COMPUTER PROGRAMMES TO CALCULATE THE INVISIBLE AND SHADOWED SECTORS 
In the annexes 1 and 2, two versions of a programme are presented which are bas 
ed on the above considerations, to calculate the (in)visibility and sunlit/shad 
ow conditions of slopes in both right- and leftlook SPOT imagery. 
Calculations start at slope angles corresponding to either the viewangle or the 
sun elevation, whichever is less. 
In the practice of image analysis and interpretation and field checking, accura 
cies of one degree or better are normally neither reached nor required. 
The programmes therefore truncate the input values and round-off the output 
values. In the first program, named Iglospot, a semi-graphic output in steps of 
5° for slope angles and exposure directions is provided. In the second program, 
named Spotslopes, a tabular output of invisible and of shadowed slopes is pro 
vided in steps of one degree. 
Both programs are written in Pascal; they will be published in full in the ITC 
Journal (Gorte & Nossin, in prep.), and are available during this conference 
(Annexes 1 and 2). Users could simply retype one or both and use it after compi 
lation on any machine using standard Pascal. The retyping should be done very 
exactly. 
PROGRAM IGLOSPOT (Annex 1) 
The following program Iglospot calculates the visibility of slopes and their 
sunlit/shadow status in both left- and rightlooking SPOT imagery. It offers a 
visual output of these properties which would represent the shadowed and the in 
visible areas on the iglo. The shadow status and the slope invisibility can be 
read from the output graph. The steps chosen for angles and directions in the 
output graph, are of five degrees. 
PROGRAM SPOTSLOPES (Annex 2) 
Based on the same principles, program Spotslopes provides the output in a tabu 
lar form, for slope invisibility and for shadow. In west-look imagery, any over 
lap in values for shadow and (in)visibility is easily read from the table. 
In eastlook imagery, the shadows are normally visible because they are cast in 
a general sense towards the satellite, while invisible (i.e. east-exposed) 
slopes are normally sunlit. 
EXAMPLES 
As examples, subscenes of SPOT stereo-imagery from China and from Turkey are 
shown in figures 3, 4 and 5. The image shown in fig. 3 is a subscene from east 
ern China, some 30 km south of the city of Hangzhou. 
The terrain shows signs of folding and faulting, with strong lineaments trending 
more or less parallel to the flight path, thus more or less perpendicular to the 
lookdirection. In the righthand image, representing a westlook of only 18.2°, in 
certain hill sectors most of the topographic detail of the terrain is lost, 
which becomes evident on comparison with the leftlook image. 
In figure 4, another subscene from the same SPOT image is presented, where the 
effect of loss of detail in the rightlook image is especially striking in the 
northwest faces of the slopes of the main ridges. 
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