Guides Scale in Aerial Images

Scale in Aerial Images

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. It is an important factor when describing and understanding vertical aerial photography. Knowing the scale of the photo under examination, can affect how you perceive or interpret what appears in the image. Scale also allows features in the image to be measured.

A small-scale colour aerial photo of Glasgow. Grids of streets are visible across the image, interspersed with green parks to the left, and with extensive green spaces in the top right corner. A river runs along the bottom edge.

Small-scale and large-scale aerial images

Scale in aerial photography is presented as a ratio, like 1:24000 or 1:50000, in the same way that it appears on maps. If an image like the one above has a scale of 1:24000, this means that 1 centimetre in the image represents 24,000 centimetres (or 240 metres) on the ground. This is a small-scale photo showing central and west Glasgow, and beyond - it covers a large area of ground but doesn't show a lot of detail.

A medium-scale colour aerial photo of Glasgow. The river from the previous image now occupies the centre, running diagonally from top left to bottom right. There are industrial buildings on the riverbank, with houses and parks further away.

This image shows a portion of the same area as the first photo, roughly equal to the lower left quarter. Now all that is visible is the University of Glasgow at the top right, across the docks to Ibrox Stadium at the bottom left. That is because it is a larger-scale image at a ratio of 1:10000. Although the area covered is smaller, the amount of detail that is visible is greater.

A large-scale black and white aerial photo of Glasgow. The river from the previous images now occupies almost the whole frame. At the left side, ships are clearly visible docked next to the river. There are blocks of flats in the bottom left corner.

This final image is at an even bigger scale of 1:3000. While 1 centimetre was the equivalent of 240 metres on the ground in the first image, here 1 centimetre represents just 30 metres on the ground. It only covers a small area of the docks visible in the previous two images, but in great detail. At this scale, cars and even people are clearly visible.

Focal Length

The 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 long focal length lens 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.

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