70
wide angle camera we then obtain the approximate following weight coeffi
cients
Qcc = 0.57
Qxx = 0.48
Qyy = 1.00
The observations are given equal weights.
Using a camera with c = 150 mm the scale of the negative varies between
1:1300 and 1:5200. Depending on the location in the picture, 1 /vm on the negati
ve corresponds to 1.3—5.2 mm on the ground. If the ground co-ordinates have a
standard error of 10 mm this means 1.9—7.7 /vm in the negative, and we may
assume the standard error of unit weight to be some 6 //m and the principal
point will have a standard error of approximately 7 /vm radially, and the princi
pal distance will have a standard error of approximately 5 /vm. Having 30 deg
rees of freedom a 95% confidence interval would be ± 10 jam for c, and
— 15 /vm for the principal point. It would be worth while to establish this test
field in order to check the elements of interior orientation under operational
conditions.
8.4.2 Radial Distortion
The two given examples of test fields for aerial cameras are not suited to de
termine camera constant, principal distance and radial distortion in one and
the same procedure, unless several more points on the ground are added.
Because of this practical reason it seems to be better to control the radial dis
tortion by photographing test fields with more points but leaving the principal
point and! the principal distance out of the adjustment. The Oland test field is
very well suited to this.
8.5. TOTAL CALIBRATION FROM SEVERAL PICTURES
A metric camera is often said to have ’’fixed and known interior orientation”.
But the image co-ordinates atre measured in the picture and then it is the inte
rior orientation of the picture that is interesting for the reconstruction of the
bundle of rays. In chapter 8.1 the hypothesis of ’fixed interior orientation” is
tested. From the different pictures it is then possible to estimate an average
orientation for pictures from the camera. Another way to do this is to compute
the elements of the interior orientation in one and the same adjustment using
all measurements in all pictures.
In chapter 3.2 a calibrating technique developed by Bergenvik [2] is briefly
described.
