PHOTOGRAMMETRIC ENGINEERING 
da = (1 +50) sto (5) 
fds! 
P+ 92 
For a better determination of dw, a number 
of points at the same distance from the 
camera can be used and the average of the 
different dw-determinations computed. Ob- 
viously different weights can bé introduced 
for computing the average. 
The deformations dz can then be ob- 
tained from the expressions (4). For an 
arbitrary point Ps: 
dw = (6) 
z;" Frd20" 
PL ^ 
If the standard errors m;' of the measure- 
ments dz’ are known and free from correla- 
tion, the standard errors of the deforma- 
tions can easily be determined from (7), by 
the special law of error propagation. At 
once it is evident that the accuracy in- 
creases with increasing f and that conse- 
is Tas — (1+ TE 
quently long principal distance cameras 
should be used for the photography. 
To increase the accuracy of the measure- 
ments of dz', special signals in the feature 
points should be used, for instance .steel 
balls, and flash lights, synchronized with 
the camera shutter. 
If one assumes the points located ap- 
proximately along the camera axis, the 
same standard error m; in all measure- 
ments, and no correlation, the standard 
error of the deformations m,, is found to be 
m,= nva ms (8) 
f 
Assuming 7:,; —0,005 mm. and f —- 200 mm. 
there is obtained 
0.035 
Mas Yi 
: 1000 
The standard error 0,005 mm. in the meas- 
urements has been found in connection 
with measurements in a modern mirror 
stereoscope. Of course, need exists for glass 
plates and high resolving power of the 
emulsion.