Experiments by the Ordnance Survey in the Use of Air Photographs (Especially for GATE-Geospatial 2022)

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Experimental surveys in England in 1925 - 6 and 1928 - 9 tested the applications of a simple method of plotting from vertical photographs. From this has been evolved the Arundel Method, very largely due to the work of Major M. Flotine, R. E. From the military point of view, it was found that plans at six inches to one mile and contours at three inches to one mile could be produced in country where the variations of height arc not greater than ten per cent of the flying height. Ground control points were required every five miles or so, and spot heights at about six per stereoscopic overlap. While the preparation of a plan at this scale was satisfactory, the accuracy of contouring was not so certain. The military desideratum of contoured plans for artillery action on enemy areas could not be satisfied where the ground control was non-existent. Efforts were made to produce reliable contours, but the recent tendency has been to depend more and more upon instruments of precision to bridge over non-controlled areas.

At this time imperfections of flying with regard to tilt and indeterminate height made the application of this simple form of air survey for large scales somewhat doubtful. The Ordnance Survey was actively interested, particularly about revision of 1/2,500 plans which had dropped sadly behind owing to depleted personnel and funds.

The First Experiments

The first experiments were made in 1925, and many difficulties were met. The first Report of the Ordnance Survey on Air Survey (1927) makes very interesting reading in view of present conditions. It is stated:

“Delays and wasted flights were caused by defects in the sights, films and other equipment, and were in no way due to the contractors, (The Aircraft Operating Company Ltd.) through the extra expense caused thereby had unfortunately to be borne by them. The first films were almost ruined by static markings …”

In comparing times taken for the ground revision for 1/2,500 plans and that required when the photographs were available, the Report goes on to say: “The above figures show that by the use of aerial photographs the field work was reduced by about forty-nine per cent. It might even have been further reduced if the revisers had been able to get away from their old habit of visiting all the ground. It was only after each man had gone over several Field Traces that he was convinced that the Photographs had given information of all the existing improvements.”

It was found as a result of this experiment that the air method cost forty-five per cent more than the ground method. The area covered was near Eastbourne and included a general variety of country and detail.

It was concluded in this report that aerial photographic methods already had advantages over ground methods for towns in fiat country and areas such as estuaries or tidal flats and it was decided to initiate a series of experimental revisions from air photographs.

The Second Report

The second report in 1930 states:

“The experiment shows that the use of air photographs as a reconnaissance is uneconomical in open downland and in suburban areas of rapid growth. It is thought that this will be true also of all other classes of property, except closely built town areas. The use of air photographs as a form of reconnaissance will tend to increase the cost of revision work, compared with methods now in use … In order to maintain our present standard of revision, the whole of the ground must be visited by the reviser.”

The report of the Davidson Committee on the Ordnance Survey issued in October 1938, shows that conditions have changed somewhat since 1930. While it is still true that in sparsely developed areas it is cheaper to continue with ground methods, it is pointed out that in areas where there has been much development it may be both economical and useful to employ air photography to expedite revision of the 1/2,500 plan. It is also stated that in the proposed plan series, all plotted on a standard national projection, air photographs should be of value in the consequent “overhaul” of plans.

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