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What is the difference between scale, and medium and high-resolution, when used in describing aerial photography?

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 mapâ€”1 inch or 1 centimeterâ€”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.

Resolution refers to the ability to distinguish the smallest visible objects on a photograph. Resolution is a result of the combination of film type and the camera lens system.

Medium-resolution aerial photographs are available at approximately 400 dots per inch (63.5 microns). The average file size for a 9-inch black and white (B/W) aerial photo is 15 MB and a color photo is 45 MB.

The standard high-resolution aerial photographs are available at 1,000 dots per inch (25 microns). The average file size for a 9-inch B/W photo is 120 MB and a color photo is 360 MB.

On-demand scanning at 1,800 dots per inch (14 microns) is also available for aerial film sources that are held in the USGS EROS film archive for a service fee of \$30 per frame.

USGS Map Scales

USGS Help Information (Map Scale)

National Aerial Photography Program

National High Altitude Photography Program

Earth Resources Observation and Science Center (EROS) - Aerial Photography

Maps, Imagery, and Publications

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