originally were photographs taken at 
an altitude of 4 600 meters. The altitude of Spot will be 832 km which means 
that there will be quite a difference in air pollution and haze. 
167 
EL