The Basis and Types of Arial Photography (Vertical, High and Low Oblique) (Especially for GATE-Geospatial 2022)

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The potentialities of aerial photography as a tool in geography cannot be fully appreciated if the geographer fails to grasp the basic concepts of photogrammetry. it is not possible to give a full treatment of photogrammetry in this chapter, but attention is drawn to the more important topics.

By definition, photogrammetry is ‘the science or art of obtaining reliable measurements by means of photography’ ! This term first appeared in a German paper published in 1839 by Meydenbauer. The principles of photogrammetry are applicable to ground photographs (known as terrestrial photogrammetry) as well as aerial photographs (known as aerial photogrammetry) . For geographical applications, as in topographic mapping, aerial photographs are usually preferred.

Types of Aerial Photography

There are basically two types of aerial photography: vertical and oblique.

  • Vertical aerial photography is taken with the optical axis of the camera held in a vertical position. However, an unintentional and unavoidable inclination of the optical axis from the vertical usually occurs, thus producing a tilted photograph. Today, most commercial survey companies can reduce the tilt to less than 2° from the vertical.
  • Oblique aerial photography is taken with the optical axis of the camera intentionally inclined to the vertical. There are two types of obliques, depending on how large the angular inclination is from the vertical.
    • In high-angle obliques, the camera inclines at a much larger angle from the vertical. Thus, it points only slightly downwards and the photograph will normally contain the apparent horizon of the earth.
    • In low-angle obliques, the camera inclines only at a small angle from the vertical. Thus the camera points steeply downwards and the photograph will not include the apparent horizon at all.

Despite this classification, all aerial photographs are perspective or central projections formed as a result of the light rays passing through a fixed point - the camera lens. This gives rise to scale change throughout the resultant photograph. The oblique photographs emphasise this perspective effect more clearly than the vertical photographs, with distinctive scale changes from the front to the back and from one side to the other.

Types of Aerial Photography

It is possible to combine vertical and oblique views together in one shot by using two or more cameras in synchronisation. Notable examples include the Trimetrogon photography of the United States Air Force, which utilised three Metrogon wide-angle cameras (f = 152mm) in one assembly with the central camera pointing vertically downwards and the other two cameras pointing to the left and right of the flight line at a depression angle of about 300.2 the nine-lens camera of the US Coast and Geodetic Survey with the central lens (f = 209.3mm) pointing vertically downwards to be surrounded by the eight other lenses pointing at an angle, of 38° away from the vertical; and the split vertical photography of the British Air. The force which made use of one or more pairs of cameras fitted on the aircraft to point slightly outwards from the vertical) These multiple camera installations have been designed specifically for military reconnaissance purposes and are therefore not particularly favoured in mapping because of the more complicated geometry involved. However, geographers may find these systems useful for their wide coverage and for macro-scale applications where only qualitative data are required.

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