Overlap in Areal Photos and Flight Lines: Exposure Stations (Especially for GATE-Geospatial 2022)

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  • Airphotos are taken so that the images overlap by approximately 60 % along flight lines (overlap) and 20 % to 30 % between flight lines (side-lap) . The exposure station is the position of the front nodal point of the lens at the instant of exposure.
  • The distance between the exposure stations of two successive images is called the air base and is equal to the ground distance between the principal points of the two images.
  • The overlap between images is essential to allow three-dimensional viewing of airphotos, but it is also needed to determine the direction of the flight line and to allow construction of mosaics that contain little distortion. This requires using only the central portion of each image since distortion increases towards the edge of the image due to perspective effects.
Approximately 60 % Along Flight Lines
  • While it might be expected that the flight line would coincide with the X-axis of the image, this is rarely the case. The flight line would only coincide with the X-axis if there was no wind and the plane was able to fly straight along the flight line. Under windy conditions, the pilot must compensate by flying slightly into the wind to stay on course. This process is known as crabbing and affects the area of overlap between images.
  • The flight line can always be determined by plotting the principal point of one airphoto on the area of overlap with the adjacent image. This point is known as the conjugate principal point. In the example below, the first image has a tree at its principal point and the second image has a building. The location of the same building on the first image identifies the conjugate principal point. The flight line is given by connecting the principal point and the conjugate principal point with a straight line.
Illustration 2 for Overlap in Areal Photos and Fli …

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