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amply proves that the remarkably economical technique of aerial mosaics is an answer to many 
requirements of the practice. The advantage that the entire information content of the aerial photo 
graph is available in the aerial mosaic is apparently more important in this case than the drawback 
of small departures from true map position. For these uses, the possibility of producing a rectified 
print within a few minutes is obviously more interesting than the advantages of precise stereophoto- 
grammetric mapping of the same area, which may require a few days work. 
Under this aspect, two photogrammetric techniques and their end products are thus facing each 
other : 
1. the aerial mosaic, and 
2. stereoplotting. 
It is only logical that attempts should be made to develop a third method which would combine 
the advantages of the two conventional techniques. The result should be 
3. a photomap, 
which should combine the wealth of details contained in the aerial photo with the horizontal-position 
accuracy of the map. 
The techniques for the production of a photomap are based on the process of transforming the 
perspective view of the aerial photograph into an image corresponding to parallel projection. For 
practical reasons, this transformation is made by “rétification in strips”, for which the ground 
elevation of each point along the resultant “profiles” must be known. These ground eUvations should 
best be determined in a sterephotogrammetric plotter. The accuracy of a transformation derived in 
this manner increases as the width of the strips is reduced. 
Before discussing related research work, the result of which - the GZ 1 Orthoprojector - is suited 
to add the production of photomaps (or “orthophotograms”) as a valuable new technique to the 
present methods of rectification and streeoplotting, a few remarks are indicated on the status of 
technology. 
2. Prior solutions 
Leaving aside Scheimpflug’s “Zonal Transformer”, the idea of building instruments with which 
it would be possible to prepare such ortophotograms was probably conceived first by Lacmann [1]. 
Ferber’s publication [2] from the same period is another example of the duplicity so often found 
in technology in the conception of ideas for which the ground is prepared. 
The prototype instruments built according to Ferber’s and Lacmann’s suggestions apparently 
did not come up to general expectations. This may be one of the reasons why it was not until 25 years 
later that these ideas were taken up again or even re-invented independently (Bean, Orthophoto 
scope, 1955 [3]). 
Ever since, work on this project has been carried on. Additional research was undertaken primarily 
in the United States and the USSR (where the instrument is called “slit rectifier”). 
The following Table is intended to summarize these developments, as far as they have come to 
the kowledge of the authors. The references at the end of the paper have intentionally been limited 
to such publications which report about instruments actually manufactured.