4 
Perhaps it would be worth while looking at these points for a moment. There 
cannot be much doubt that the monocular measurement apparatus is cheaper, but 
perhaps not as cheap as might be expected. The system requires in all three optical 
systems instead of two and much of the expense of a rationally designed stereo- 
-comparator lies in the measurement and read-out system which is not duplicated. 
Again a flase idea of economy is introduced by the (possibly subconscious) idea 
that the marking is only preparation and is not to be compared with precise mea 
surement. Nothing could, of course, be farther from the truth for everthing de 
pends upon the marking which must be carried out as conscientiously and as 
carefully as any stereoscopic measurement. Furthermore, on reflection, it cannot 
surely be maintained that the routine is any more «tidy» for a marking operator 
than it is for a stereoscopic operator making measurements. The former has to 
identify a point on as many as six pictures lying in two different strips, taking 
five pictures and bringing them successively into stereoscopic correspondence with 
the fifth. This can be done with virtually no more difficulty in a stereocomparator 
in which any arbitrary point on one picture can be brought into stereoscopic corres 
pondence with any arbitrary point on another. And, in a stereo-comparator, once 
the correspondence has been achieved, the coordinates are as easily recorded as 
the marks made. It is, indeed, difficult to see where the economic advantage lies 
with the monocular measurement: rather the contrary. Of course it is a great 
nuisance to have to break the routine observation of a strip to deal with pictures 
in adjacent strips, but the nuisance arises, not from having to measure pictures 
in adjacent strips but from having to set up stereoscopic correspondences with 
pictures in adjacent strips and that is common to both methods. 
There remains the problem of accuracies. Monocular measurement cannot be more 
accurate than binocular measurement for a given precision in the measuring equip 
ment, for it cannot be more accurate than the placing of the marks and these 
depend, at the very least, upon binocular acuity. But, of course, there are prima 
facie reasons for considering that it will be less accurate. The errors of the mark 
ing device are additional errors as is the pointing to the mark in the monocular 
measuring instrument. Even, however, if these can be reduced to negligible 
amounts there are pschycological reasons why the human errors may be greater 
in routine work. This is even anticipated by the way in which monocular methods 
are described. The stereoscopic process is «only marking» not precise measure 
ment which comes later. But if the marking is not done with the attention and 
concentration of a first class observer the horse will be out of the stable when we 
come to shut the door. And can we seriously expect a routine operator who is 
not responsible for the measures themselves to pay the same attention? Of course 
we can check gross errors, but who has ever thought that geodetic observations 
could be checked for quality by a second operator looking through the telescope 
to see if the signal was bisected properly? 
Everyone will agree that a binocular measurement to a pair of unmarked photo 
graphic points is the most accurate measurement that can be made unless the 
photographic point is a pre-marked ground point identifiable unequivocally on 
both pictures. What is sometimes overlooked is the more elementary fact that