Short note: Planning and taking aerial photographs

Aerial photography is taken by an aerial camera fixed in an aircraft. Good planning decides cost, accuracy and coverage.

Planning (flight planning)

1. Purpose and scale: the scale of the photographs (and so the map) depends on the use. Photo scale $S = f/H$, which fixes the flying height $H = f/S$ for the camera focal length $f$.
2. Area and boundaries: the project area is marked on an index map and flight lines are drawn parallel to the longer side to reduce the number of turns.
3. Overlap: forward (end) overlap of about 60 % for stereoscopic vision and side overlap of about 25–30 % between adjacent strips.
4. Photo coverage and number: ground coverage of one photo $=$ photo size $\times$ scale denominator; the air base $B = (1-\text{end lap})\times$ ground coverage; the number of photos is computed from the strip length and strip count.
5. Ground control: control points are planned and marked (signalised) before the flight.
6. Season and time: clear sky, little cloud and haze, the sun about 30–40° above the horizon to avoid long shadows, and a season with minimum foliage if the ground is to be seen.

Taking the photographs

• The aircraft flies along straight parallel lines at constant height and speed with the camera axis vertical (tilt less than 3°).
• The camera is operated by an intervalometer so that exposures give the required overlap.
• Crab and drift are corrected by rotating the camera; the exposure, film speed and filters are adjusted for the light.
• The strips are flown, the film is processed, and the photos are indexed and checked for gaps, tilt and quality.