4 
1. Basic Principles for Testing Photogrammetric 
Instruments and for Establishing Tolerances 
1.1 Test Measurements and Adjustments 
The usual method of testing a photogrammetric instrument in practice is to 
use it to measure given data of high geometrical quality. It is obviously 
desirable that the quality of such test data be expressed in well defined con 
cepts and terms. The commonly used data consist of coordinates of the inter 
sections of regular and closely spaced grid lines on glass plates. 
If the grids are not observed orthogonally to the surface, it is also necessary 
to know the flatness of the grid surface i. e. the third coordinate of the grid 
lines intersections must be given with high and known quality. 
In practice, the data for the intersections on the glass grid must be deter 
mined originally by measurement in instruments of the highest known quality. 
Independent measurement in at least two instruments with different operators 
is always desirable. Then the geometrical quality of the results obtained can 
be determined from numerical comparisons of the different, independent sets 
of coordinates. 1 ) In principle the geometrical quality of the given data should 
be so high as to be regarded as errorless in comparison with the instrument to 
be tested. Obviously, compromise in this requirement is sometimes necessary. 
The general principles of the instrument tests are as follows. 
With the instrument to be tested the coordinates of the grid are measured 
under operational, as distinct from special laboratory, conditions. The orien 
tation of the grid should usually be such that the differences between the 
measured and the given data are sufficiently small to be treated with linear 
differential formulas. The necessary calculations are then simple. Sometimes, 
however, there are reasons to use a completely arbitrary orientation or com 
bination of orientations of the grid. The computations in such a case usually 
must be of iterative character. If possible, the grid points to be measured 
should be chosen in a regular pattern around the center point. 
The basic measurements must always be made with great care. The operator 
must use a certain “warming up” period of at least 10— 15 minutes in the instru 
ment. Enlargement and measuring marks should be personally chosen, if 
1 ) An example of this procedure has been given in Lycken 1956. 
In this case the coordinates of all grid intersections were given by the manufacturer of the 
grid. Independent measurements of the coordinates of the grid were then performed in a 
SIP-comparator and coordinate transformations including adjustment according to the 
method of least squares were made. Assuming the same order of magnitude of the errors 
for both sets of coordinates the standard error of each set became about one micron. The 
grid had the total format 9" x9". The flatness of a grid is preferably measured according to 
the microscope method, which does not require the surface to be touched. See Hallert 1960 a.