Effect of Photographic Scale, Focal Length of the Camera, and Flight Altitude on Aerial Photograph (Especially for GATE-Geospatial 2022)

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Selecting Scale of Representation and Size of Objects That to be Detected

  • The choice of the photograph՚s scale depends on the scale of representation (the scale of the plane that is going to be represented) and the size of the objects that are supposed to be detected; this is the first problem that needs to be solved.
  • The relation between the photograph՚s scale (Mb = 1/mb) and the scale of the plane (Mk = 1/mk) that is supposed to be obtained through photogrammetric methods, comes from the abacus in the figure.

The Relation between the Photograph՚s Scale and the Cartography՚s Scale

The Photograph՚s Scale and the Cartographys Scale

Such photograph ′ s scale limits the detection of the objects ′ size. Considering that the limit for visual perception is 0.2mm if we observe through image magnification appliances, the limit becomes 0.02mm, so that each scale would give us a different minimum size of the objects.

Minimum Size of an Element with a Restitution Element

The Minimum Size of an Element Detected with a Restitution Element
1: 5,0001: 10,0001: 20,0001: 30,000
0.10 m0.20 m0.40 m0.60 m

Effect of Focal Length of the Camera

The focal length is an inherent fact of the camera that is used in the flight. Such magnitude is a calibrated datum that corresponds to the distance from the optical centre of the lens to the focal plane where the image is captured. The flight altitude is determined by the two previous magnitudes

Effect of Flight Altitude

As shown in below figure, such height (H) is the average height above the ground, which has nothing to do with the flight altitude above sea level H0 (altimeter data) .

Flight Altitude Above Sea Level

Flight altitude above sea level

Basic Principle of Photogrammetry

The basic principle of photogrammetry is the radial motion that a point undergoes in the reference frame due to its altitude.

Referance Level

The Motion of a Point Due to the Relief

Considering the above figure, the height of the point above sea level can be calculated as follows:

Height Above Sea Level

Height of the point above the reference level is:

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