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construction of successive transparent overlays on which values were 
represented by graduated tones or colored shadings that indicated the 
relative value of a particular factor within a given geographical area. 
In this manner, optimum areas were visually located with respect to some 
given decision criteria. 
In composite computing mapping, the overlaying of tonal transparencies 
is replaced by the algebraic combination of two or more matrices whose 
elements have numerical values corresponding to the grey toned densities. 
The GMAPS compositing capabilities are quite extensive, and include both 
arithmetic and logical compositing procedures. Arithmetic compositing 
is a simple extension of the tonal overlay procedures, but allows much 
more varied analyses using combinations of addition, subtraction, mul- 
tiplication and division in conjunction with the ability to weight some 
components more heavily than others. Logical compositing is even more 
flexible because it allows a detailed examination of the conditions with- 
in each map cell and the creation of a resulting composite map which 
reflects these conditions. 
The most important decision made in the compositing process is the 
determination of the relative values or weights given to two or more 
factors which are to be combined. Such weightings are called external 
weightings. All conditions shown within a single map must also be given 
desirability values. These are called internal weightings and range from 
0 to 9. 
Weights are assigned whenever possible based on known cost or established 
scientific relationships. On issues where a clear public preference has 
been established through public opinion surveys, weights are assigned to 
reflect these findings. When qualitative judgments are necessary, as in 
the case of evaluating scenic sensitivity, an interdisciplinary team is 
utilized. Additionally when a difference in weights might substantially 
affect an analysis, a number of different weightings are used so that 
the results can be compared. 
The GMAPS programs require no specialized computer equipment for either 
data input or display. The procedures were designed specifically for 
use by non-technical personnel at field office locations. They have been 
applied to a variety of studies at many scales. Studies have been con- 
ducted in areas as small as 30 square miles while others have covered 
the entire United States. 
Optional data sources, such as census data tapes and LANDSAT digital 
imagery classifications, can be entered into the GMAPS data bases. Dis- 
plays are most commonly created using the standard line printer. These 
can be photographically reduced and printed in black and white or color 
using normal offset printing techniques. Examples of these products 
are included in this paper. However, more sophisticated displays have 
been produced in situations where specialized equipment and needs existed. 
These include CRT displays, direct production of 35 mm color slides, use 
of electrostatic plotters, and the production of color prints from the 
slides via xerox or color photographic processing.