- 972-- 
Data Extraction and Recording 
Using a Carl Zeiss Interpretoscope the black and white 
.aerial photographs were stereoscopically examined to 
identify those units of open space as recorded in the 
notation (Figure 2). 
A transparent grid overlay was placed over the photographs 
so that the data could be recorded on a grid square basis. 
The grid represented a 20m x 20m square at ground scale, and 
that use which fell under the cross at the centre of the grid 
was the use recorded for the entire cell. 
Figure 4 illustrates the procedures followed in recording 
the data. Where the cross landed cleanly in a particular use 
then that use was the recorded one. Where the cross landed 
on a boundary the decision followed a set pattern, as can be 
seen in Figure 4. 
The location of each grid cell was recorded on the coding form 
so that spatial information could be retrieved for mapping 
purposes. 
The colour infra red photography of the three enumeration 
districts were also stereoscopically examined and similar 
information was derived from them and entered on the coding 
forms. 
The two sets of survey information - one from the black and 
white photography, the other from the colour infra red 
photography - were then fed into a computer data bank. 
Computer programmes were developed to provide a variety of 
statistical data from the information fed into the system. 
Some consideration was given as to whether to use a grid square 
or a single point (in this case a cross +) as a basis for recording 
the information. 
In the former case the 'dominant use! of the grid square would be 
recorded, whereas in the latter case the use directly under the 
cross would be recorded. It was found that the latter was very 
much quicker and, unlike the former did not involve any judgement 
decisions or estimates of areas. 
Some random tests showed that there was no advantage in accuracy 
in using the 'dominant use! of the grid square. An important 
factor was the density of the observations - in all the work 
carried out in this study a density of 625 points per Km^ was 
used to record the data. 
At a subsequent stage a programme was developed to carry out a 
large number of random samples, from 625 to 10 points per Km2, 
and the results of this are now being analysed. 
The aim of this particular part of the investigation is to give 
the planning authority some idea of the relative accuracies 
attainable when using grids of varying density. 
        
   
   
    
    
    
     
     
    
    
  
  
  
  
  
     
    
  
    
    
   
  
    
  
    
   
  
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