THE STEREO-SLOPE COMPARATOR 
An Instrument for Measuring Angles of Slope in Stereoscopic Models 
By 
* 
Robert J. Hackman 
U. S. GEOLOGICAL SURVEY 
ABSTRACT 
This paper describes a new instrument, the Stereo-slope Comparator (also 
known familiarly as the Super Duper Dipper), and a method for rapidly determining 
angles of slope in the stereoscopic model from two vertical aerial photographs. The 
only required information is the focal length and photo base of the photography. 
The important feature of the instrument is the syncronized targets. Adjustment knobs 
on the instrument control their separation, horizontal swing, and slope angle. Two 
additional devices are used with the instrument: (a) The Supplementary Slope Model, 
a constructed stereoscopic model that is related to photography of different focal 
lengths and photo bases; and (b) The Slope Conversion Chart, which shows the re- 
lationship of the true angle of slope, the exaggerated angle of slope, and the ex- 
aggeration factor. 
In measuring angles of true slope with the Stereo-slope Comparator, the 
exaggerated angle of the Supplementary Slope Model is first determined. According 
to the selected photography, the true angle that this slope represents is read from 
the table that accompanies the Supplementary Slope Model. With these two angles 
known an exaggeration factor is determined from the Slope Conversion Chart. Angles 
of exaggerated slope are then determined in the stereoscopic model of the photographs. 
These angles of slope are referred to the Slope Conversion Chart, with which, by 
using the exaggeration factor established above, they are converted to angles of true 
slope. 
* Publication authorized by the Director, U. S. Geological Survey