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Aviation History
1955
1955 - 1476.PDF
586 FLIGHT MODERN AIR SURVEY TECHNIQUE Points from the Commonwealth Survey Officers* Conference THE papers submitted to the 1955 Commonwealth SurveyOfficers' Conference, held recently at Cambridge, were ofa highly specialized nature, those on air survey subjects dealing more with surveying techniques and with the resultsyielded thereby than with the Hying aspect. Indeed, the aeroplane was a notable absentee at "the otherwise comprehensive exhibitionof survey instruments displayed for the conference—partly, no doubt, for purely practical reasons and partly because many of theaircraft used in civilian survey work are adaptations of tamiliar military types; the Lockheed P-38 Lightning, D.H. Mosquito,and B-17 Hying Fortress are examples that come to mind. Tne C-47 Dakota has also been widely used for this purpose, and itwas of particular interest to read in a paper entitled An Aerial Survey Operation Overseas, by W. P. Smith (Air Survey Co., Ltd.)that a Dakota was used for high-altitude photographic survey in the Zambesi River valley last year. The operating height was25,000ft and the aircraft was fitted with Pratt and Whitney R-1830-90C engines.A point that emerged in the discussions at the conference is the fact that aircraft are playing an essential part in mappingoperations in roles other than the principal one of carrying the surveying camera; in addition to transporting men and equip-ment over the long distances involved in the routine work of Colonial surveying, aircraft of both fixed- and rotary-wing typesare being increasingly used for flying surveyors directly to the points on high ground from which the basic visual triangulationobservations have to be made. The helicopter is already making a major contribution in this and similar roles, and one which willfurther increase when performance under tropical conditions and at high altitudes is improved. The papers submitted to the conference were necessarilyesoteric in their appeal, but there emerged from them, and from the ensuing discussions, two points of wider interest; first, the wayin which different countries have adapted the techniques of navi- gation and survey photography to their own conditions andtechnical resources and, secondly, the progress that has already been made, and will evidently continue, in the use of electronicdevices for navigating the survey aircraft and for securing the actual survey data.Both these points were well brought out in a paper—Modern Survey Methods Adapted to Canadian Conditions—presentedby Mr. W. H. Miller, Director of the Surveys and Mapping Branch of the Department of Mines and Technical Surveys,Dominion of Canada, and prepared by Mr. E. J. White, Topo- graphic Survey, and Lt.-Col. J. I. Thompson, Army SurveyEstablishment. The first part of this paper dealt with a new photographictechnique developed in Canada and known as the bi-camera method. In this technique, the aircraft carries two cameras, onepointed vertically downward in the conventional manner and one mounted obliquely and directed in the line of flight with itsaxis depressed approximately 30 deg from the horizontal. The angle of depression of the oblique camera is such that the photo-graph will include both the horizon and a small portion of the area covered by the vertical camera. The effect of this arrange-ment, coupled with the process of photogrammetric analysis, is that the pictures of the terrain ahead, taken by the forward-lookingcamera and later photographed again by the vertical camera, markedly reduce certain errors which accrue when surveymeasurements are carried out along a strip of photographs. Use of Shoran The second part of the paper was devoted to the use of theShoran secondary-radar position-fixing system in Canada since 1948. Shoran consists essentially of an airborne transmitterwhich triggers-off two (or more) ground beacon stations, receiving and display equipment in the aircraft enabling the distances fromit to the beacons to be precisely determined. By flying along the right bisector between two stations, the system can be used tomeasure the inter-station distance; the measurement of lines in this way, averaging about 200 miles in length, is the basis oftrilateration surveying (as opposed to triangulation), and the technique has been used over a large area of Canada with greatsuccess. Most of the mainland of Northern Canada has been covered by Shoran trilateration nets, over 90 stations havingbeen established. Shoran is also used as a position-fixing system for the aircraft,whereby the aerial photographs taken on a survey can be assigned their correct positions on the earth's surface. Since 1948 Shoranhas been extensively used in Canada for the control of air photo- graphy, first by the R.C.A.F., and later by contract to civilianfirms. About 55,000 square miles were covered in the first contract, and in 1953 the same contractor made the most of good photographic weather and of the lessons learned from the pre-vious year, covering over 132,000 square miles. The 1954 season was equally successful, and in three seasons a lattice of Shoran-controlled photography spaced at about 20-mile intervals covered 80 map-sheets of 1/250,000 scale—an area of 330,000 squaremiles. The Canadian paper also referred to the increasing use of radio altimeters for "vertical control" in air survey. Electronic aids to surveying have come into being only in thedecade since the war, and the recent Survey Officers' Conference was the first of a series held at roughly four-yearly intervals toinclude a paper devoted entirely to this subject. (Radio Aids to Surveying, by Mr. C. Powell, of the Decca Navigator Co., Ltd.)After a review of current radio and radar position-fixing systems, in which it was shown that the wavelength on which the systemoperates determines within close limits its operational scope, the author described the Decca Navigator system in detail, withspecial reference to its current use in various types of survey operation. On the air side, an interesting example was the con-trol of the accurate navigation required for large-scale air photo- graphy over the U.K. for map revision by Ordnance Survey. Theuse of Decca had been specified for this work for some years past and was made necessary by the close spacing of the parallelphotographic tracks and the high accuracy of tracking re- quired (e.g., plus or minus 20 yards at the large photographicscales used). The Decca Navigator, being a long-wave system, could be usedby low-flying aircraft which would be unable to receive the trans- missions of systems based on radar techniques. For this reason,Decca lent itself for use in geophysical survey operations and for the navigation of helicopters and other relatively low-flying air-craft flying on survey operations over featureless country. Com- paratively little experience had so far been obtained on the useof the system for the control of high-altitude photo-survey air- craft, but it showed considerable promise in this direction, par-ticularly in view of the light weight of the airborne equipment. Trials were at present in progress. Survey in Africa Recent and current aerial survey work on the African con-tinent was described in a paper (Aerial Photography in Nigeria, by Mr. H. A. Stamers Smith, Director of Federal Surveys). Thefirst aerial photography of Nigeria of any magnitude was by the trimetrogon process, using three airborne cameras, carried out bythe United States Army Air Force during World War 2, and covering some 250,000 square miles. After various abortiveattempts under extremely bad weather conditions, the Royal Air Force started in September 1949 to cover further large areas of thecountry and by 1951 had mapped a total of about 150,000 square miles.Radar tracking had been used in much of this work, and a lesson learned was that an aerial survey in a country of variable andcapricious weather should be planned from the outset to cover a large area, tackling it in sections as weather permitted, rather thanspecifying a relatively small region in which photography might be impossible for long periods.Concurrently with the photography by the R.A.F., the Survey Department of Nigeria chartered a de Havilland Dove from WestAfrican Airways Corporation for departmental aerial photo- graphy, and this work was continuing up to the present time. Thetotal photographic coverage by the Survey Department had been about 124,600 square miles. The aircraft was fitted with aWilliamson survey sight by which drift was measured and the necessary course correction made, and an order had been placedfor a navigating sight designed specifically for aerial photography, also manufactured by Williamson. Despite the stability of theDove visual tracking was a considerable problem. The papers referred to have been concerned with the means ofaerial surveying. It is perhaps appropriate to conclude this brief review of a few of the conference papers by mentioning one ofthose that dealt more with the end-product of the survey. The paper, entitled Aerial Photography and Resources Surveys, wasby Mr. T. D. Weatherhead, director and general manager, Hunt- ing Aerosurveys, Ltd., and Mr. V. C. Robertson, manager,Resources Surveys Department, Hunting Aerosurveys, Ltd. The principal product of an air survey is a topographical map, andthis, die authors pointed out, was the essential basis of all develop- ment work in the area concerned. Equally important, however,was the preparation of maps recording land use, soils, forestry and geology, these being grouped under the general heading of"resources surveys." Aerial photography played just as vital a part in the production of a resources survey as in the making of atopographic map, and the paper included examples of this type of survey operation. •fe
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