(2) 
1e 
from 
photo and the 
) becomes 
@") 
small scanning 
rthophotos 
ize the effect 
hophoto pair. 
troduction of 
of profiling 
zed in the 
1e experiments 
vertical infor- 
setting as 
The following parallax errors are then found from equation (2): 
for x°=-"0Ap = 0 
x 
for x. €41/28Ap "0213 mm 
x 
for x^ -0B5 Ap = 0.26 mm 
x 
The above example shows that when profiling errors are different for 
the orthophoto and the stereomate they can have a serious effect on 
the x-parallaxes in the stereo-orthophotos. The results presented 
next are affected by this type of error but considerably improved 
results are expected from production-oriented stereo-orthophoto equip- 
ment, such as is presently being developed at the N.R.C. 
Experimental Results 
  
a. Individual points 
Two stereo-orthophoto models of the Alnwick NRC test area, produced 
on the modified SFOM equipment at the scales of 1:4000 and 1:2000, were 
used for evaluating the accuracy of X, Y and Z coordinates of individual 
points. Distinct features such as road intersections, field and fence 
corners, were used as reference points. The x, y coordinates of these 
points on the orthophoto were measured using an A7 coordinatograph. 
Their elevations were determined from the stereo-orthophotos by measuring 
the x-parallaxes in the Stereocompiler. These stereo-orthophoto 
coordinates were compared with corresponding coordinates measured in 
the stereomodel which was reconstructed in an A7 plotter. Both sets 
of coordinates were then transformed into a common system by a linear 
conformal transformation. Standard errors were computed from the 
. coordinate differences, assuming the A7 coordinates to 0e free from 
errors. They are listed in Table I. 
Table I 
Errors of individual points in stereo-orthophotos 
  
  
  
  
Orthophoto Scale of Planimetric errors in Height errors in 
scale original | orthophoto scale orthophoto scale 
; photographs 
FA M m number of | m |number of 
X y points z points 
1:4000 1:16000 {0.16 mm|{0.18 mm 94 0.14 mm 40 
1:2000 is 8000 ° 10.16 0.16 57 0.22 56 
  
  
  
  
  
  
  
  
b. Contour lines 
The accuracy of contour lines plotted from stereo-orthophotos 
using the Stereocompiler was estimated in conjunction with a large-scale 
mapping project of the Grande Cache Area in Alberta. Stereo-orthophotos 
at the scales 1:2500 and 1:5000, each composed of two stereomodels of 
the original 1:10,000 and 1:20,000 photographs, were used. The contour 
intervals were 10 feet and 20 feet respectively. 
-10-