3. The number of linear features ; 
4. The tonality of the linear features. 
The headlines of the study were carried out as follows : photo- 
graphic prints were made on different scales : 1/250.000, 1/100.000 and 
1/50.000 in order to find a suitable scale to draw up an inventory. The inven- 
tory capacity for scales between 1/250.000 and 1/50.000 proved, by experiment 
to be the same. As for larger scales, lines were detected purely caused by 
speckle effects. Further enlargement causes a break-up of the image. 
As 1/250.000 and 1/100.000 seemed rather unpractical to draw the lines on an 
overlay, photographic prints at a scale of 1/50.000 were used to draw up the 
inventory. 
The image was "line-scanned" visually along azimuthal, range and 
diagonal directions. The human eye doesn't detect pixelwise but rather has a 
simultaneous overview on a part of the photograph or the whole photograph. 
Confirmation of this statement is found in the "Gestalt"-psychology. So the 
visual detection of linear features on photographic prints is mainly based on 
the contrast of the pixels on line with their neighbouring pixels. Consequent- 
ly a logical distinction was made between pale lines neighboured by darker 
pixels and dark lines neighboured by pale pixels. The use of a more detailed 
division was out of request, due to the lack of continuity in grey value 
of neighbouring pixels, caused by speckle. 
The overlays on which all linear features detectable, show that 
most of the lines are not longer than 2 cm. This means that only important 
features as highways and  shipcanals are detectable as continuous lines. 
Comparison was made with the groundtruth, based on terrain observation, 
maps and air photographs. It is impossible to reconstruct the existing network 
of all kinds of roads except highways, without further additional information. 
This is shown by comparison of overlays with image information with overlays 
based on map information concerning roads canals and motorwaycomplexes. 
Distinction among the motorwaycomplexes is based on : 
1. the importance and width: of each way, according to the 
map legend. 
2. accompanying features as tree-rows and ditches. This infor- 
mation is found on airphotographs from approximately the same 
date as the SEASAT imagery. 
In order to quantify the interpretability of the linear features 
according to their nature and their relative orientation, several tests have 
been made in which variables were compared two by two. 
Comparison was made as follows : we draw up an inventory of 
white lines on a transparent overlay on the image. Independent of the first 
overlay we draw up a second one with the black lines. These two overlays 
are superposed and adjacent black and white lines are counted. 
The intention was to know whether linear features are neigh- 
boured by other contrasting linear features or whether they are neighboured 
by contrasting patches. We also wanted to know if the way of detection by 
human eyes does work bi-directional or not, fore as this bi-directional 
detection might show an example of optical illusion. It was found that not 
five percent of the lines (as well in number as length) were overlapping 
on another. The coincidence of white and black lines are mostly caused by 
motorwaycomplexes with accompanying trees and/or canals. On the other hand,if 
we lookat the map information ‘first the greater part of motorway complexes were 
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