Terminology of Airphoto Geometry (Principal Point, Nadir, Isocenter) , Vertical and Tilted Air Photos, Flight Path and Overlap (Especially for GATE-Geospatial 2022)

Glide to success with Doorsteptutor material for IMO-Level-2 : get questions, notes, tests, video lectures and more- for all subjects of IMO-Level-2.

Examrace Books on Mapping, GIS, and Remote Sensing prepares you throughly for a wide range of practical applications.

Airphotos used for mapping purposes is usually vertical airphotos, although oblique airphotos are often used to aid in visual interpretation or in mapping mountainous areas. EMR reflected off the surface of the Earth is directed back toward the camera lens. The lens focuses the reflected EMR on the film in the back of the camera. The camera focal length is the distance from the front of the lens to the film. Using a normal lens, this distance is approximately 153 mm. The camera focal length and the altitude of the lens above the ground determine the scale of the Airphoto.

Because the EMR reflected off the Earth passes through the camera lens, the image formed on the film is a negative image. The rays of reflected EMR represented by the diagonal lines expose opposite sides of the film - i.e.. , the right edge of the field of view on the ground appears as the left edge of the film and vice versa. However, an imaginary positive image plane exists at a distance equal to the camera focal length in front of the lens.

Fiducial Marks

The airphoto prints also show fiducial marks located midway along the edges of the print. These are V-shaped notches that are used to locate the X and Y axes of the image. The intersection of the X and Y axes is the principal point of the airphoto. This point is the centre of the airphoto. For a vertical airphoto, this will be the point that was directly below the centre of the lens at the instant of exposure. The x-axis most nearly defines the direction of flight.

Airphoto Geometry

The y-axis nearly defines the flight line. The intersection of the fiducial marks represents the ‘’ principal point ‘’ of the photograph.

Principal Point Photograph

Three terms need defining here, they are Principal Point, Nadir and Isocenter. They are defined as follows:

  • Principal Point: The principal point is the point where the perpendicular projected through the centre of the lens intersects the photo image.
  • Nadir: The Nadir is the point vertically beneath the camera centre at the time of exposure.
  • Isocenter: The point on the photo that falls on a line half- way between the principal point and the Nadir point.

On a true vertical aerial photograph, all three of these would be at the same point. There is no such thing as a true vertical aerial photo. All air photos have some degree of tip or tilt.

Advantages of Vertical Air Photo

The basic advantages of vertical air photos are:

  1. The scale is essentially constant.
  2. 0 ′ 0 Measurements of directions are easier than on oblique photograph. Directions can also be measured more accurately.
  3. Within limits a vertical aerial photograph can be used as a map (if grids and marginal data are added) ; arid.
  4. Vertical aerial photographs are often easier to interpret than oblique and are better for stereo (there is no masking) .

Advantages of Tilted Air Photos

Given a constant altitude and camera you can cover a much larger area on a single photo;

Aerial Photography
  • The view of some objects is more familiar to the interpreter;
  • Some objects not visible on vertical photos may be seen on oblique.

High and Low Oblique Air Photos

High and low oblique airphotos differ in that the horizon is visible on a high oblique airphoto (e. g. a view of the Earth from a space shuttle) , but not on a low oblique airphoto.

Vertical Airphoto

Developed by: