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for a useful purpose. For some purposes identification of features alone 
is sufficient, but the amount of detail and recognition of small individual 
features can be dangerously misleading for practical purposes. For 
thematic mapping, the amount of detail recognisable in a photograph must 
have regard to: 
i. the consistency of recognition over all the photograph and 
ii. the degree of accuracy required in the delineation of 
boundaries. 
It may be a demonstration of photogrammetric expertise to recognise churches, 
quarries or airfields, for example, on a small-scale photograph, but the 
information is not useful for inventory purposes or monitoring changes 
unless all such land-uses can be recognised and plotted. These problems in 
mapping information were encountered in the DOE survey of 'developed areas' 
which is described in the following sections. 
THE NATIONAL SURVEY 
  
The data source for the survey was 1:60,000 panchromatic, vertical aerial 
photography. A Williamson F49 camera was used at an altitude of 30,000 feet. 
Full stereo cover was obtained. Ninety per cent of the photography of 3,500 
prints was obtained during 93 sorties, flown in 1969. 
The first task in the general project to develop air survey techniques was a 
pilot study to assess the suitability of photography at this scale for the 
identification, mapping and measuring of urban land-uses. 
The quality of the photography is excellent and it was found even at this 
small scale to be possible to detect up to 28 individual land-uses even as 
small in area as 0.25 hectares. However, after further tests by the DOE it 
was concluded that information in this detail and size of area could not be 
consistently interpreted to ensure that all areas in each of the 28 categories 
would be distinguished, Moreover, accurate measurement of areas only 0.25 
hectares in size raised serious problems, in particular for reaching the 
required standards in monitoring changes of areas as small as this. Although 
the definition of 'developed area' could be closely prescribed and followed 
absolutely in the interpretation and mapping of the areas, the definitions of 
the main land-use categories within developed areas were determined largely by 
the photography. The outcome of the pilot study was the conclusion that a 
classification of five main land-uses could be mapped at five hectares and 
above. 
The term 'developed area' was preferred to 'urban area' because the latter is 
strictly applicable only to the areas of cities and towns. For planning 
purposes a wider net of land committed to urban uses is required. Sporadic ox 
scattered development separated from the main urban areas would be excluded 
from the general meaning of 'urban area' but it was essential for the purpose 
of the mapping to include, for example, small areas of development in Green 
Belts and industrial sites in rural areas such as quarries and tips. 
The source data for the mapping is mainly based on photography which limits 
the detail of information about activities in individual buildings. Thus the 
definitions of the five main land-uses within developed areas are definitions 
in physical terms rather than in terms of activities, as in the National Land 
Use Classification.