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Commission II Invited paper 
Classification of Photogrammetric Instruments 
by W. SCHERMERHORN 
I.T.C., Delft. 
We all know the usual expressions of first, second and third-order plotting instru- 
ments. This classification suggests that it is based on a difference in precision between 
various types of instruments. This short introduction to a discussion on the London Con- 
gress is to show that such a classification already now is not justified. The following 
considerations show the difficulties of the usual classification. 
The so-called first-order plotting machines are such as: 
Nistri Photostereographo Model Beta II 
Poivilliers SOM Stereotopographe Type B 
Santoni Stereocartografo IV 
Wild Stereo-Autographs A7 and A9 
Zeiss Stereoplanigraph C 8. 
These are all instruments in which aerial triangulation can be carried out without 
the necessity to move the diapositive, common to two successive models, from one pro- 
jector to the other. This method can be applied to all instruments where we have a pos- 
sibility of observation with base inside and outside and of offering the right image to 
the left eye and reverse. Therefore all these instruments are considered as typical aerial 
triangulation instruments. In many services these instruments are purchased in parti- 
cular for aerial triangulation in addition to other types, which are used especially for 
plotting. 
Nevertheless, this method of characterizing the instruments is not entirely satis- 
factory since also several other instruments can be used for aerial triangulation and in 
some cases aerial triangulations are carried out on so-called first-order plotters with a 
constant position of the base, which means without making use of the possibility of meas- 
uring with base inside and outside. Therefore I propose to replace for these instruments 
the expression “first order” by that of “universal restitution machines”. In the past there 
has been some controversial discussion between the manufacturers of instruments with 
the system of optical projection, such like the Stereoplanigraph, and those with mechan- 
ical projection, like the systems of Wild and Santoni, regarding the expression universal 
plotting instruments. We believe, however, that, since the instruments with mechanical 
projection are provided with devices for the observation of convergent photography, there 
cannot be any longer a great objection to the proposal to use the expression “universal 
» 
plotting instruments” for all instruments mentioned above. 
The second category in the list are instruments like: 
Galileo-Santoni Stereosimplex III 
Thompson Watts Plotter 
Drobyshew Stereograph 
Romanowsky Stereoprojektor SPR/2 
Wild Stereo-Autograph A8. 
Although I do not know from personal experience the precision of the Russian in- 
truments, according to a statement of M. D. Konshin in an article in No. 11 of 1957 in 
the Russian Review Geodezija i Kartografia these instruments show a precision of 
0.19/,, of the flying height and are considered by him to belong to the group of instru- 
ments with first-class precision. 
The difference with the universal plotting machines lies in general not in the preci-