Air Photo TypesAir Photo Scale
To identify the two types of air photos. To learn how to estimate the scale
of an air photo when paired with a topographic map.
Let's look at the two categories of air photos and learn how to estimate the scale of an air photo.
Types of Air Photos
1) Oblique: the skyline / horizon is visible.
2) Vertical: the photo is taken at a 90 degree angle to the ground.
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 A
S = F / H
S = scale of
eg If an aircraft was flying at an altitude of 2500 m and the focal length of the camera was 15 cm, what would be the air photo scale?
S = 15cm /
S = 0.15m /
S = 1m / 16
1:16 666 is the scale of the air photo.
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.
The scale of the aerial photo
Step 1: Ask
yourself, on the air photo have you zoomed in or zoomed out in
comparison to 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 Air
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
The scale of the St. John's air photo is 1:25 000 (b).
1) Utilizing the steps from above, the scale of the Tofino air photo is approximately
a) 1:25 000 b) 1:50 000 c) 1:75 000 d) 1:100
2) Utilizing the steps from above, the scale of the New Glasgow air photo is approximately
a) 1:25 000 b) 1:50 000 c) 1:75 000 d) 1:100 000 ?
New Glasgow Air Photo
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