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182 Commission II Invited paper 
The Recent Development in Theory and Design 
of Approximate Instruments 
by H. C. ZORN 
LT.C., Delft. 
Reviewing the communication of the 1952 congress in Washington, we do not find a 
single publication on the subjects of 3rd order instruments or approximate instruments. 
Certainly at that time there were many different instruments of this nature used in 
many countries. 
Remember the K.E.K.-plotter, the Kail Radial Line Plotter, the Wernstedt-Mahan 
plotter in the U.S.A. and the Nistri Stereographometer and different outfits with paral- 
lax bar and parallel guidance mechanism in Europe. 
The 1956 congress has shown us two new instruments: the Stereotope of Zeiss and 
the Cartographic Stereomicrometer of Santoni. About the former a paper was presented: 
“Theory and Practice of the Stereotope" by Dr. H. Deker. This gave some detail about 
the construction and some results which had been obtained. 
Besides these two instruments the Hilger & Watts Radial Line Plotter and the Nistri 
Stereographometer were shown. I did not notice new publications about these instruments. 
In the period from 1956-1960 some important papers were published: 
Dr. H. G. Jerie — The Stereotope 
In this paper, Dr. Jerie computed the theoretical errors of the Stereotope due to the 
approximate character of the correction devices. These theoretical errors were mainly 
due to 
1. the fact that the parabolic cylinder is not corrected; 
2. all altimetric corrections refer to flat terrain, so there are, in cases of moun- 
tainous terrain noticeable deviations from the hyperbolic paraboloid, which is 
used as a basis for correction in the Stereotope. 
In 1959 the dissertation of Dr. Deker was published. Dr. Deker gave, in addition to 
critical calculations about errors in the Stereotope, some details on 3rd order plotters in 
general and possible improvements on the Stereotope. 
In the derivation of his formulae Dr. Deker took terms like ¢2 into consideration. 
They are of the order of 36.10—9. But if such small quantities are mentioned is it not 
‘AZ \2 |Z 
then necessary to mention also | Z | 9. If Z 20%, this value is larger than terms 
V0 “0 
like g?. 
Prof. Schwidefsky has suggested that Dr. Jerie in his paper considered too large 
height differences. The numbers Dr. Jerie used were 10, 20 and 2596 of the flying 
height. Mr. Y. Ozaki, Chief Photogrammetrist in Japan, however suggested to take also 
1Z 
the 3rd order of —— into account. He has shown me a map made with the Stereotope in 
70 
which height differences of 3096 were no exceptions. 
Another publication of Dr. Jerie: Stereogerüte 3. Ordnung in der S.U. gave details 
about the Russian instruments. Some of them have complete correction according to the 
hyperbolic paraboloid. Planimetric corrections are not applied. The Russian mapping 
procedure makes that unnecessary. 
Since the beginning of this year a Stereometer Drobyschew (STD-2) is at the 
LT.C., so I have some experience with it. The instrument corrects for the terms x2, xy, 
%, y, * Ap and y Ap. It is normally used in such a way that part of the elements of