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index for the color to be drawn; this eliminates the need 
for comparisons to decide what color should be used, by in 
corporating an array 'COLOR' (by the number of classes) 
initialized at the beginning with the proper colors assigned 
to it. Thus a call to the color routine is accomplished via 
the element CLASS. 
Contour Map (DCQMTOUR) 
The traditional method for the display of elevation data, 
the contour map, is also available in DEMGS. In this case 
the isopleth is the boundary between two elevation class 
intervals. The algorithm used in DEMGS only requires that 
the contour interval be known. This is supplied by the user 
and can be specified in either feet or meters. The contour 
interval is then matched to the data. 
The basic process moves a control point through the data and 
checks to see if the elevation of the control point is in 
the same class as two of its neighbors. If not, the element 
is drawn. The algorithm used in DEMGS emphasizes speed and 
makes use of the grid size to resolution relationship to 
avoid costly interpolation. The movement of the control 
point also allows for increased speed by only checking two 
neighbors, one "above" and one to the "left". Another 
method used to speed up tne algorithm is to compute the 
classification of each element when the band is read in. 
by doing this the classification process is only done once 
instead of each time tiie element is used in the comparison 
process. 
Anagraph Map (PARAGRAPH) 
It requires training to view contour lines correctly. Many 
other methods have been utilizea to allow a user to see the 
three dimensional form of terrain from a map. A few of 
these are block diagrams, profiling, shaded relief, raised 
relief, stereo plots and anagraphs. Some of these tech 
niques are available or being developed in DEMGS and are 
discussed in the next section of this paper. This section 
concentrates upon the stereo-plot/anagraph display. 
Jensen (1980) proposed the use of stereo plots for thematic 
maps as a method to enhance the communication of the phen El 
ena to the map reader, while keeping one map planimetrically 
correct. One drawback of the stereo plot is the limited 
coverage due to the various methods that are used to view 
them. This is not a problem with the anagraph technique. 
The anagraph procedure is very simple: the image is first 
drawn in its planimetrically correct position in red, and 
then offset by an amount dependent on the elevation and 
drawn in blue. When this is viewed with red/blue glasses 
the image appears to float out of the screen. 
In order to accomplish this in DEIIGS the basic contour pro 
gram is used, however, when a pixel is drawn (in red) the 
corresponding blue pixel is offset and drawn as well. The 
amount of offset of the blue pixel is calculated by P = HK, 
where P is the parallax displacement, il is the class inter 
val derived from the minimum elevation with equal steps, and 
K is the amount of vertical exaggeration (Jensen).