happens mostly unconsciously except during experiments set up in the study 
of optical illusions. To day, optical illusions are considered to support 
the Gestalt theory. The misinterpretation of the visual sensation is caused 
by the interaction between the elements of the image, proving that the whole 
is experienced and is different from the quantitative sum of these elements. 
J. The impact upon visual image interpretation. 
As already discussed, the first perception of a photographic 
image happens in a similar way as watching television. Only after the differen- 
tiation of the Gestalt, systematic "reading" becomes operational. The Gestalt- 
abilities have many advantages. Similar shapes and patterns may be recognized 
and grouped easily even without a previous identification of the composing 
parts.Structures which are only partially registered may be completed and 
recognized. Coherent, compact, regular and geometric shapes, having a distinct 
boundary and a high contrast, are considered as "foreground" objects and are 
always differentiated first. On small-scale imagery (satellites) they consist 
of landforms, water surfaces and larger vegetation units. On large-scale image- 
ry (aerial photographs) they consist of all anthropogenic constructions, 
the vegetation- land use pattern, road-network and tree rows or-patterns, 
as shown by their proportion of recognition in the index of differentiation 
(table I). The index of differentiation is a simple measure and expresses the 
ratio of the total number of features to the total number of images observed 
by a group of students. In the test, all features has been written down in the 
chronology of their recognition. No significant differences were found between 
the indices of satellite imagery (Landsat, Seasat ; different Spectral bands) 
and aerial photographs (different scales and emulsion-types). 
TABLE I : Index of differentiation, nature of the features recognized and 
their proportion in the differentiation (9) for different image- 
types. (Further explanation, see text). 
  
  
  
  
  
Nature of the features differentiated upon : satellite aerial 
imagery photographs 
Land form 45,5 3,0 
Vegetation/Land use 12,5 16,9 
Water bodies (surfaces) 11,6 2,8 
Towns agglomerations, cities 4,5 2,8 
Parcelment 0,9 5,5 
Rivers, canals 10,7 6,6 
Roads, streets, railways 2,7 42,6 
Anthropogenic constructions : industries, 0,9 19,5 
bridges, parking-lots, chimneys. 
Tree rows, isolated trees, orchards 0,0 12,4 
Isolated houses, farms 0,0 2,4 
Marks (fault lines, soil marks, agric.marks) 1,8 5,3 
Shadows 1,8 1,6 
Clouds, haze 4,5 0,0 
Snow 2,7 0,0 
Index of differentiation 10,18 13,66 
  
On the contrary, the nature of the features which were differentiated varies 
a lot. Finally, the gradual differentiation makes it "natural" to start with 
macro-units and go on with micro-units, thus proceeding from small-scale 
reconnaissance to large scale detail. 
Disadvantages are mainly residing in faulty image completion 
especially for "geometric" image elements which are used to build "anthropoge- 
nic" or other "logical" structures which aren't really there. 
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