sating glass plates. In fig. 2.4:4 no compensation plate was used and in other tests plates were used
during the printing procedure for the correction of optical radial distortion, atmospheric refraction and
curvature of the earth. Effects of lack of flatness of the diapositive glass plates are evident.
2.5 Tests of the Accuracy of Glass Grids
Glass grids are often used for tests of instruments in practice, but it is seldom that the quality of the
grid coordinates is specified or tested. To determine this basic factor in instrument tests several grids
used regularly were measured in the Wild StK 824 stereocomparator. See section 2.1.
Twenty-five points were measured in each grid.
The computations follow the procedure described in reference 2.5:1.
The results are summarized in table 2.5:1.
The measurement were made by different operators, but mostly by P. Kaasila, MSE, who was also
responsible for most of the computations.
Reference. 2.5:1. Hallert, B.: Test Measurements in Comparators and Tolerances for Such Instruments.
Photogram. Eng. March 1963.
Table 2.5:1. Results of tests of glass grids
Grid
number
Instrument
S o
microns
dm v -dm„
X y
io- 6
Conf. lim.
5 % level
io- 6
d3
cc
Conf. lim.
5 % level
cc
1
Zeiss stereocomp.
2.7
+ 16.3
26.9
-12.9
17.2
2
18X18, 1939
2.6
+ 34.0
26.3
— 14.0
16.8
1
Wild autogr.
3.4
— 4.2
27.3
-81.5
17.4
2
A6, 1939
3.4
- 8.2
27.1
+ 25.1
17.2
1
Wild autogr.
1.7
+ 16.1
17.2
+ 1.1
10.9
2
A7, 1953
1.5
+ 3.3
15.2
+ 0.5
9.5
788
Wild StK 824
0.9
- 5.4
7.1
- 0.2
4.4
789
1962
1.2
-12.2
10.1
— 0.4
6.5
423
Wild A8
1.3
+ 14.0
9.8
+ 8.7
6.2
424
1.4
- 4.6
10.8
+ 6.2
6.8
696
Wild A8
1.2
+ 21.3
12.5
+ 3.9
7.9
697
1.1
+ 24.5
10.9
+ 2.7
6.9
822
Wild A8
1.9
+ 10.3
14.2
+ 11.8
9.0
823
2.2
+ 6.2
16.4
+ 6.7
10.4
943
Wild A8
1.2
+ 4.2
9.0
+ 11.9
5.6
958
1.3
+ 11.0
9.8
+ 31.8
6.2
The accuracy of the glass grid coordinates has improved considerably since 1939. Glass grids belonging
to the A8 autographs were received in recent years. The average of the standard errors of unit weight
of the 12 grids delivered since 1953 is 1.4 «m, According to section 2.1, the standard error of unit weight
of the StK 824 stereocomparator is about 0.6 qm. The basic accuracy of the glass grid coordinates tested
can therefore be described as having a standard error of unit weight of 1.3 qm. Further, it should be
noted that in 6 tests out of 16 the expression dm K —dm y exceeds the confidence limit on the 5 per cent
level and in 6 tests out of 16 the expression d(3 exceeds the confidence limit on the 5 per cent level. The
coordinate residuals are normally distributed in 10 cases out of 16 according to the chi 2 -test on the 5
per cent level.