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methods, which are still useful in the classification process. 
Diazo Method ; A number of colors may be assigned to both 
positives and negatives of ERTS bands in various combinations to 
highlight features of interest when the films are sandwiched to 
gether. This method, which can produce a full range color composite 
from only two bands, usually results in a less expensive poorer 
quality image than may be obtained through the normal photographic 
process or from the color additive viewer. (Warrington and Ryerson 
1974) . 
Color Additive Viewer ; The principle of the color addit 
ive viewer is similar to the diazo, although the "overlay" is accom 
plished optically in a viewing device. The method has the same 
flexibility as the diazo method with more easily reproduced results, 
better resolution, and consequently a more useful image for inter 
pretation. Costs are considerably increased over the diazo method. 
Density Slicer : The analogue density slicer is a unique 
way of studying the density variations within a single band of the 
image. 
Density steps are identified (sliced) and to each of these 
a color may be assigned. The investigator must establish the sigr 
nificance of the resulting density variations and then interpret 
these for his problem. Although the method uses only one band, the 
density slicer is the most sophisticated hardware used to assist 
visual interpretation. 
Digital Enhancement Methods : Digital enhancement methods 
are concerned with enhancing the digital MSS data in such a way that 
maximum visual discrimination between chosen land-use classes can 
be achieved. At CCRS we have developed an interactive software- 
hardware system (Goodenough et al 1974), called the MAD (Multi- 
spectral Analyzer Display) system with which one can rapidly perform 
all of the functions in color of the diazo method, color additive 
viewers, or density slicers. 
In addition, one can "add", "subtract", "and/or" any three 
of the four ERTS bands, generate three theme classifications, and 
produce photographic or digital copies of the results. The MAD 
system enables a user to quickly ascertain the image combinations 
which will be most useful for photointerpretative purposes. 
A second approach to digital enhancement is the method 
developed by Martin Taylor (1974) in collaboration with the staff of 
CCRS. Taylor derives colors for an ERTS image by forming three in 
formationally independent linear combinations of the four MSS bands. 
These three linear combinations are mapped into the three visual 
color dimensions of brightness, red-to-green ratio, and blue-to- 
yellow ratio. One computes the linear combinations by selecting an 
area on the image representative of the object types to be discrimi 
nated. Digital or photographic reproductions of the results can