angle cosines rarely approach one or zero, and images pro 
duced using the "physically correct" illumination typically 
are dominated by only three or four of the six gray shades 
which are used in DS1IADE. 
In order to provide better contrast, DSII/'.DE allows the user 
to specify a minimum (A) and maximum (B) angle between 
which the six gray shades are to be distributed (see figure 
4). Current default values for A and B arc 20 and 70 
degrees respectively. Values of 0 and 90 degrees would 
re-establish the physically correct shading. 
SUMMARY AND PROSPECTS 
DEMGS has proven to be quite a useful tool for the rapid 
inspection, manipulation and display of digital terrain 
models. The 'package' approach, the 'virtual memory emula 
tor', the 'run-length/run-time' display, and the interac 
tive operation, makes the system quite powerful in its 
capabilities. 
Currently at the Geographic Information Systems Laboratory 
DEMGS is being expanded to 1) be a transportable system to 
any type of hardware installation by modularizing the 
'system-dependent' routines to the lowest level of the 
system, 2) incorporate overlaying capabilities of Digital 
Line Graphs with the Digital Elevation Models, 3) permit 
arbitrary point-to-point profiling, 4) utilization of 
fractals for image enhancement, smoothing and interpolation 
procedures, 5) permit windowing (selection of a specific 
area within the displayed DEM), 6) incorporate feature 
extraction capabilities (peaks, pits, ravines, etc.), 7) 
real time display/rotation of DEM three dimensional block 
diagrams, and 8) DEM composites (edge matching). 
REFERENCES 
Brassel, Kurt (1974 ), "A Model for Automatic Ilill Shading", 
The American Cartographer, vol. 1, number 1, pp. 15-27. 
Frieder, Gideon (1980) , "A Virtual Array Manager for the 
Data General Corporation Eclipse Computer", State Univer 
sity of New York, Computer Science Department, Technical 
Report No. MIPG44. 
Jensen, John R. (1980), "Stereoscopic Statistical Maps", 
The American Cartographer, Vol. 7, Number 1, pp. 25-37. 
Mark, David M. (1979) , "Phenomenon-Based Data-Structuring 
and Digital Terrain Modelling", Geo-Processing, Vol. 1, 
Number 1, pp. 27-36. 
USGS (1980), National Mapping Division, Computer Files for 
Digital Elevation Models: USGS National Mapping Division, 
520 National Center, Reston, Virginia 22092.