The figures above indicate an Increase in error as the angle of slope 
steepens. This increase in error as the angle of slope becomes greater is due pri- 
marily to the relatively large increase of the tangent value of steep slopes. The 
result of this Increase is shown in the following example: With an exaggeration 
factor of 3.5, a 17 degree exaggerated angle of slope would represent a 5 degree 
angle of true slope. If the exaggerated angle of slope were erroneously determined 
as 16 or 18 degrees, the true slope would be read as 4 3/4 or 5 1/4 degrees. The 
plus or minus | degree error in reading this low exaggerated angle of slope would 
still give approximately the correct answer for the true angle of slope. On the other 
hand, an 85 degree exaggerated angle of slope would represent a true angle of slope 
of 73 degrees. If this angle of exaggerated slope were erroneously determined as 
84 or 86 degrees, the true angle of slope would be read as 76 |/4 or 69 3/4 degrees. 
Thus a plus or minus | degree error in this steeper exaggerated slope would cause an 
error of plus or minus three degrees in the true angle of slope. 
The increase in error as the angle of slope steepens is also due to the 
greater difficulty involved in orienting the stereoscopic target. As the angle of 
slope increases the plan view of the slope in the stereoscopic model commonly de- 
creases, making proper alinement of the target less accurate. 
15 INT.-DUP. SEC., WASH., D.C. 96246