THE COST OF THE WORK.

Evening Post, Volume CI, Issue 15, 18 January 1921, Page 2

THE COST OF THE WORK.

In reckoning costs, it is assumed that a . Survey Department is a Government office, that aeroplanes are, or will shortly be, owned by every Government, and hence that the cost of purchase, equipment, personnel, and other items, need not be charged ~to the Survey Department. This being accepted, the only cost of making a particular flight is that incurred by payment of petrol, oil, insurance, and depreciation. Two hundred square milos can easily be photographed ill two flights of two hours, plus the time necessary to reach a height o£ 10,000 to 15,000 feet, and to get from anil to the. aerodrome to and from the country to bo mapped. £10 per hour more than covers the above-mentioned costs; then for 200 square miles, say, five shours, or £50, will cover the actual cost of flying. This gives 5s per square mile; add the cost of the negatives, developing, printing, and attendant processes at two negatives per square mile—say, 5s to 6s per square mile —and one has obtained most of the detail of a map at a cost of 11s per square mile. To measure the detail for a map on a scale of 1-5000 accurately on the ground would cost from £5 per square mile of open country to £1100 per square mile of intricate city. Even if the Survey Department bought its own aeroplanes and paid its own pilots, in many 'countries there would still be a saving, since the most suitable plane for this class of work, is a slowflying machine somewhat on the lines of the pre-war 35-h.p. Caudron, which would cost £500. For' accuracy, all maps must be.based on trigono'metKcally fixed points and measured traverses. The cost of this framework will be the same, or less for air photo maps. It has been found much cheaper to reproduce the photo retouched by photogravure than to draw the map and print in the usual v:ay. A photographic map of Damascus City done in October, 1919, on a scale of 1-5000, took thirty-one working days to complete, including the taking of the photographs, trigonometrical work, enlarging, toiiching-up, and reducing. The total cost was about £50 (estimated as its share of a larger map of 200 square miles, which was not completed, owing to the political situation). Three Italian engineers took three years to produce a map on the same scale, with muchness detail, at a. probable cost of well over £6000. WORK FOR THE R.A.F.

It is to the advantage of the Air Force to undertake as much of tliis class 01 work as possible. In the first place, itgives their personnel, who are young, keen, and only fao anxious for some useful work, a. practical and definite object in flying with certain conditions to fulfil. After they have been of such enormous practical value in the war, it is sometimes depressing to them to have only a, few aimless, practice flights. Secondly, the art of photographing .country requires training, and in war time has proved invaluable. The opportunity ot practising should not be given over to an entirely civil department. The Air Force, too", must keep its photographic plates ar.d material ready for war purposes. These require renewing every few months, especially in hot climates. Hence let them be used on some definite practical plan of value to the State. The negatives, which can be developed by the Air Force or by commercial photographers, or preferably by the Survey Department, should be kept filed and indexed by the latter, because they must always keep a permanent staff at a pwi manent place, and are in a position to make us© of and publish photographs. So valuable can air photographs become to a variety of scientists that a.t least the bast selection should be preserved in a. Government library, and the photograph. published. Accuracy, assuming the country to be flat, by this process depends mainly on .the fixing of the enlargement to the trigonometrical points. Distortion .tad. other optical inaccuracies can bo dealt with, provided certain rules are followed by the airmen and photographer. There is far greater accuracy of small details; the chances of personal error are greatly reduced, both with tli© surveyor,' the plotter, and the draughtsman, who tends to draw straight linos. H<» would draw a face like an Egyptian hieroglyphic. Oil the other hand, cadastral surveyors contended that measurements taken from a photo maf> are not sufficiently accurate for legal purposes, owing to boundaries being less well defined, a matter for further experiment and proof. A good photograph can always be enlarged, even ben times its original scale, arid' be accurate : a drawn map cannot be enlarged. This fact alone gives a great advantage to the former. The speed of the method, requires no demonstration. In an hour's flight' almost 100 square miles can be photographed, gaining detail tha.t would take weeks or .months to collect otherwise. The- trigonometrical work takes the same time iji both methods., and the reproduction is quicker by photography. Tt has been proved that accurate maps can be ma<le cheaply from 1-2000 to 1-60.000 scales, provided tha country to ha mapped is flat. If it" is uneven or hilly no satisfactory method of trotting absolute accuracy has yet been demonstrated, but it certainly caii be done by overlapping photos and the stereoscope. Experiments for mapping in war time in an enemy's country, when one cannot fix points on the ground, for reconnaissance of large areas on small-acaJo work (bearing in mind the cost), in determining the most practical instruments to use, camera lenses. enlarsinjr auuar.i.tua, etc.. and in various other matters, must be studied further.