Map scale is ordinarily interpreted as the ratio of a map distance to the corresponding distance on the ground. In similar manner, the scale of a photograph is the ratio of a distance on the photo to that same distance on the ground. On a map, scale is everywhere uniform because a map is an orthographic projection. An aerial photograph, on the other hand is a perspective projection and, as will be demonstrated herein, its scale varies with variations in terrain elevation.
Scales may be expressed as unit equivalents, dimensionless representative fractions, or dimensionless ratios. If, for example, 1 in on a map or photo represents 1000 ft (12000 in) on the ground, the scale expressed in the aforementioned three ways is
Unit equivalents : 1 inch = 1000 ft
Dimensionless representative fraction : 1/12000
Dimensionless ratio : 1:12000
It is helpful to remember that a large number in a scale expression denotes a small scale, and vice versa; e.g. 1 inch = 100 ft is a larger scale than 1 inch = 1000 ft.
Scale can be calculated in the following three ways
(a) Average scale of photography = f/h where
F = focal length of camera
H=(H-havg) where H is the flying height of camera above MSL (Mean Sea Level), havg is the average height of ground.
(b) By comparing the distance between two objects on photograph and the corresponding distance of the objects on the map.
Scale of photography = map distance X map scale factor / photo distance
By comparing the distance measured on ground between the objects to the corresponding photo distance
For example :
100000 units —— 1
10 units ——- ?
10/100000 = 1/10000