Scale in Aerial Photography
Scale is the ratio of the distance between two points on an image to the actual distance between the same two points on the ground. Scale is an important describing factor of vertical aerial photography. It is important to know the scale of the image under examination, as this can affect how you perceive or interpret what appears in the image. Scale also allows features in the image to be measured.
Small-scale and Large-scale aerial images
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Small scale images, with a ratio of 1:25000 or 1:50000 for example, are those which cover a large area with less detail. A large scale image, around 1:3000 or 1:5000 for example, will cover a smaller area but will show ground features in more detail.
This photograph was taken in 1988 during the All Scotland Survey. It has a nominal scale of 1:24000, which means that every centimetre on the image represents 24,000 centimetres, or 240 metres on the ground.
The former docks area, now known as Glasgow Harbour, is visible in the lower-left quarter of the image.
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This photograph shows the former docks area of Glasgow at a larger scale, 1:9800. In this image, every centimetre on the image represents 9,800 centimetres, or 98 metres on the ground.
The area covered by this image represents a very small part of the lower-left corner of the 1988 photograph of Glasgow, above. The buildings and landmarks surrounding the Glasgow docks area can be seen in much more detail in this larger-scale image.
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Focal Length
Focal length is the distance between the camera lens and the film. The focal length of the lens affects the scale of the image captured. A camera with a lens of long focal length will produce images which look as if they have been taken close to the ground. Conversely a short focal length lens will produce smaller scale photographs which can look as if they have been taken at altitude.
The scale of a photograph (S) can also be expressed as the ratio between the focal length of the camera (f) and the altitude of the platform aircraft (H) in the equation S = H/f