Another possibility of obtaining aerial information is the use of 
35-mm cameras for making oblique airphotos. Oblique photographing has 
the advantage that no technical changes have to be made in the airplane 
and that almost any light and cheaply operating airplane can be used. 
The advantage of the 35-mm camera is that camera equipment, films, 
developing chemicals and enlarging equipment are almost everywhere 
available and the cost is very low. Oblique airphotos are, however, 
not practical for mapping purposes. Although there are methods to 
overcome the difficulties of the changing scale from foreground to 
background (IMHOF, 1966 and SCOLLAR, 1975), these are normally beyond 
the capabilities of the staff of an urban planning department. A pre- 
requisite for the successful use of obliques is therefore the avail- 
ability of a good topographical map, with sufficient landmarks, and/or 
vertical airphotos. How old these are is less important than how well 
made. The newly built-up areas are normally surrounded by 'undisturbed' 
areas, either urban or rural. And in the process of building-up an 
area, the new land uses are adapted in their location to existing 
features like roads, old ribbon developments, canals etc. These 
features can serve in locating the newly built-up areas properly on 
the map. 
The possibility of using obliques for mapping purposes is indicated 
and demonstrated in several publications. (See BRANCH, 1971, WILSON, 
1975 and Photography from light planes and helicopters, 1971). 
However, no reports have been found that answer the interesting 
question of how accurate 'normal' photo interpreters are interpreting 
and mapping from obliques. In order to find an indication of this 
accuracy, & test has been carried out at the ITC-URBAN SURVEY 
department with the available material from a Middle Eastern city with 
several million inhabitants. 
Test interpretation 
1. The material 
A sample area of 1 km2 was selected at the fringe of the city. For 
this area was available: 
a) a topographical map scale 1:10,000 from 1965 (buildings grey) 
b) vertical airphotos scale 1:10,000 from 1968 (black and white) 
c) three 35-mm camera low obliques from 1976 (black and white). 
The time gap between the obliques and the map that has to be updated 
is eleven years. The obliques have been taken with a 35-mm camera with 
55-mm lens, through the window of the airplane. The optical axis made 
angles with the vertical between 35 and 45 degrees. The Kodak Tri-x 
negatives (24x36 mm) have been enlarged 7 diameters to 18x24 cm. The 
image scale of the enlargements is: 
— foreground 1:1000 - 1:1100 (negative' scale 1:7000 and 1:8000) 
  
    
  
  
  
  
  
  
  
  
  
  
  
  
  
  
    
  
   
  
  
  
  
  
  
  
    
   
    
  
    
    
  
  
  
   
  
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