Aerial survey

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Historical background

The first person to take an aerial photograph was Nadar, who used the wet-plate process from a captive balloon, in 1858. The first recorded still photographs to be taken from an aeroplane were not made until 1909. During the First World War, many developments took place with air cameras and mapping, but the maps were crude and it was not until 1924 that the first aerial survey took place. Using 12.7 × 10.2 cm (5 × 4 in) plates, a DH9A of RAF 84 Squadron made a survey at the request of the Anglo-Persian Oil Company of its Abadan refinery. At about this time the British Air Survey Company (f. 1923), from 1929 a subsidiary of Fairey Aviation, made important surveys in the Middle and Far East. In the same period, assisted by the French air force, the aerial archaeologist Antoine Poidebard was making aerial surveys of Roman remains in the Syrian desert.

Aerial survey subsumes two major areas, aviation and photogrammetry. The chief difference between aerial photography and aerial survey is that whereas the former can include obliques and photographic reconnaissance, aerial survey is a very precise operation involving photogrammetry.

Photogrammetry

Photogrammetry involves measuring x, y and z of a terrain from an image, as required for the production of planimetric and topographic maps. Mapping cameras usually require to be calibrated every two years and before a mapping contract can be signed. Vertical photography is used for topographic mapping. But although the layman assumes that the vertical photograph is the same as its map counterpart, the map differs from the vertical photograph in a number of important ways. Not least of these is the geodetic accuracy of the map when compared to the photograph. A photograph is a perspective view, in contrast to the strictly orthogonal view of a map. The survey camera is of a very large format, 23 cm (9 1/10 in) square, and weighs upwards of 100 kg (220 lb). The two major manufacturers are Zeiss with the RMK A series plus the latest RMK-TOP, and Leica with the Wild RC-30. The camera is operated from its own mounting and sophisticated navigation sight, and is expensive (c. £150, 000-400, 000).

Camera calibration is usually done by the manufacturer and consists of finding the exact focal length of the lens (f) to two decimal places, now known as the principal distance or p.d., and at least four fiducial marks calibrated to an accuracy of Sum or better, from which the principal point can be found by intersection. The calibration certificate will also include information concerning tangential as well as radial distortions of the lens.

Today, softcopy (or digital) photogrammetry is the route to a faster and more economic solution to map-making. The film with its 60 per cent forward overlap of images, once processed and dried, is scanned into a computer at a scan pixel size of about 15 μm. The Zeiss PHODIS SC photogrammetric scanning system, for example, has a basic scanner integrated with an image-processing system using the same computer platform. Positive or negative representations, full colour, and all data such as tilts and scale, aerotriangulation, and fiducial coordinates allow for the generation of digital terrain models (DTMs) and orthophotos. In the 21st century there has been growing interest in the Zeiss/Intergraph Digital Modular Camera (DMC) and the Leica-Helava Airborne Digital Sensor (ADS 40). These cameras do not use film, therefore eliminating the need for scanning, and feed directly to the computer; their cost is upwards of c. £1 million.

Survey photography is a highly disciplined procedure. Flying straight and level means that the aircraft should be well trimmed and on course. The effects of drift (due to wind) must be noted, and the camera on its drift mount compensated accordingly. The aircraft's ground speed must be accurately known. These requirements are easily satisfied with the Global Positioning System (GPS) used by modern survey operators.

Weather is the greatest problem. Survey photography requires clear air, not only cloud-free but clear air free of haze and pollution. On average there are only fifteen days per year of such conditions in northern Europe. At 2, 400 m (8, 000 ft) and above, haze can obscure fine ground detail, and the mission will therefore be wasted.

Satellite imagery

Images from satellites such as LANDSAT have been available since 1973. The various types of satellite use scanners as their sensors and these are either multi spectral scanners (MSS), or thematic mappers (TM). The sensors send their images down for subsequent processing and today deliver very good quality. They are not, however, of air camera standard, and are mainly used for geological, meteorological, and earth resources interpretation. Although satellite images can be used for making maps, they are not as accurate as air camera maps, but are good enough for military usage at 1:25, 000 scale, and for large-area mapping of deserts etc. Satellites are invaluable for the mapping of inaccessible terrain or, by the use of radar, through total cloud coverage.

— Ron Graham

Bibliography

• Graham, R. W., and Read, R. E., Manual of Aerial Photography (1986).
• Warner, W. S., Graham, R. W., and Read, R. E., Small Format Aerial Photography (1996).
• Etacci, C., Introduction to the Physics and Technology of Aerial Reconnaissance and Survey (1997).
• Collier, P., ‘The Impact on Topographic Mapping of Developments in Land and Air Survey: 1900-1939’, Cartography and Geographic Information Science, 29 (2002)

i. The use of aerial cameras and/or other photogrammetric instruments for making maps and charts.
ii. The visual survey of the ground or an objective from the air, usually to assess damage during natural or human-made calamities.

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