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