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REMARKS ON THE PENNINGTON PROPOSAL FOR A STANDARD 
TEST OF STEREOSCOPIC PLOTTING INSTRUMENTS 
by W. SCHERMERHORN, Delft, Holland. 
As aerial photogrammetry is more and more being applied to map production 
the demand for all kinds of plotting instruments has increased accordingly. Official 
and private organizations have not only to decide on a certain plotting method, 
but also on the instrument most suitable for their purposes. In view of the high 
costs of the instruments, attempts have been made to compare the various types of 
instruments; therefore literature is often consulted for indications as to their 
accuracy. Sometimes the future buyer is disappointed, as it proves impossible to 
make an objective comparison of the various accuracy indications, owing to the 
fact that the test conditions vary. 
It stands to reason, however, that attempts are made to compare the instru- 
ments on the ground of the accuracy of the results obtained. Especially during its 
period of development, i.e. till 1940, photogrammetry had to compete with terres- 
trial survey. The surveyor doubted whether the accuracy would be sufficient for 
his purposes: manufacturers and photogrammetrists tried to dispel his doubts by 
submitting accuracy indications. Therefore the accuracy always played an impor- 
tant role. Of course the evaluation of the accuracy of the whole method nearly 
always coincided with that of the instruments. 
Since then this attitude has not changed much and the question which of two 
instruments has the greater accuracy is still the main point. It is still believed that 
this accuracy can be objectively expressed by one or two figures applicable to all 
kinds of aerial surveys. 
If this assumption were correct, some very simple instructions might be drawn 
up for a final testing method. Application of this method to all kinds of instru- 
ments would provide a possibility of comparison. As shown below, the problem 
of comparing instruments is not quite so simple; even the accuracy, however 
important, only forms part of a total comparison. 
The second problem — which only arises after an instrument has been taken 
into use — is the regular control of the adjustment and of the optical-mechanical 
quality of the instrument. Actually these are two problems. Each manufacturer of 
(nstruments indicates a method of adjustment, although some differences in the 
instruments do not always entail a different method of adjustment. It is inappro- 
priate and also undesirable to prescribe the same adjustment method for all in- 
struments. It should be left to the manufacturer to indicate those adjustment 
instructions which he considers best. 
Furthermore there is the problem of loss of accuracy in the course of time. 
Although there are instruments which even after 100,000 working hours still 
produce reliable results (thus when the instrument is carefully handled the loss of 
serious) there is no doubt that after some years their 
accuracy decreases. Besides, it is well known that certain differences in accuracy 
exist between instruments of the same type. It is gratifying that the fine-mechanical 
industry has succeeded in improving the precision of its instruments. Consequently 
accuracy can never become 
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