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4.6. RATIO MAPPING 
Ratio mapping eliminates effects due to variable illumination of 
landscape as well as its shadowing. The latter is based on the 
assumption that the spectral distribution of shadows is the same 
as that of normally sun-illuminated landscape. Hence taking the 
ratio between grey levels of two spectral bands results in a 
picture which is independent of sun illumination. Fig. 8 shows 
the ratio (MSS 4 - MSS 6) / (MSS 4 + MSS 6) multiplied by a 
factor of 32. 
4.7. CLASSIFICATION 
Based on statistical data obtained so far criteria for classi 
fication may be obtained. Fig. 9 shows the last picture which 
was obtained on our old system before it went down. In this case 
the problem was classification of land and water in the presence 
of a fairly heavy cloud cover. It was possible to separate water 
from land under light clouds, while heavy clouds have been classi 
fied separately.