photograph was taken in 1985. The 1:10 000 Ordnance 
Survey map of the same area represented on the photograph 
was used to digitise the field boundaries. This map was 
surveyed at 1:2 500 scale between 1969-72 and revised for 
significant changes in 1973, the publication date is 1975. 
The vector file obtained was processed as described in 
section 2.1. The maximum resolution on a map is 0.2mm, 
therefore a precision greater than 2m can not be expected 
from a measurement taken from a 1:10 000 map. This means 
that even if a resolution smaller than 2m per pixel is used, 
this will not increase the accuracy of the measurements made 
on the digital map. Taking this into account and at the same 
time trying to minimise the amount of data to be processed, 
both the map and the image were scaled to a pixel size of 
3.92m, thus resulting in a -660x660 pixel image. In this case 
the highest point on the photograph has a height of 100m, 
even if this point was at the extremity of the photograph 
where the relief distortion is maximum, this distortion would 
be 0.6mm, which corresponds to 1.4 pixels on the digital 
image. Since the image was reduced 8 times, this distortion 
is irrelevant. The digital photograph was prepared using the 
procedure described in section 2.1: it was smoothed, 
thresholded, the edge detector was applied and finally the 
smallest polygons were removed from the image. The results 
of each step of this process are shown in Figure 2. 
Figure 3 shows both the image and the map prepared for the 
matching process, the polygons are numbered according to 
the order used to read them. A record of the frequencies of 
the directions between neighbouring pixels, the frequencies of 
the change in direction segments, the area of the polygon, the 
perimeter of its boundary, the width and height of the 
bounding rectangle and the coordinates of the centre of 
gravity of each polygon were derived. Figure 4 shows an 
example of the information recorded for one of the polygons 
represented in the Isle of Wight image. 
  
  
Polygon number = 1 
perimeter = 135 
area = 227 
Code=61 31 13 39 13 3 | 
Differences = 93 23 1 0 0 0 017 
Width of bounding rectangle = 65 
Height of bounding rectangle = 9 
Centre of gravity (X) 2377.8 
Centre of gravity (Y) 2 3.6 
  
  
  
  
  
  
Figure 4. Information extracted from a polygon 
represented in the Isle of Wight image 
A program was written to compare the information relating 
to each of the polygons extracted from the map and from the 
image. For each polygon in the map differences were 
computed between each of its attribute values and the 
corresponding attribute values for all polygons in the image. 
After adding all the differences, the minimum value was 
found and the pair of polygons that correspond to that 
minimum is considered matched. As mentioned by 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
  
  
Figure3. The Isle of Wight map and the image prepared for 
matching 
Abbasi-Dezfouli and Freeman the chain code approach 
followed in this study is very often a reliable way to 
distinguish between different shapes. However the method 
by itself is not infallible and the other parameters used to 
characterise a polygon should provide the information to 
compensate for a possible uncertainty derived by the 
frequencies of the chain code direction segments and change 
in direction, if these frequencies were used alone. 
  
   
  
  
   
  
  
   
  
   
  
  
  
  
   
  
  
  
  
  
   
  
   
  
  
   
  
  
   
  
   
   
  
  
  
  
  
  
  
   
  
   
   
   
   
  
  
   
   
  
   
   
  
  
   
   
  
  
  
    
  
  
   
  
   
   
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