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intensity) effects are eliminated and the colour information is brought 
out (compare Fig. 6). Three spectral hand ratio images are generally color 
composited in a false color display. Most interband correlation is coupled 
with scene intensity. Hence colour context of a scene appears to be strongly 
improved.* Where intensity information is crucial, ratio should not be used. 
  
Band 1 
  
Figure 6: Principle of ratio processing: points P1, P2 may have the same 
color (be the same type of terrain cover), but have different intensity. 
Ratio B1/B2 is color information: values for P1, P2 are identical. 
(After Quiel, 1975). 
4.2.3 Direction Cosine and Equal Vector Length Transforms 
This transform results in enhancing the colour information like the 
ratioing method does. Each pixel has associated with it a feature vector, 
for Landsat with lh elements BJ, B5, Bg, By. First the vector length (x) 
of each pixel k is calculated; 
x, = (BR + BB «Qe 82), 1/2 
and is stored as a transformed data set related to intensity Or spectral 
brightness, Vector length output produce imagery with enhanced topography 
due to shadow effects. Then the direction cosines cos ©; /x, &re calculated. 
Particularly in mountainous areas the direction cosine and equal vector 
length method are powerful preprocessing methods (Donker and Meijerink,1977). 
The direction cosine and band ratio techniques are nearly equivalent. Fi- 
gure T includes an example of a vector length image. 
4.2.4 Differencing 
Teking the differences between two spectral bands of the same multiple 
image serves according to Goetz et al., (1975) to produce an enhanced image 
for interpretation. Its usefulness as a pre-processing method is reported 
to be comparable to that of ratioing. 
4.3 Data Display 
The registered, compressed and enhanced images must be displayed to the 
interpreter as several black and white images, or as a composite color 
image depending on the purpose. In addition a feature space plot may be 
useful. Imagery on TV screens is useful only as a check during the pro- 
cessing stage but generally inappropriate for actual interpretation. 
  
r 
An illustration of the performance of band ratio color composites and 
comparison with PCT (unsupervised) would require color. Cost considerations 
prevent us from including such illustrations,