**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).**