(409) 
As far as control extension is concerned, I believe that for small scale 
mapping the slotted template system is adequate for the planimetry, and that 
the ease with which aneroid spot heights can be obtained in the field with mo- 
dern instruments, obviates the necessity for height control extensions longer 
than can adequately be dealt with by less expensive plotting instruments. 
The use of the slotted template system in conjunction with multiplex for 
small scale work has further advantages which will be dealt with Jater in this 
paper. 
For larger scale mapping however, whilst it is often cheapest to break down 
the initial trigonometric framework by ground survey methods, it is realised 
that this is by no means always the case, and there is a big field for precision 
photogrammetric control extension in relation to work at the larger scales. 
5. An outstanding development of precision photogrammetry in Great Britain. 
The Ordnance Survey of Great Britain were quick to appreciate the possi- 
bilities of photogrammetry for their specialised large scale mapping, and exper- 
ments were started about 1925. The provision of minor control networks, and 
the surveying of large scale detail by ground methods is particularly expensive 
and difficult in Great Britain on account of the enclosed nature of the country 
and the need to consult a large number of small landowners. This gave greater 
urgency to the search for a suitable alternative. 
In 1935, as a result of previous experimental work, E. H. Thompson, and 
the Cambridge Instrument Company designed and produced a stereocomparator 
to meet the special requirements of the Ordnance Survey for the provision of a 
close network of air triangulated points. 
Experiments with the Cambridge Stereocomparator were completely inter- 
rupted by the war but since 1945 the work has been resumed and has resulted in 
a technique which has been very successful and is in my opinion the greatest 
advance that has been made in photogrammetry in Great Britain for a long 
time, and likely to have big repercussions on other types of survey as well as the 
very large scale. 
The observing technique in the comparator has been greatly simplified by 
the use of a centimetre reseau engraved on the pressure plate of the camera so 
that it appears on each photograph. Points lying on reseau intersections are used 
for the control extension. 
This means that the co-ordinates of these points on one of the photographs 
are the same as that of the corresponding point on the reseau, and that it is only 
necessary to measure the co-ordinates of the homologue of the point on the other 
photograph. Base lining or very tedious adjustment of the photographs in the 
comparator is avoided and observing time is very considerably reduced. Another 
advantage in using the reseau is that it is possible to avoid the necessity of using 
glass plates; as the reseau provides the means for accurately and automatically 
correcting for any film distortions, camera lens distortions, or lack of flatness 
of the film, without involving any extra work to enable this to be done, other 
than the initial calibration of the camera and reseau. The use of film instead of 
glass plates without any loss of precision is of great practical value. 
Consideration is now being given to the design of a special instrument for 
these measurements, much simpler and easier to use than the standard stereo- 
comparators. Simplification becomes possible because no long measurements are