> 
LS 
1S 
"y 
n 
|S- 
17. 
     
is determined by a planimeter and the area found is then divided by the length of 
the contour line in order to produce the average error. The results of these 
determinations for individual contour lines are listed in Table II. 
Table II. Comparison between A-7 and automatic contouring 
for 1:3000 photographs 
  
  
  
Average 
Average Contour dec Errors, em? >| |, ay 
slope, % height, m error, mm positive negative cm? error, m 
4 172 3.0 2.9 7.8 10,7 0.07 
16 176 6.0 33.4 15.6 49.0 0.58 
20 180 4.4 3548 734 42.9 0.53 
20 184 3.1 17.6 14. 7 32.3 0.37 
10 188 3.9 23.8 14.3 38.1 0.24 
10 192 1.6 5,5 14.9 20.4 0.10 
8 196 3.0 6.3 19. 4 25.7 0.14 
8 200 4.6 23.9 14.9 38.8 0.22 
8 204 3. 6 1.8 15.4 17.2 0.17 
10 208 6.9 0.9 26.8 27.7 0.42 
14 212 4.8 3.9 14.3 18.2 0.40 
20 216* 7.2 24.7 0.0 24.7 0.86 
18 220 3.3 1.2 2.8 10.0 0.36 
16 224 4.7 2,0 12.2 14.2 0.45 
  
probably made by the operator. 
further analyses. 
" The contour at 216 m is shifted systematically because of some mistake, 
This contour should not be included in 
   
   
   
     
        
  
  
  
  
  
  
  
  
  
The actual accuracy of automatic contouring is obviously better than 
can be concluded from the figures in Table II. 
It is of interest to analyze the contour lines in relation to the slope of 
the terrain. The accuracy of photogrammetric contour lines is often expressed by 
the formula 
mp = a +b tan « 
where a is the accuracy of spot elevation for a given photographic scale and b is 
the planimetric plotting accuracy that corresponds to approximately 0.3 mm in 
the model scale. The theoretical error of contour lines, computed by assuming 
that a = 10 cm is shown in Fig. 10, together with the differences between the 
A-7 and automatic contours. 
On assuming that the curve of theoretical plotting accuracy represents 
the errors of the A-7 contour lines it seems that the accuracy of automatic con- 
touring, which is comparable for flat terrain with an operator's contouring 
accuracy on a first order plotter, deteriorates with steeper slopes. 
  
There may be several reasons for this drop in contouring accuracy in 
steep terrain. The designers of the automatic plotting device have already