JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year II Semester REMOTE SENSING AND GIS Previous Question Paper (November/December - 2020 - R16)

The Principle of Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring, and interpreting photographic images. At its core, it relies on the geometry of perspective projection.

The fundamental principle is based on the collinearity condition, which states that the perspective center of the camera lens, the object point on the ground, and its corresponding image point on the photographic film or sensor must all lie on a single straight line. By capturing overlapping images from different positions (stereoscopic coverage), photogrammetry allows for the reconstruction of 3D spatial coordinates (X, Y, Z) of terrain features.

Aerial photographs are primarily classified based on the orientation of the camera axis at the moment of exposure:

• Vertical Photographs: These are taken with the camera axis held as nearly vertical as possible. They are the standard for mapping and GIS applications because they provide a relatively uniform scale and are easier to interpret geometrically.
• Tilted Photographs: If the camera axis deviates slightly from the vertical (usually by less than 3 degrees), the image is considered tilted. While often unintentional, they require mathematical rectification to be used for precise measurements.
• Oblique Photographs: These are taken with the camera axis intentionally inclined at an angle. They are further divided into:
  • Low Oblique: The horizon is not visible. These provide a better view of building facades and terrain relief.
  • High Oblique: The horizon is visible in the photograph. These cover a larger ground area and are useful for reconnaissance and visual orientation.

The Scale of an aerial photograph is defined as the ratio of the distance between two points on the photograph to the corresponding distance between the same two points on the ground. For a vertical aerial photograph, the scale is primarily determined by the focal length of the camera lens and the flying height of the aircraft above the terrain.

Where:

• S is the photo scale.
• f is the focal length of the camera.
• H is the flying height above mean sea level.
• h is the elevation of the terrain above mean sea level.

In areas of varying relief, the scale varies across the photograph because different points on the ground are at different elevations relative to the camera. If the terrain is flat (h=0), the scale is constant and expressed as S = f / H.

Height measurement on a single vertical photograph is performed using the principle of Relief Displacement. Relief displacement is the shift or position change of an image point from the nadir point caused by the object's elevation above a reference datum. The displacement (d) of a point is directly proportional to its height (h) and its radial distance (r) from the principal point, and inversely proportional to the flying height (H) above the base of the object.

Where:

• h is the height of the object.
• d is the relief displacement measured on the photograph (the distance between the top and bottom of the object).
• H is the flying height above the base of the object.
• r is the radial distance from the principal point to the image of the top of the object.

Note: This method assumes the photograph is truly vertical and the principal point is accurately identified. For precise height determination, stereoscopic parallax measurements are typically preferred over single-photo relief displacement.