To be most useful, a map must show
locations and distances accurately on a sheet of paper of convenient size. This
means that all things included in the map—ground area, rivers, lakes, roads,
distances between features, and so on—must be shown
proportionately smaller than they really are. The proportion chosen for a
particular map is its scale.
Simply defined, scale is the relationship between distance on the map and
distance on the ground. A map scale might be given as a
drawing (a graphic scale), but usually it is given as a fraction or a
ratio—1/10,000 or 1:10,000.
These "representative
fraction" scales mean that 1 unit of measurement
on the map—1 inch or 1 centimeter—represents 10,000 of the same units on the
ground. If the scale were 1:63,360, for instance, then 1 inch on the map would
represent 63,360 inches or 1 mile on the ground (63,360 inches divided by 12
inches equals 5,280 feet or 1 mile). The first number (map distance) is always
1. The second number (ground distance) is different for each scale; the larger
the second number is, the smaller the scale of the map.
"The larger the number, the
smaller the scale" sounds confusing, but it is
easy to understand. A map of an area 100 miles long by 100 miles wide drawn at
a scale of 1:63,360 would be more than 8 feet square. To make the map a more
convenient size, either the scale used or the area
covered must be reduced.
If the scale is
reduced to 1:316,800, then 1 inch on the map represents 5 miles on the ground,
and an area 100 miles square can be mapped on a sheet less than 2 feet square
(100 miles at 5 miles to the inch equals 20 inches, or 1.66 feet). On the other
hand, if the original 1:63,360 scale is used but the
mapped area is reduced to 20 miles square, the resulting map will also be less
than 2 feet square.
Such maps would be easier to handle.
But would they be more useful? In the small-scale map
(1:316,800), there is less room; therefore, everything must be drawn smaller,
and some small streams, roads,
and landmarks must be left out altogether. On the other
hand, the larger scale map (1:63,360) permits more
detail but covers much less ground.
Many areas have been mapped at
different scales. The most important consideration in
choosing a map is its intended use. A town engineer, for instance, may need a
very detailed map to locate precise sewers, power and
water lines, and streets. A commonly used scale for this purpose is 1:600 (1 inch on the map
represents 50 feet on the ground). This scale is so
large that many features—such as buildings, roads, and railroad tracks—that
are usually represented on smaller scale maps by symbols can be drawn to scale.
The U.S. Geological Survey (USGS)
publishes maps at various scales. The scale used for most United States topographic mapping is
1:24,000. USGS maps at this scale cover an area
measuring 7.5 minutes of latitude and 7.5 minutes of longitude
and are commonly called 7.5-minute quadrangle maps. Map coverage for most of
the United States has been completed at this scale, except for Puerto Rico,
which is mapped at 1:20,000 and 1:30,000, and for a few States that have been
mapped at 1:25,000. Most of Alaska has been mapped at 1:63,360, with some
populated areas also mapped at 1:24,000 and 1:25,000. Maps at 1:24,000 scale
are fairly large and provide detailed information about the features of an
area, including the locations of important buildings
and most campgrounds, ski lifts, and water mills. Footbridges, drawbridges,
fence lines, and private roads are also shown at this scale. Usually these
features are omitted from maps in the 1:50,000- to 1:100,000-scale range; these
maps cover more area while retaining a reasonable level of detail. Maps at
these scales are most often produced using the 30- by
60-minute quadrangle formats.
Small-scale maps (1:250,000 and
smaller) show large areas on single map sheets, but details are limited to
major features, such as boundaries, parks, airports, major roads, railroads,
and streams.
Contours Buildings Land Survey Systems Mines and Caves
Railroads Boundaries Rivers, Lakes and Canals Roads Utilities
Vegetation Submerged Areas Marine Coastal Surface
Glaciers and Permanent
Snowfields Bathymetric Control Data and Monuments
