The collection holds approximately 1.5 million monochrome, vertical aerial photographs. Each one is survey-standard and suitable for viewing stereoscopically in 3D. Most are at nominal scale 1:30 000 – 1:60 000 and in 230 mm by 230 mm format. Mostly panchromatic, though there are infrared, monochrome prints, off-colour, infra-red negatives, and a few colour prints.
Nearly all the photography was taken for topographic or land use mapping purposes. Sources and scales varied over the years; dating back to flights taken by United States Army Air Force and the US Navy over Caribbean and Pacific islands during the Second World War.
Almost all photography of Africa, Aden, Malaya, and Borneo flown between 1946 and 1953 was taken with Fairchild® K17 cameras, and from 1951 with Williamson F49 cameras, both using the same focal length of 152 mm. These were carried in aircraft (mainly Lancasters) of RAF photographic reconnaissance squadrons. The planned scale of photography was 1:30 000, selected as a compromise that would provide specialist departments (geology, forestry, agriculture) with photography that both enabled interpretation of thematic information, and suited topographic mapping.
In specific cases, photography was needed at a scale larger than 1:30 000. For example, the Volta River delta and dam site was taken at 1:5000 scale in 1946. Gee-H radar was used in Africa and Aden to navigate a series of concentric flight lines around a ground radar station. Visual, straight-line navigation was used occasionally in Africa, for large-scale photography of the Freetown Peninsula, Sierra Leone, and in British Somaliland for some 1:30 000 scale cover.
Contract photography was first taken in 1949 and predominates from 1953 onwards, flown by British commercial air survey firms using visual navigation methods. Over 200 contracts were let by DOS. Each usually covered the area of a proposed mapping or land use project, sometimes extending over several countries such as in the Caribbean, and at several different scales. In 1954 the scale of photography for 1:50 000 scale changed to 1:40 000. From the 1960s, with increasing use of super-wide-angle (89 mm) lenses, photo scales were often smaller, 1:50 000 or 1:60 000. Large-scale mapping areas still required larger scales, such as 1:12 500 photography for 1:2500 scale mapping blocks. Photography was sometimes flown at scales larger than 1:40 000 for specialist interpretation, such as 1:25 000 for forestry interpretation.
In addition to contract photography, the collection occasionally holds cover obtained by the national survey departments, and other aid agencies such as the Canadian International Development Agency.
RAF photography at varying scales is held, along with Royal Navy photography. Additional records exist for more than one set of cover at various dates and is available for environmental research, geomorphology, vegetation, communications, and settlement, across a 40–50-year time span.
Most of the photography used by DOS for mapping, including the RAF and contract cover, is indexed on record reference cover diagrams, generally at 1:500 000 scale. The written catalogue is a Roneodex card index, with one card per film recording:
- Camera, lens type and number
- Calibrated principal distance
- Film type
- Flying heights
- Dates of photography
A small-scale guide to this photography is included in the DOS Annual Reports from 1951 to 1984, to be held by The National Archives (TNA). Other photography in the collection is shown on non-DOS cover diagrams or sortie plots produced by the originator of the photography. Summary diagrams for each country illustrate the location of all photography. The principal points of photos used in the mapping are shown and numbered on almost all DOS 1:50 000–1:125 000 scale topographic maps and provide accurate indications of the location of individual photos. Air photo mosaics and print laydowns (uncontrolled mosaics produced as map substitutes on standard sheetlines in advance of the regular mapping) are archived at the National Collection of Aerial Photography.
Technical records include camera calibration certificates, film reports with dates and times of photography, and contract documents. The film negatives of RAF and RN photography are held by the RAF; those of contract and other cover are held by the national mapping agency concerned, except in the case of British Antarctic Territory, for which the films are held by the British Antarctic Survey, Cambridge. Some satellite imagery, Landsat and SPOT is also held.