20
For a point located in a corner of the image where the coordinates are
assumed to be
c
and for the same s 0 =2,5 microns the formulas give the results
s a ^16 cc or 5"
15 cc or 5"
The slight decrease of the standard error is naturally due to the use of one
average value for the standard error of unit weight all over the plate.
The standard errors of the directions derived here depend on a definition
of the camera axis as the straight line between the (inner) perspective center
of the lens and the principal point. The geometrical quality of the direction
in space of this axis is of course important for the determination of the geo
metrical quality of the oriented directions reconstructed from the photograph.
It is evident that the relation between the camera axis and some fixed direc
tions, for example the circles of a theodolite with which the camera is joined,
in general are determined with the aid of measurements in photographs and
readings of the circles. Consequently, the standard error of the determination
of the absolute horizontal and vertical directions of the camera axis can be
expected to be of the same order of magnitude as the relative direction of a
ray with respect to the camera axis. The geometrical quality of the circles of
the theodolite will, however, be of great importance in this case.
Conclusions
The investigations have shown that the determination of directions of rays
from photographs with the aid of interpolation principles can be accomplished
with considerably higher geometrical quality than with the aid of the elements
of the interior orientation.
The great importance of a well defined procedure for the determination of
the elements of the interior orientation of the camera (or photographs) and the
non-projective regular errors of the image coordinates has been emphasized.
The calibration must be made under actual operational conditions as well as
from specific tests of the lens under laboratory conditions. The test points
should be chosen so that the elements of the interior orientation can be deter
mined from point combinations in circles around the principal point, partic
ularly because the basic geometrical quality of the image coordinates can be
compared for different radial distances from the center. Preliminary tests have