The DP-1 Double Projector, a stereoplotter of Il b order 
by Dr.-Ing. M. AHREND and Dipl.-Ing. G. DREYER, Oberkochen 
1. General remarks 
The principle of objective optical projection in its application to a photogrammetric stereoplotter is 
extremely attractive due to the simplicity of the resulting design. The number of instruments developed 
on this principle is relatively large. The table on page 17 — which does not make any claim to com- 
pleteness — may therefore be of interest as a survey. 
Some of the instruments listed in the table have been continuously improved over the years. Thus 
the original Kelsh Plotter is now designated as the Model K5, the Nistri Photo-Multiplex as DIII and 
the Nistri Photocartograph as the Model VI. Further instruments would have had to be included 
if the table were also to cover types leaving room for an extension of the design principle, e. g. by 
optically movable elements (as, for example, in the case of Ferber Gallus). Finally, many rather interest- 
ing remarks regarding design details, for instance attached projection mirrors, illumination systems 
composed of lenses, centrical or eccentric ellipsoidal mirrors, type of projection carrier or material of 
plotting table, etc., had to be omitted. 
The question of who was the first to develop such instruments has been the subject of many 
discussions in the course of the past decades. Today it is relatively difficult to comment on this 
question. It may be said, though, that SCHEIMPFLUG did try out the principle of double projection 
with make-shift optical equipment [1] and that GAssER developed a first special instrument in the form 
of the Gasser Projector [2]. It seems equally certain that the Zeiss Multiplex by BAUERSFELD and 
all subsequent models developed from it as well as the American Kelsh Plotter with all the versions 
derived from it are or were the photogrammetric plotters most widely used throughout the world. 
The following advantages and drawbacks apply to simple double projectors: 
Advantages: Drawbacks: 
Simple principle Different plotting cameras required 
Limited instrument outlay for different focal lengths 
Considerable ruggedness Limited depth of field 
Great clarity of operating Operation in darkroom. 
principle 
It was above all the disadvantages of this design principle which were widely discussed over several 
decades. However, in all discussions it was generally overlooked that a good instrument is primarily a 
good compromise between the different advantages and drawbacks and far less an ideal — that is 
costly — solution. Consequently, it has not been fully recognized that the double projectors which were 
able to conquer the market were those which in all operational details were as simple as the original 
principle had promised. And in this connection the required "limitation" not only refers to the speci- 
fications of the basic instrument, but just as much to the accessories and auxiliary equipment required, 
e. g. the use of simple filter spectacles instead of extensive stereoscope systems. 
In view of these considerations, the Zeiss Works at Oberkochen decided to resume the production 
of double projectors in order to satisfy the requirement for a general and inexpensive plotter suited for 
map compilation, map revision and instruction. 
In this connection, extremely helpful suggestions were received from Prof. Dipl.-Ing. G. Krauss 
of Bad Godesberg who was also very cooperative in finding a suitable solution, among other things 
by experiments with the old Aerosimplex and testing a prototype DP-1 as well as specially designed 
“Zoom Stereopret" [5]. Besides the authors, above all Mr. Josef SEEH had a major part in the design 
of the final DP-1. 
16