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Fig. 4 
Theme analysis: extraction of a river (80 - 100 m wide) 
from ERTS data» 
4a - Multispectral classification of open-water bodies. The 
Petit Rhone River appears discontinuously and is surrounded 
by many other (undesirable) water bodies. 
4b - Extraction of the Petit Rhone River (from original data) 
by topology analysis.