EMPIRE AERIAL SURVEY

Otago Daily Times, Issue 23806, 12 May 1939, Page 4

EMPIRE AERIAL SURVEY

TASK FOR CIVIL AVIATION WORK WILL LAST TWENTY YEARS (From Our Own Correspondent) LONDON. Apl. 14. An important extension of aerial survey and map-making throughout the 13,900,000 square miles of the British Empire is foreshadowed in the purchase of two specially equipped biplanes which will be used for survey flying in Britain during the northern summer and will supplement later a fleet of four survey craft employed in Afncs This new enterprise oi a private concern—the Aircraft Operating Company— follows close upon the recommendation of a departmental committee under the chairmanship of Lord Davidson that an air survey unit be

formed in Great Britain. Other and far-reaching plans affecting a great part of the Empire, only a tiny fraction of which is adequately mapped, are being made.

In the years since the war much has been done, notably in Canada and India, to secure through the medium of aerial photography information on which small-scale maps and statistics needed in regional development and government might be prepared As recently as 1935, however, only about 1 per cent, of the Empire was covered by first-class mapr on a scale of 1-25,000 or larger. The rest of the world showed a yet smaller proportion. The supply of this immense deficiency is a major task for civil aviation during the next 20 years. In England air survey may be usefully applied to local development, such as town planning and road laying. In Australia, Africa, Canada. India, South America, and elsewhere air survey is used advantageously in geological study, including prospecting for oil and minerals; for rail, road, and watershed development for archaeological exploration; for forestry; for the important science of ecology (study of soil and vegetation); and for many other purposes. ORDNANCE SURVEY CO-OPERA-TION The speed of aerial photography and map-making may, 1 . example, enable the exten' of floods to be gauge! almost before they have reached their climax. Country afflicted by forest fires may be accurately mapped, and the damage estimated in time to adopt speedy recovery measures. In England during the past ten ,r ears more than 2,000,000 houses and 4000 miles of new roads have rendered the majority of Ordnance Survey maps ob-

solete, so that the aeroplane can help enormously In mapping, the new Britain.

In the Irrawaddy delta accurate air photography of 1400 square miles of flat terrain composed largely of dense forast, mangrove swamps, soft and deep mud, and other features to which ground surveyors heartily object was completed in five months, as compared with the Burmese Government’s official estimate of three to four years for the same work by ground methods.

Sequence of operations in production of an air map calls for high skill from pilot and air photographer, and for an aircraft of unimpeachable stability of flight. Areas to be plotted are covered by a series of photographs which, jointly, make up a “ pictorial mosaic.” Photographs are taken with a longitudinal overlap of 6 ■ per cent, to ensure full coverage and sufficient prints for stereoscopic examination Special electrically-operated cameras are used. Their, normal charge is between 100 and 200 films. The readings of an interna instrument panel are automatically photographed on the film by an arrangement of lights and small lenses inside the camera each time a picture is taken. This identifies the time at which the photograph was taken, the height, the fore and aft and, lateral trim of the aircraft, and the number of the exposure. The camera is set to take pictures at regular time intervals, the overlap beine calculated in terms of the time taken by the aeroplane to fly a given distance ovei the ground. For example, a camera with 7in lens, carried in an aeroplane moving at 90 miles an hour over the ground, needs one photograph every 45 seconds from a height of 15,000 feet to give the requisite overlap of 60 per cent. *