Your base map has a scale of 1 inch equals 500 feet. If aerial photos were taken from a height of 3,000 feet with a 6-inch focal length, what is the scale of the map?

• A. 1" = 500'
• B. 1" = 1000'
• C. 1" = 250'
• D. 1" = 400'

Answer: (A)

Scale on an aerial photograph is set by the geometry of the camera: the ratio of focal length to flight height. Specifically, for a vertical photo, the scale equals f divided by H, when both are in the same length units.

Here the focal length is 6 inches and the flight height is 3,000 feet. Convert the height to inches: 3,000 ft × 12 in/ft = 36,000 inches. Then ground distance per inch on the photo is 36,000 inches ground / 6 inches photo = 6,000 inches ground per 1 inch on the photo, which equals 500 feet per inch (since 6,000 inches ÷ 12 = 500 feet).

So the aerial photos produce a map scale of 1 inch representing 500 feet, matching the base map scale. The other options would require different height or focal length values to produce those scales.