(411) 
any international yardstick, or any unquestionably best technique to fit all 
circumstances. 
In this country we claim to have very considerably reduced sources of error 
by the Ordnance Survey technique already briefly described but we are by no 
means content with this. The biggest advance we should like to see is a very 
considerable increase in the resolution and detail depicting powers of aerial 
photographs. At the moment continental countries appear to be in advance of 
us in this connection, but research continues in this country. Improvement in 
the depiction of detail is at present very much more important than the 
reduction of perspective distortions. After all the distortion of the perspective 
can be measured by calibration and corrected for (corrected, incidentally, much 
more easily and accurately by the Ordnance Survey reseau technique than in 
any plotting machine) but if the detail is not adequately depicted, this is a 
disadvantage which cannot be overcome. The cost of any air survey is very 
largely determined by the limits of detail depiction, and in my opinion this 
is the most pressing item towards which research should be directed. 
I am inclined to feel that at the moment we have reached somewhat of an 
impasse in this matter due to the assessment of photographic resolution in terms 
of lines per millimetre on a standard contrast test chart. I doubt if there is suffi- 
cient correlation between what we want — good detail depiction — and resolu- 
tion measured in this manner. There is also too much effort directed towards 
keeping the lens distortions small solely to serve the requirements of plotting 
machines. 
I would suggest that the most pressing item for research is to obtain better 
detail depiction by improving the emulsions and the optics quite regardless of 
the distortions; then if the distortions are intolerable for some aspects of photo- 
grammetry let us see if we cannot modify our methods to enable us adequately 
to compensate for them. In any case detail depiction should take first priority. 
7. Work at the Directorate of Colonial Surveys. 
I should like to say something more about small scale mapping, and how 
we are tackling this problem in the British Colonial Empire. 
We have a central Organisation known as the Directorate of Colonial 
Surveys charged with the Geodetic and Topographic surveys of the Colonial 
Empire, to which each of the Colonial Governments can, if it wishes, farm out 
its topographical mapping. This leaves individual Colonial Survey departments 
free to concentrate on other aspects of survey, such as the cadastral. 
Such is the pressure of Colonial development that the maps are usually 
required in a great hurry. As usual, the mapping of the relief takes much longer 
than the mapping of the planimetry so in order to get some type of map out as 
quickly as possible it is frequently necessary to publish first a provisional edition 
showing only the planimetry, and to follow this up later with a second edition 
showing both planimetry and contours. A method must thus be chosen which 
does not allow the contouring to delay the planimetry. 
In undeveloped countries, where the detail is in any case quite sparse, the 
planimetry can be mapped most rapidly by radial intersection methods on a 
framework supplied by slotted template adjustment controlled on existing, new, 
or supplemented triangulation. The compilation scale for this work is equal to 
the mean scale of the photographs, generally 1:30,000; the compilation being