apparent. Also to be noted is a slight curvature of the lines, with the vertical 
lines showing a tendency to a reverse curve. 
No manufacturer of precise photogrammetr c plotting instruments has yet 
claimed a precision of better than 7 microns in the plane of the photograph, and 
most operators are well pleased if they can obtain 10 microns. Therefore it is 
apparent that for all practical purposes, the grid plates which are now produced 
(at least these two samples) are adequate for testing and adjustment of instru- 
ments. Since for the most part only comparative measurements are used in 
adjustment, the minor discrepancies in scale have no effect. About the only place 
where absolute values enter is in the determination of focal lengths, and in this 
computation the Z scale of the instrument must also be taken into account. At the 
same time that we observe that present grid plates are adequate, it is obvious that 
the gap between instrument and test plate precision is rapidly being narrowed, 
so there is no reason for complacency on the subject. 
One further recommendation arising from this study is that, if possible, the 
thickness of the grid lines be considerably reduced. The floating mark (in the 
A-5) has a diameter of 40 microns, which is completely covered by the grid lines. 
Lines of half the present width would improve the precision of pointing. 
The author is grateful to Mr. Pieter de With of the Geodetic Institute at Delft, 
who made the observations, and to Mr. Mohammed Ebrahimi, of the International 
Training Centre for Aerial Survey in Delft, who assisted in the calculations.