Photogrammetry and Remote Sensing
This domain tests aerial and satellite imaging, LiDAR, and derived mapping products. Expect questions on photo scale, flying height, relief displacement, orthophotos, ground control, and accuracy standards.
Core rules
- Photo scale equals focal length divided by flying height above ground: S = f/H. A 6-in focal length at 6,000 ft AGL gives 1:12,000.
- Relief displacement is d = r h / H, where r is the radial distance from nadir, h is object height, and H is flying height. Elevated objects displace radially away from nadir.
- Standard aerial photography uses about 60 percent forward overlap and 20 to 30 percent sidelap for stereo coverage.
- An orthophoto is differentially rectified to remove relief displacement and tilt, but building lean and hidden facades can remain.
- LiDAR produces point clouds; bare-earth filtering yields a DEM, and first-return data can produce a DSM.
- Ground sample distance is not the same as accuracy. Check-point RMSE against independent control is the accuracy measure, following ASPRS standards.
Worked example
A camera has a 6-in focal length (0.5 ft) and flies at 6,000 ft AGL. Scale = 0.5 / 6,000 = 1:12,000. A 100-ft tower appears 3.0 in from nadir on the photo. Relief displacement = 3.0 x 100 / 6,000 = 0.05 in. The top of the tower is displaced 0.05 in radially outward from nadir.
Traps
- Mixing height above ground with elevation above sea level.
- Treating GSD as a guarantee of map accuracy.
- Displacing elevated objects toward nadir instead of away.
- Assuming an orthophoto removes all building lean.
- Using check points that were also used in the adjustment.