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- 243 - 
In practical air photo interpretation there are cases where 
model deformation will not be found disturbing at all: When 
measuring short distances, minor level differences, small areas 
or volumes, distortion of the stereo model will hardly have any 
noticable effect on the measurement result. 
On the other hand, there are situations where model distortions 
will introduce considerable errors, for instance when measuring 
elevation, route profiles or slope angles. In order to make 
allowance for these differences in measurement problems there 
are different coordinate transformation programs for the 
STEREOCORD, designated COORD A, COORD C, and COORD D (Fig.4). 
In COORD A $=wWw=0 is assumed; in other words, no allowance is 
made for photograph tilt, just as in measurement with a parallax 
bar. The advantage of this approach is that the user may start 
measuring in the stereo model right away without recourse to 
the orientation programs, no control points being required. 
However, accuracy will only be satisfactory when measuring 
short distances or small areas or volumes. As soon as longer 
distances are involved, model deformation will introduce con- 
siderable errors. As an example, Fig.4 shows a test model flown 
at 2,600 m, with photograph tilt around 18 - 28, The diagrams 
show the distribution of terrain-coordinate errors over the 
model surface. For greater clarity, the arrows are exaggerated 
by the factor 8 in relation to the model surface. 
COORD A COORD C COORD D 
and parallax bar Sing=¢; sinw=w sing ;sinw 
P= wW=0 
cos@=cosw=1 COS Q ; cos w 
  
50m 
deviations exaggerated by factor 8 
50m ^X 
Fig.4: Accuracy of coordinate transformation using the 
transformation programs COORD A, COORD C, COORD D with 
a test model (camera focal length 153 mm, flying height 
2,600 m, Q1 -1.38, 92 = +1,08, (1s 1,48, 02 = 1.72). 
For more accurate computation of terrain coordinates, the 
COORD C transformation program has in the past been available 
for the 9810 A calculator by Hewlett Packard. Since standard 
equipment of that calculator did not include trigonometric 
functions, allowance for photograph tilt was made to a first 
order approximation in COORD C. As a result, the accuracy in 
the test model was increased by about the factor 5 compared to 
COORD A.