Photogrammetry, Remote Sensing, and Mapping
What this domain tests
Expect scale computations, overlap and flight planning, orthophoto and LiDAR products, and accuracy standards for mapping deliverables.
Core rules
- Photo scale = focal length ÷ flying height above ground. A 6-inch (0.5 ft) lens at 3,000 ft gives 0.5/3,000 = 1:6,000, so 1 inch on the photo equals 500 ft on the ground.
- Standard aerial photography uses roughly 60% endlap and 25–30% sidelap to support stereoscopic compilation.
- Digital GSD = (pixel size × flying height) ÷ focal length. Keep units consistent.
- DSM includes vegetation and structures; DEM/DTM represents bare earth. LiDAR classification determines which product you can deliver.
- Horizontal and vertical accuracy are reported as RMSE; the 95% confidence value is about 1.73 × RMSE for horizontal and 1.96 × RMSE for vertical (ASPRS conventions).
- UAS mapping requires compliance with FAA Part 107 and adequate ground control or RTK/PPK.
Worked example
A 5.2-micron sensor with a 50-mm lens flies at 200 m above ground. GSD = (5.2 × 10⁻⁶ m × 200 m)/0.05 m = 0.0208 m, about 2.1 cm. Reasoning: convert everything to metres first, and remember the height is above ground, not above the datum; using ellipsoid height inflates GSD over high terrain.
Traps that cost marks
- Using height above sea level instead of above ground.
- Assuming an orthophoto has no lean on tall structures.
- Treating a DSM as a bare-earth surface.
- Quoting RMSE as if it were a 95% value.
- Flying without adequate ground control and calling the result survey-grade.