Air Photo Scale

The scale of an air photo varies according to the focal length of the camera used and the height above the ground from where the picture was taken.

Scales for most air photos in this course will vary from 1:10 000 to 1:60 000 (i.e. large scale).

An example of a small scale air photo would be a weather satellite picture.

Calculating Air Photo Scale - Method B

During the course, you will not be given the focal length of the camera nor the height of the aircraft. Consequently, you must incorporate a different method. This method requires an accompanying topographic map.

When this question is given on the government exam, it is presented as a multiple choice question. Moreover, you are not allowed to use a calculator on the government exam. Therefore, this method allows you to simply approximate the scale of the air photo.

eg View the air photo of St. John's and the topographic map of St. John's .

The scale of the aerial photo is approximately
a) 1:10 000 b) 1:25 000 c) 1:50 000 d) 1:100 000 ?

Step 1: Ask yourself, on the air photo have you zoomed in or zoomed out in comparison to the topographic map?

If you have zoomed in, you are looking at a smaller area in more detail. As a result, the scale number for the air photo will be smaller than the one for the topographic map.

If you have zoomed out, you are looking at a larger area in less detail. Therefore, the scale number for the air photo will be larger than the one for the topographic map.

If you cannot clearly tell if you have zoomed in or out, do not panic, simply move onto Step 2.

With the St. John's air photo, you have clearly zoomed in. Consequently, the answer must be less than 1:50 000 which is the scale of the topographic map. Thus, you can automatically eliminate choices c and d. Step 2 will allow to determine whether the answer is a (1:10 000) or b (1:25 000).

Step 2: Pick two common points that you can easily recognize and measure between on both the topographic map and the air photo. Wherever possible, pick two points on the topographic map that are at least 3 cm apart. Measure the distance (in cm) between these two points on both the air photo and topographic map.

St. John's Topographic Map:
The Distance Between South Head (636689) and Spriggs Point (639668) is 4.4cm.

St. John's Air Photo:
The Distance Between South Head and Spriggs Point is 8.5 cm.

Step 3: Approximate the factor difference between the two distances by dividing the larger distance by the smaller one.

8.5cm / 4.4 cm = Approximately 2.

Step 4: If you have zoomed in on the air photo, divide the topographic map scale by the factor difference.

If you have zoomed out on the air photo, then multiply the topographic map scale by the factor difference.

(Note: If you have a factor difference very close to one, then the air photo has the same scale as the topographic map.)

St. John's Topographic Map = 1:50 000
Zoomed In on Air Photo so divide by the factor difference of 2.
Therefore, 50 000 / 2 = 25 000.

The scale of the St. John's air photo is 1:25 000 (b).