Stereo and Non-Stereo Imagery
Stereoscopic imagery is the result of overlap, which is the amount by which one photograph includes an area covered by a neighbouring photograph. Air photo coverage is generally designed to provide about 60 percent forward overlap between photographs. This allows stereoscopic, or 3D, viewing when the two overlapping photos are used with a stereoscope. In addition, from 20 to 40 percent lateral (side) overlap is allowed when complete coverage of an area is required. For mapping, inventory and vegetation studies, for example, a survey is flown in a series of to-and-from parallel strips with side overlaps between strips over the entire area.

For non-stereoscopic coverage, used in crop sampling or pollution detection, the photographer may choose a 20 percent forward overlap.

Scale and Aerial Photography
A photographic scale is basically an expression that states that one unit (any unit) of distance on a photograph is a representation of a specific number of units of actual ground distance. Scale is expressed three ways:

• Unit Equivalent
• Representative Fraction
• Ratio

A photographic scale of 1 millimetre on the photograph represents 25 metres on the ground would be expressed as follows:

• Unit Equivalent - 1 mm = 25 m
• Representative Fraction - 1/25 000
• Ratio - 1:25 000

Two terms that are normally mentioned when discussing scale are:

Large Scale - A large scale photo simply means that ground features are at a larger, more detailed size. The area of ground coverage that is seen on the photo is less than at smaller scales.

Small Scale - A small scale photo simply means that ground features are at a smaller, less detailed size. The area of ground coverage that is seen on the photo is greater than at larger scales.

The National Air Photo Library has a variety of photographic scales available, such as 1/3 000 (large scale) of selected areas, and 1/50 000 (small scale).


By The National Air Photo Library of Natural Resources Canada