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Appendix to No.
THE VALUE AND USE OF AIR-PHOTOGRAPHS.
To survey from air-photographs, to extract the three dimensions of nature from the two of the photograph; requires study, training and skill. At present this Survey Department is in no position to undertake it. On the other hand to revise existing maps of flat ground, such as the paddy field areas of the 32 and 16-inch to the mile cadastrals, or as the flattish parts of the 1/20,000 map, is simple and very economical. For both these purposes photographs should be used if they can be obtained.
2. The 16 and 32-inch cadastral plans are ideal for air-photography because :---
(a) No marking on the ground is required.
(b) The field divisions show up well.
(c) The areas are flat.
(d) A very high order of precision is not required.
The following steps should be taken :—
(e) In ordering photographs postulate single prints, never a mosaic. Overlap in line of flight 60 per cent., at right angles 20 per cent. Prints as nearly as possible at scale of map in question (although the contact scale need be only one-third of this.) Straight and level flying.
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(f) The cadastral sheets should be connected to the survey by traverse or trig., and then the sheet itself carefully gridded in the Victoria Peak co-ordinate system (say 500 feet squares). For procedure see " Survey Computation. (9) The co-ordinates of field corners can now be measured graphically and the
scale of the photograph determined.
(h) Put each pair, consecutively, in the stereoscope, and avoid surveying on slopes, i.e., determine the areas on which perspective holds true and use them only.
(i) Use the simple perspective methods given in the 1929 manual of map read- ing and field sketching, and in the first professional paper of the Air Survey Committee.
A model of the Barr and Stroud topographical stereoscope should be ordered, and another should be made locally on the lines of the makeshift stereoscope illustrated in the manual of map reading and field sketching.
3. In revising the topo. maps remember that the hillslopes here are very steep. Scale will be much distorted on them and, at present, no effort should be made to use the photographs except in flat, or nearly flat, country. If topo. maps are revised in this way send duplicate photographs to the War Office.
4. Send some surveyor when on home leave to the War Office or to the Ordnance Survey for a month's attachment to learn air-photographic surveying.
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