169
of the measuring-mark so that the corresponding direction can be
read.
To obtain the correct readings the instrument has to answer in the
first place the following requirements with respect to the adjustment:
1. The point in the photograph which is chosen as centre of directions
has to coincide with D.
2. The line AB has to pass through D.
3. The centre of the division on the circle has to coincide with D.
The influence of er
rors according to the
requirements sub 2and
3 can be eliminated by
measuring the directions
in two positions of the
plate-carrier which differ
in amoufit of 200 grades.
The point P (see fig.
1) can be brought 011
AB by a rotation a1; as
well by a rotation a2. In
the mean value of the two readings the above mentioned errors are
eliminated.
Errors due to a contravention of the first requirement can not be
eliminated. With respect to the estimation of the influence of these
errors on the results may be referred to an article of the author on
„The accuracy of the measurements of directions in radial-triangula-
tion" which will appear in the next number of „Photogrammetria".
At the practical execution of a radial-triangulation the coincidence of
the chosen measuring-centre (principal-point, isocentre or plumb-
point) with the rotation-centre D is reached by three manipulations,
namely
a) The chosen point is marked on the photograph by a little hole or
cross on the emulsion-side.
b) The rotation-centre is marked on a fixed glass-plate in the plate-
holder by an etched cross.
c) The photograph is so adjusted on the plate-holder that both marks
coincide.
The accuracy of the first manipulation is independent of the con
struction of the radial-triangulator and is only affected by the accuracy
by which the elements that fix the place of the chosen measuring-
centre, are known and by which the marking of this point can be
executed.
The accuracy of the operations b and c is, however, indeed dependent
on the properties of the instrument and they will therefore be sub
mitted to a closer examination.
The marking of the rotation-centre.
This marking has only then sense, when each plate-holder is always
put in the same position on the carrier. To reach this purpose all
