Photogrammetry and Remote Sensing
Geometry of aerial photography
For a vertical photograph the scale is S = f / H, where f is the focal length and H the flying height above the ground (or above mean terrain), giving a representative fraction such as 1:25 000. Flying height above the datum must be reduced by the terrain elevation first. Relief displacement is d = r h / H, where r is the radial distance from the principal point and h the object's height; displacement is zero at the nadir (principal point of a truly vertical photo). Overlap is normally 60 percent endlap and 25 to 30 percent sidelap to permit stereo viewing; the base-height ratio governs height precision.
Orientation and products
Interior orientation uses fiducial marks and camera calibration (principal distance, fiducial coordinates); exterior orientation gives the six elements of each exposure. Aerotriangulation densifies control; digital photogrammetry produces orthophotos, DEM, DSM and DTM. In remote sensing, know the four resolutions: spatial (ground sample distance), spectral, radiometric and temporal, plus platforms, sensors (multispectral, SAR, LiDAR) and interpretation elements such as tone, texture, pattern, shape and association. NDVI contrasts near-infrared and red bands to indicate vegetation vigour.
Traps
Using flying height above datum in the scale formula, treating the principal point and nadir as identical under tilt, expecting relief displacement at the photo centre, and confounding DEM with DSM.
Worked example. A camera with f = 152 mm flies at 3 800 m above the ground. Scale = 152 mm / (3 800 m x 1 000 mm/m) = 1/25 000, written 1:25 000.