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INTRODUCTION 
In this paper, we analyze the visibility and shadow state of steep slopes in 
sidelook SPOT imagery. 
The presence or absence of a slope can, practically, only be determined in ster 
eo-imagery; throughout this paper therefore, we presume the context of stereo 
observations. 
We shall consider the slope direction as the horizontal projection of a line 
normal to the contours, viewed in the down-slope direction. 
The satellite flies in the southward direction on the daytime overpass. 
The rightlooking position therefore looks to the west of the satellite, the 
leftlooking position, to the east. 
Stereo imagery is obtained if the same scene is imaged by the satellite twice 
from different angles. For each image of a stereopair, the user has to know the 
lookangle and direction, and the date of acquisition of the data. This informa 
tion is routinely supplied in the leader file of the CCT's. 
Stereopairs in any case do not show simultaneous situations in both images. 
We shall use the term 'eastward' for slope expositions eastward of the flight 
path direction; similarly, westward will be used to indicate expositions west 
ward of the flight direction. 
The look-angle of the satellite (i.e. the angle of the mirror with the vertical) 
differs from the incidence angle, which is the angle that the line of view from 
the satellite makes with the surface normal at the scene centre; this difference 
is due to the earth's curvature. 
We shall call the complement of the incidence angle, the viewangle. 
In side-look SPOT imagery, slopes steeper than the viewangle, exposed in the 
lookdirection, are not visible to the satellite, and hence, not imaged. 
Slopes with a westerly exposure in the direction (sun azimuth + 180°) steeper 
than the sun elevation angle, are in shadow. 
Slopes with a slope angle less than the sun elevation and exposed in the same 
direction, are sunlit. 
On left-look (i.e: eastlook) images, slopes exposed in the lookdirection of the 
satellite, i.e. slopes with an eastward exposure component, are generally all 
sunlit, but slopes with a slope angle > viewangle are not visible to the satel 
lite. 
In west-look Spot imagery, slopes exposed in westerly directions, different from 
the lookdirection of the satellite or the direction (sun azimuth + 180°), may be 
or not be imaged, while sunlit or in shadow, depending on slope angle, exposure, 
sun azimuth, and sun elevation. 
The combined effect of sunlit status for certain slopes, and non-visibility of 
shadowed slopes, may give rise to marked absence of contrast in west-look ima 
gery of steep terrain with steep westfacing slopes. 
This effect is especially noted in strong west-look imagery. 
It was first noted and described for images of steep karstified terrain in 
Turkey (Nossin, 1989) and analyzed by the same author,(1990). 
Images from several areas are shown to serve as illustrations of this effect.