JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III REMOTE SENSING AND GIS Previous Question Paper (August - 2022 - R16)

Aerial photography relies on the principles of photogrammetry, which involves obtaining reliable measurements from photographs. The core principles include:

• Perspective Projection: Unlike maps which use orthographic projection, aerial photographs are central projections where all light rays pass through a single point, the camera lens (perspective center).
• Scale Variation: Due to terrain relief and camera tilt, the scale of an aerial photograph is not constant across the entire image.
• Relief Displacement: Objects above or below the datum plane appear to lean radially outward or inward from the principal point, which is a function of the object's height and its distance from the nadir.
• Stereoscopic Vision: By capturing overlapping images (typically 60% forward overlap), the principle of binocular vision is used to perceive depth and create a 3D model of the terrain.
• Geometric Fidelity: The relationship between the image coordinates and ground coordinates is governed by the collinearity condition, where the perspective center, the image point, and the ground point lie on a single straight line.

Height measurement using relief displacement is based on the geometric principle that the image of a vertical object is displaced radially from the nadir point. The procedure is as follows:

1. Identify the Nadir Point: Locate the nadir point (the point on the ground directly beneath the camera lens) on the photograph. In a vertical photograph, this is approximately the principal point.
2. Measure Displacement: Measure the radial distance (d) from the nadir point to the top of the object and the radial distance (r) from the nadir point to the base of the object. The relief displacement (Δr) is the difference between these two measurements.
3. Apply the Formula: Use the relationship between the displacement, the flying height, and the object height. The formula is:
   h = (Δr × H) / r
   where h is the height of the object, Δr is the relief displacement, H is the flying height above the base of the object, and r is the radial distance from the nadir to the top of the object.
4. Calculate Flying Height: Ensure the flying height (H) is known relative to the base of the object. If the altitude above mean sea level is known, subtract the elevation of the object's base.
5. Refine Results: For high-precision requirements, account for camera tilt and atmospheric refraction, though for standard vertical photography, the radial displacement method provides a reliable estimate of object height.