with a minimum of confusion. To this end, color coding, 
stereo, and shaded contours were all employed. The detailed 
shape of the watershed is clarified with the stereo depic 
tion of topography. The shaded contours, in addition to 
implying relief, provide a surface texture that enhances 
the stereo effect. The detailed surface texture makes a 
precise correlation between corresponding points in a 
stereo pair possible. This correlation in turn is required 
by the visual system to deduce the apparent elevations. 
In this investigation, a determination was made of the 
rate of increase in total land area occupied by irrigated 
crop land using Landsat imagery recorded in 1975 versus 
similar data in 1979. As a means of evaluating the source 
of irrigation, the location of irrigation canals and the 
distribution of irrigation wells were also plotted. 
Another important parameter to be considered in regional 
agricultural development is the slope of the terrain. The 
various slopes were calculated from the topography files 
and shown in increments of five degrees (5°). 
Colors were chosen in a fairly realistic way for some fea 
tures and to provide high contrast in others. The irri 
gated farm land in the reference year (1975) is green. 
However, to show the growth in time clearly, the new farm 
land was depicted in red. Slopes are shown in shades of 
brown and streams and most boundaries in white for high 
visibility. Generally, the choice of colors in a complex 
map is a combination of many factors. The process is 
usually longer and requires more trial and error than 
might be expected. 
Figures 3 and 4 are stereo pairs of the Valles Caldera in 
northern New Mexico. The upper pair is an example of a 
raster digitizing and merging portions of two 1:250,000 
scale topographic maps. This merged map file was regis 
tered with a digital terrain data set of the same resolu 
tion. Contour shading was added to the stereo pair to 
accentuate the relief features. The vertical exageration 
is a function of the relative displacement between corres 
ponding picture elements and can thus be adjusted as 
desired. This example illustrates how existing maps may 
be digitized and treated as image data sets. Figure 4 is 
a Landsat scene from January 14, 1981 for the same region. 
The image has been contrast stretched, edge enhanced and 
resampled to register it with the map image. Ground con 
trol points were identified to provide accurate registra 
tion with the map and terrain data files. The stereo pair 
was produced with the same vertical exaggeration as the 
topographic map and no artificial shading. 
A variety of geodata products have been prepared in which 
geologic and/or geophysical data e.g. gravity, magnetic, 
seismic, and hydrologic data have been combined with topo 
graphic and Landsat data, but to date these products have 
not been cleared for publication and in many cases are 
deemed proprietary by our clients. 
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