12. Photogrammetry
Photogrammetry
This domain tests aerial image geometry, stereoscopy, orientation, ground control and product accuracy. Expect scale and height computations, overlap and parallax concepts, and the role of bundle adjustment.
Core rules
- Photo scale = focal length / flying height above ground; it varies with terrain height.
- Endlap is commonly about 60% and sidelap about 20-30% for stereo coverage.
- Parallax is the apparent displacement caused by relief and perspective; it enables height measurement.
- Orientation includes interior, relative and absolute stages; ground control fixes the model to the datum.
- Bundle adjustment jointly estimates camera parameters, station positions and object coordinates.
Worked example
A camera has focal length 152 mm and flying height 3,040 m above ground. Photo scale = 0.152 m / 3,040 m = 1/20,000, written 1:20,000. A 1,000 m ground line appears 1,000/20,000 = 0.05 m = 50 mm on the negative. If the terrain rises 100 m, the local scale becomes larger because the height above ground is smaller.
Traps
Using flying height above sea level rather than above ground; assuming one scale over variable terrain; ignoring radial displacement; using too few ground control points; confusing orthophoto with true orthophoto.