Geography Reference
In-Depth Information
1 mm on a map is equal to 25,000 mm on the ground. There are 10 mm in a
centimeter, so 1 mm on a map is also equal to 2,500 cm. With 100 cm in a
meter, we can finally convert the relationship of map measurements to
ground measurements, or 1 mm equals 25 m. Knowing this relationship, we
can easily calculate how long a bridge 4 mm on the map is on the ground.
Knowing how many millimeters are in a centimeter, we can also easily calcu-
late how wide a 6-cm field is—or any other distance for that matter.
When using inches, feet, yards, and miles, the mathematics are more
complicated, but the principles are the same. The main numerical relation-
ship to know is that the 5,280 feet, or 1,760 yards, of a single mile are equal
to 63,360 inches. In other words, at a scale of 1: 63,360, 1 inch on the map is
equal to 1 mile on the ground. The calculation of distance measurements
from a map or visualization to the equivalent ground distance when using
standard or imperial distance units must always first determine the number
of feet on the ground in 1-inch distance measured on the map. For example,
1 inch measured on a map in the United States at the scale of 1: 24,000 is
equal to 24,000 inches on the ground, or 2,000 feet. If the distance between
two intersections on a 1: 24,000 scale map is 6 inches, the distance between
them on the ground is 12,000 feet or a little more than 2.25 miles. You
should note that this imprecision in reporting distances is just one of the
many reasons for using the metric system for distance measurements. The
ease of calculations is perhaps the second reason.
Scale can also be expressed by using a bar that indicates a single dis-
tance or multiple distances on the ground. A visual comparison offers easy
approximation of distances and the use of a ruler to determine the equiva-
lent distances on the map makes it possible to determine the representative
fraction and to do similar calculations.
Scale is also a very significant indicator of the detail shown by a map or
visualization. Generally, smaller scales show less detail, while larger scales
show more detail. Although plenty of cartographers will disagree, many
would agree that maps at scales greater than 1: 50,000 scale are called small-
scale maps, while maps at scales between 1: 100 and 1: 50,000 are called
large-scale maps. There is some disagreement among cartographers about
this, but this division provides a meaningful starting point for the discussion.
Finally, different disciplines work with a distinct scale or possibly a set of
scales. These scales are usually related to established disciplinary conven-
tions. Because of the cost of GI and the standardization of topographic maps
in most European countries, many European planning activities rely on
scales of maps created by their national mapping agencies. For smaller areas,
they rely on scales standardized for cadastral mapping. This is starting to
change as GI becomes more common, but the 1: 25,000 and 1: 50,000 scales
are still prevalent.
DETERMINING SCALE BY OTHER MEANS
If you don't know the scale of a map or visualization, you can figure it out by
using one of three techniques.
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