Laboratory Manual for Introductory Geology 4e

How does this variation affect our use of a compass? To determine the true
direction (or bearing) from one place to another, you have to adjust from magnetic
north to true north by adding the declination to or subtracting it from the
azimuth reading. Geologists use a compass that can be adjusted to account for
declination.
8.3.3 Map Element 3: Distance and Scale
A map of your campus, or of the entire world, can fit onto one sheet of paper—if
we scale the Earth down so it fits. A map scale indicates how much an area has been
scaled down so that we can relate inches on the map to real distances on the ground.
Map scale may be expressed verbally, proportionally, or graphically, depending on
the purpose of the map and the accuracy desired.
8.3.3a Verbal Scales A verbal scale, used on many road maps, uses words, such as
“1 inch equals approximately 6.7 miles,” to describe the scaling of map and real
distances. This kind of scale is good enough for the purpose of allowing drivers to
estimate distances between cities.
8.3.3b Proportional Scales The most accurate way to describe scaling is with a
proportional scale (FIG. 8.10), one that tells exactly how much the ground has been
scaled down to fit onto the paper. For example, a proportional scale of 1:100,000
(“one to one hundred thousand”) means that distances on the map are 1/100,000
of distances on the ground. This ratio is the same for all units of measurement, so
that 1 inch on such a map corresponds to 100,000 inches on the ground (1.58 miles)
and 1 cm on the map corresponds to 100,000 cm on the ground (1 km). The scales
in Figures 8.10b and c, 1:62,500 and 1:24,000, respectively, indicate that those maps
have been scaled down less than the map in Figure 8.10a. The larger the number in
a proportional scale, the more the map has been scaled down, and the less space is
needed to portray an area on the map.
Note that the more a map is scaled down, the less detail it can show. Thus, Figure
8.10a is accurate enough to show the general locations and outlines of the lakes,
but not accurate enough for a canoer looking for the exact point closest to a road.
Figure 8.10c, which shows the irregularities of the shorelines much more accurately,
is far better for that purpose.
The metric system is ideally suited for scales such as 1:100,000,000 or 1:100,000
because it is based on multiples of 10. In the United States, distance is measured in
miles, but map distance is measured in inches. Unfortunately, relationships among
inches, feet, and miles are not as simple as those in the metric system. There are
63,360 inches in a mile (12 inches per foot 3 5,280 feet per mile), so the proportional
scale 1:63,360 means that 1 inch on a map represents exactly 1 mile on the
ground. Old USGS topographic maps use a scale of 1:62,500. For most purposes, we
can interpret this scale to be approximately 1 inch 5 1 mile, even though an inch on
such a map would be about 70 feet short of a mile. Other common map scales are
1:24,000 (1 in. 5 2,000 ft), 1:100,000 (1 in. 5 1.58 mi), 1:250,000 (1 in. 5 3.95 mi),
and 1:1,000,000 (1 in. 5 15.8 mi).
8.3.3c Graphical Scales Map scale can also be shown graphically, using a bar scale
(FIG. 8.11). A bar scale can be more accurate than a verbal scale, depending on the
map scale and how carefully you measure, but is not as accurate as a proportional
scale. Graphical scales on USGS topographic maps typically show distances in miles
and kilometers, but sometimes show distances in feet for maps covering small areas
(that is, maps that are scaled down less).
8.3 MAP ELEMENTS
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