This threshold is determined by successive approximations com- 
paring the signs plot with maps and/or aerial photographs and 
relying on the assumed geographic conception of the built-up 
domain. With the aid of the defined threshold, the photograph 
technician finally prints, by the Agfacontour process, an equi- 
density corresponding to the built-up areas. This last step is 
nevertheless not indispensable if the densitometer sweeping has 
adequate resolution (practically 20 4m) and if the character 
choosen for the signs plot has an homogeneous cover. It is suf- 
ficient in this case to photograph directly and at the right 
scale, the signs plot corresponding to the selected threshold. 
The Agfacontour process was used in our study mainly as a proved 
technique for comparison. 
Results. 
The advantage of the method which has just been des- 
cribed is that it makes use of relatively modest technical means. 
It generally yields good results (ex: Bruxelles, Lille, Roubaix, 
Tourcoing). 
But this method does not always give full satis- 
faction, as it is limited in certain cases by various factors 
such as: 
a) quality of the photographs: some photographs are defective 
because of a desynchronizing of the signal at its reception. 
The missing lines are then replaced by the six preceding 
ones. Such photographs can therefore not be used. 
b) geographical factors in connexion with the stucture of 
ground occupation: in this matter the problems are varied 
and complex. It is difficult to delimit a densely built- 
up area because it is often settled in an environement 
that is characterized by an extraordinary mixing of the 
structures of ground occupation; these one have a reflec- 
tance similar to that of the built-up areas, but they do 
not have their nature. 
It is the case of slag-heaps, the colleries, the marshal- 
ling yards and the industrial equipments, the more as the 
built-up areas imbricated there, especially the mining 
villages, have highly variable structures and densities. 
(ex: Lens, Geleen, Sittard). 
It is the case too of certain wooded areas contiguous to 
urban areas (ex: Libercourt). 
Finally, the exemple of Gent shows that it is sometimes 
difficult to distinguish water from habitations. 
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