VISIBILITY AND SHADOW STATE OF 
STEEP SLOPES IN STEREO-OBSERVATION 
OF SIDE-LOOK SPOT IMAGERY 
JAN J.NOSSIN 
International Institute for Aerospace Survey and Earth Sciences ITC Enschede, 
the Netherlands. 
ABSTRACT: 
In side-look SPOT imagery, slopes steeper than the complement of the incidence 
angle, exposed in the lookdirection, are not visible to the satellite, and 
hence, not imaged. 
A method has been developed to determine the (non)visibility of slopes in other 
exposure directions (Nossin, 1990). 
Similarly, slopes steeper than the sun elevation angle are in shadow, if exposed 
in the (azimuth + 180°) direction, and a method to calculate the sunlit/shadow 
condition of other slope exposure directions has been developed. 
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. 
Strong westlook imagery may not show steep west-facing slopes, whether sunlit or 
shadowed, and the arc over which this occurs for corresponding slope angles, is 
determined. 
Slopes with an eastward exposure component are generally all sunlit, but slopes 
with a slope angle > (90°- incidence angle) are not visible to the satellite on 
left-look (i.e: eastlook) images. 
On eastlook imagery, in general, slopes with a westward exposure component are 
visible; slopes steeper than the sun elevation angle are in shadow, and visible. 
The contrast in eastlook imagery therefore, in general is more pronounced than 
in westlook imagery. 
Annexed to this paper, two versions are presented of a computer programme that 
calculates the (in)visibility of slopes with specified slope angles and their 
exposure directions, and also the state of shadow or sunshine on the slope. It 
does this calculation for any combination of lookdirection, incidence angle, sun 
azimuth and sun elevation. 
In the first program, named Iglospot, a semi-graphic output of slope visibility 
and shadow is provided in steps of 5° for slope angles and exposure directions. 
In the second program, named Spotslopes, a tabular output of invisible and of 
shadowed slopes is provided in steps of one degree. 
Both programs are written in Pascal; users could simply retype one or both and 
use it after compilation on any machine using standard Pascal. The retyping 
should be done very exactly. 
This sidelook effect is important as an interpretability factor.