Calculating Scale
Often the scale of a map must be calculated. If a base map is available that has only
an RF and the cartographer wants to know the verbal scale or to draw a linear scale
or if the scale of a map isn’t known, the scale can be determined. Table 5.1 has some
useful conversion factors.
Examples
Determine a verbal scale for a map with an RF of 1:253,440
1 inch represents 253,440 inches.
Feet
0
2000
4000
1 inch represents 2000 feet
1: 24,000
FIGURE 5.1.
Scale can be expressed in three ways: as a ratio or fraction, verbally, or as a
linear or bar scale.
scale, Compilation, and generalization 75
To find the verbal scale in miles, one divides 253,440 by the number of inches
in one mile
253,440 ÷ 63,360 = 4. Therefore, 1:253,440 is the same as one inch represents
4 miles.
To find the verbal scale in kilometers for the same map, one divides 253,440 by
the number of centimeters in 1 kilometer.
253,440 ÷ 100,000 = 2.53. Therefore, 1:253,440 is the same as 1 centimeter
represents 2.53 kilometers.
Another kind of problem occurs when one has a source map or photograph that
has no scale. Here, two known points of known distance must be found. These can
be two points on a border, two roads, or, on photographs, some feature with known
Gorham
0
2000
Feet
Gorham
0
½
1
Mile
0
2
4
Miles
Gorham
A
B
C
fIgURe 5.2.
The maps in this figure are all of the same area, but at three different scales.
Note the amount of detail that can be shown at different scales.
76 THe geOgRaPHiC anD CaRTOgRaPHiC FRaMeWORK
dimensions, such as a football field. The map distance between the points is mea-
sured and using the formula RF = Map Distance/Ground Distance the scale can be
determined. Remember that RF is a fraction expressed as 1/x. This method works
best for large-scale maps, and is only approximate for large regions depending on the
projection.
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