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for each class of residential area, find the relation (and its variance) between 
the area size on the photograph and the corresponding population size. Is it 
possible to use the results of the previous census to find this relation? How 
many years may these data be used before they have to be considered 
obsolete? Is it better to count the population in selected sample areas every 
two years? If so, what are the best rules for this sampling? How many sample 
areas are needed to estimate the variance? 
CENDATE 
Hardly any question has been answered so far. The ITC Urban Survey 
Department therefore collected material from a metropolis in a developing 
country. Census data on the population, of several millions, became available 
for enumeration districts— as small as 5000 inhabitants or less— of which 
many show only one type of house on the photographs. Also, aerial photo- 
graphs at the suitable scale of 1 : 10,000 were obtained by the ITC. Under 
the name CENDATE ("census updating"), a research project has been started 
which will last several years. A number of tests have already been made with 
different groups of interpreters, to find the variance in interpretation and 
measurement on the aerial photographs. This year, a description of the 
methodological framework and the procedures to be followed is being tested 
with another group of interpreters. Current activities aim at making the 
method operational as soon as possible; publication of research results will be 
done in the ITC Journal, while education in the method will be part of the 
standard and advanced courses. 
3.2 Urban Land Use 
The Urban Survey Department of the ITC has gained considerable 
experience in land use inventories and mapping during the last 7 years. The 
basic procedure has always been interpretation of aerial photographs, and 
subsequent field check. The inventories were made by staff and participants 
of the post-graduate courses in Urban Survey with Aerial Photography, as a 
part of the training and research programmes. A concise description of the 
different techniques and procedures for land-use surveys in city centers is 
given by Pollé (1974). Some conclusions were: "Although the use or function 
of many buildings in city centres cannot be reliably established by photo- 
interpretation, the aerial photograph is considered very useful in land use 
o