Basics of Air Photo Scale
The basic geometry of air photo consists of two similar triangles. Key to this geometry is the fact that photos (just like images captured by our eyes) have a perspective view. That is 'the world is seen from a single point ... the lens'. Note that we can use the notion of similar triangles to determine scale in 2 ways:
Photo Distance / Ground Distance (PD / GD)
or
Focal length / flying height (f / H)
Remember that:
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Focal length (f) is often provided as the first 2 characters of a photo number |
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Flying height (H) is height of the aircraft above the ground (not sea level) |
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Altitude is height of air craft above sea level (usually given in feet) |
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Thus flying height = altitude - elevation |
Handy conversions:
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1 foot has 12 inches |
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1 inch = 2.54 cm |
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1 m = 100 cm = 1,000 mm |
Open the Photo Geometry pdf to see a sketch (Figure 1) that depicts the similar triangles. Triangle a-b-L is a mini version of A-B-L. In this sketch, a-b-L is 40% the size of A-B-L. This means that a-b is 40% of A-B and that f is 40% of H. Of course with real air photos the reduction is far greater than 40%.
Scroll down for Figure 2. This sketch shows how photo scale varies when ground elevation varies. Stated more precisely ... photo scale varies as flying height (H) varies. It is an inverse relationship so that as H gets smaller (i.e. the camera is closer to the ground) the photo scale gets larger.