VISIBILITY AND SHADOW OF STEEP SLOPES IN SIDELOOK SPOT IMAGERY 
Figure 1 depicts a general situation of slope exposure, westward of the flight 
path direction. The sun, normally, is east of the scene. The satellite in this 
case is in westlook mode. 
The slope angle is SI. 
Let <|>2 be the angle between slope exposure and lookdirection, and <f>3 the angle 
between slope exposure and (sun azimuth + 180°). 
It follows from fig. 1 that 
tan 82 = tan 61 * cos <\>2 and 
tan 63 = tan 81 * cos <f>3 
{based on: cos <J>2 = BF1 / BF2 
BF1 = h / tan 81 
BF2 = h / tan 82 } 
The slope is not seen by the satellite, and therefore not imaged, if 82 > view- 
angle. 
The slope is in shadow if 83 sun elevation. This shadowed slope is seen by the 
satellite if 82 =< view angle. 
To analyze a particular slope, the slope angle 8 is given, as it can be chosen. 
The viewangle X and the sun elevation y are also known, as they are provided in 
the peripheral data of the image CCT. 
Substitution of X for 82 and y for 83 will give the corresponding values for <j>2 
and <f>3, respectively. The range of exposure directions with invisibility and 
with shadowed slopes extends over a value of <J>2 and <f>3 degrees on either side 
of, respectively, the lookdirection and the direction (sun azimuth + 180°).For a 
more elaborate discussion, the reader is referred to Nossin (1990). A wide range 
of slope expositions and slope angles can be analyzed this way. 
THE IGLO 
All slopes possible in any terrain could be represented by a dome (an iglo) 
where each point on the (hemi)sphere corresponds to a specific combination of 
slope exposure direction and slope angle (represented by the tangent to the iglo 
surface at that point). This iglo, illuminated by the sun, at an azimuth and el 
evation as given in the CCT peripheral data, would have a shadow zone which cor 
responds to the graph of all possible shadowed slopes in the terrain, under that 
particular illumination, represented again by points on the iglo surface as des 
cribed above. It is by no means certain that all possible slopes do indeed occur 
in the terrain, but if they occur then, for their specific combination of expo 
sure direction and slope angle, their shadow (if any) would fall within the 
shadow area on this iglo. 
Similarly, the iglo, looked at from the satellite in side-look, under the speci 
fied incidence angle and lookdirection, will have an invisible sector, made of 
all points on its surface which correspond to the combination of slope angle and 
exposure direction of slopes in the terrain invisible to the satellite from that 
lookangle,- again, if such slopes do occur in the terrain. 
Figure 2 attempts to elucidate this. 
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