Photogrammetry and Remote Sensing
Scale and flying height
The core relationships are simple and heavily tested. Photographic scale is focal length divided by flying height above ground: S = f/H, and for a photograph, S = photo distance / ground distance. If you need the flying height, H = f/S. Always confirm whether the height given is above mean ground, above the datum, or above sea level - using the wrong reference is the most common single error in these items.
Relief displacement, parallax, and stereo
The displacement of a point due to its height above the datum is d = r·h/H, where r is the radial distance from the principal point and h is the object height. Displacement is radial from the nadir/principal point and vanishes directly below the camera. Stereoscopic viewing uses parallax differences to recover elevations; the base-height ratio (B/H) governs vertical accuracy - a larger base gives better heights. Endlap (typically about 60%) and sidelap (typically about 30%) ensure continuous stereo coverage.
Products and platforms
Expect questions distinguishing orthophotos (displacement removed, measurable) from plain vertical photos, and DSM from DTM/DEM. LiDAR delivers dense elevation points and can penetrate light vegetation; satellite imagery is limited by GSD, revisit, and cloud cover. Ground control and aerial triangulation tie imagery to the datum.
Worked example
A camera with a 6-in (0.5 ft) focal length flying 5,000 ft above ground produces S = 0.5/5,000 = 1:10,000. On that photo, 1 in represents 10,000 in, or about 833 ft on the ground.