What is meant by the scale of the aerial photograph?

Simply defined, scale can be expressed as the ratio or proportion between a distance on the aerial photographs (or maps) and the actual distance on the ground or land surface. It is a ratio that could represent any unit of measurement and this ratio is called the Representative Fraction (RF). A scale usually is given as a fraction or a ratio (i.e., 1/10,000 or 1:10,000) and this "representative fraction" scale means that 1 unit of measurement on the map—1 inch or 1 centimeter—represents 10,000 of the same units on the ground. A scale of 1:40,000 means 1 inch on the photograph equals 40,000 inches on the ground, or 1 centimeter equals 40,000 centimeters on the ground.

If the scale were 1:63,360 then 1 inch on the aerial photograph (or map) would represent 63,360 inches, or 1 mile, on the ground (63,360 inches divided by 12 inches equals 5,280 feet, or 1 mile). The first number (map distance) is always 1. The second number (ground distance) is different for each scale; the larger the second number is, the smaller the scale of the map. "The larger the number, the smaller the scale" sounds confusing, but it is easy to understand. So, a small scale map such as 1:1,000,000 shows a large area, but shows it in less detail. A large scale map such as 1:24,000 only shows a small area, but it shows it in great detail. 

Please use the Scale field to help search for the photos that fit your needs. Scales of 1:500 to 1:24,000 are best for identifying buildings. Scales of 1:20,000 to 1:80,000 work well for agriculture and geologic interpretation. Scales in excess of 1:150,000 provide regional assessment information.

Learn more:

USGS Map Scales

USGS Help Information (Map Scale)