Photogrammetry
What the exam tests
Photogrammetry items test camera geometry, scale, ground sample distance, overlap, control, aerial triangulation, and product accuracy. Expect a scale or GSD calculation.
Geometry
- Scale = focal length ÷ flying height above ground. Use height above the terrain or mean ground, not above the datum.
- GSD = (flying height above ground × pixel size) ÷ focal length.
- Forward overlap is typically about 60%; sidelap about 20–30%, increased for urban terrain.
- Parallax in a stereo pair produces elevation.
Control and products
Aerial triangulation adjusts photo measurements to ground control. Ground control points should be well distributed and photo-identifiable. An orthophoto is relief-corrected; a DSM includes vegetation and structures; a DTM represents bare ground.
Worked example
A camera has a 152 mm focal length and flies 1,500 m above ground. Scale = 0.152 m ÷ 1,500 m = 1:9,868. With a 0.010 mm pixel, GSD = (1,500 × 0.000010) ÷ 0.152 = 0.0987 m, about 9.9 cm. Doubling flying height doubles GSD and halves map scale.
Traps
- Using flying height above sea level instead of above ground.
- Mixing millimeters and meters in the scale formula.
- Treating a DSM as a bare-earth DTM.
- Assuming an orthophoto is free of building lean.
- Using too few or poorly distributed control points.