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In this paper a description will be given of how ortho photos and 
digital terrain models (DTM) have been used in a scanner and in a computer 
for the simulation. 
Processing of an ortho photo to the scanner 
In Sweden Land Use Maps are being produced at a scale of 1:10 000. 
As a background in such a map is an ortho photo in black and white. The area 
in question is covered by 64 maps. The corresponding ortho photos have been 
reduced to paper prints at a scale of 1:50 000 and put together to one photo 
with a format of 80 x 80 cm‘. This photo has been further reduced to a nega- 
tive at a scale of 1:200 000 and the format of 20 x 20 cm?, which is close 
to the largest format the scanner can take. 
Processing of a digital ortho photo 
The ortho photo has been digitized in a scanner with an aperture 
of 50 um. In the scanner the ortho photo is splitted up in pixels in columns 
and lines. Every pixel is set to a digital value between 0-255 dependent on 
the grey scale of the pixel in the ortho photo. The digital values are 
written on a magnetic tape. With the aperture of 50 pm and the scale of the 
ortho photo, 1:200 000, every pixel will correspond to a size on the ground 
of 10 x 10 m?. 
In the scanner a photo can not be oriented exactly. For this reason 
an area well overlapping the photo has been digitized. A number of well 
defined control points have been located in the digital image and in an 
affine transformation the digital image has been rectified. In the rectifica- 
tion the pixels within the photo have been saved on a magnetic tape. In this 
way the ortho photo is digitaly stored on the tape as 4 000 lines in west- 
east direction, each line holding 4 000 pixels. 
Height information of the area 
To simulate Spot-data it is necessary to know the heights in the 
terrain of the area in question. In the production of ortho photo to the 
Land Use Map at a scale of 1:10 000 a DTM is produced and stored for every 
single map. A DTM covers 5 x 5 km? on the ground and is built up as a grid 
with the heights of the terrain. The grid size is 50 meters and a DTM holds 
101 x 101 gridpoints. In this simulation the 64 DTM's to the ortho photos 
have been put together to one DTM with the same grid size and 801 x 801 grid 
points. The DTM has been stored in the computer. 
Calculation of the simulation 
In the calculation some simplifications are made. The track of the 
satellite is set to be strictly north to south and the earth curvature is 
set to a constant earth radius. The calculations are made on a computer that 
reads one line at a time from the tape with the digital ortho photo. In the 
calculations a new line is created and every pixel is moved from the column 
of the ortho photo line to the column on the new line where Spot will 
register it. The difference in columns is dependent of the height in the 
terrain of the pixel, the distance between the pixel and the track of the 
satellite and the earth curvature. When every pixel on the ortho photo line 
is moved to the simulated Spot line, the Spot line is written on another 
magnetic tape. The computer reads next line and the procedure is repeated 
until every line has been treated. A simulated digital Spot image will then 
be stored on the other magnetic tape. 
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