When the photographs carry the reseau, coordinate measurement may be 
referred to a near reseau cross, in which case it becomes a practical advantage to 
provide for the automatic identification of the reseau reference cross. It is there- 
fore that when the floating (or measuring) mark is set on a reseau cross, an electric 
circuit would record a pair of two-figure numbers corresponding to the x and y 
coordinates of the observed reseau cross. 
In this way, the observational procedure reduces to two steps: 
(1). Setting the floating (or measuring) mark on the point of which the coor- 
dinates are required; and, 
(2). pressing a button. 
This last action by the observer starts a train of operations which record the 
readings of the appropriate micrometers to three decimal places, and the numbers 
identifying the reseau cross if this happens to be the observed point. 
Let us now consider the form in which these data are to be recorded. It is 
important to realize that in the method of automatic recording alluded to above, 
a special electric circuit is established for each figure of a number. What this 
circuit is made to do is irrelevant to the design of the comparator. The circuit may 
for instance operate the key of a type-writing machine but it can be equally well 
made to punch a card or perforate a tape for later use in an electronic or punch-card 
machine. The choice of the form in which the information is recorded can therefore 
be made to suit the available means of computing. The suggested method of record- 
ing would serve them all and equally well. It seems however that whichever method 
of calculation is employed, it is desirable to record the data in a form suitable for 
inspection before they are handed over to the computing section. The storing 
register mentioned in connection with the computing equipment can clearly be 
used in an attachment to the stereocomparator to average, type and code the data 
at the same time. 
A stereocomparator serving these purposes is unlikely to be less expensive 
than current types, but I think it would turn out much more work per man hour. 
The saving will therefore come as a result of improved efficiency even if the 
initial cost of each unit is not reduced. 
References. 
(1) Shewell, H. A. L.; Proceedings of the Commonwealth Survey Officers Con- 
ference, London, March 1951. 
(2) Thompson, E. H.; The Journal of the Royal Institution of Chartered Sur- 
veyors, Vol. XXX, No. 81, July 1951. 
(3) Wassef, A. M.; Ph. D. Thesis, University of London, March 1950. 
(4) — ; Empire Survey Review, Vol XI, No. 84, April 1952. 
(5) — ; Patent Applications, 1949. 
Acknowledgement. 
My thanks are due to my wife for helpful suggestions in the course of develop- 
ing the ideas alluded to in this paper.