Geoscience Reference
In-Depth Information
that it was generally warmer in the southern part of the United
States than in the northern part of the country, but the over-
all geographic patterns would be vague. Geographers typically
clarify these patterns by using isolines, which are lines that
connect points of equal value at predetermined intervals. In the
context of our temperature map, you may decide that you want
to show the temperature pattern in the country at 5°C (9°F) in-
tervals, or you may choose another interval. Using a 5°C (9°F)
interval as an example, you would draw an isoline connect-
ing all points of equal value at, for example, 5°C (41°F), 10°C
(50°F), 15°C (59°F), or 20°C (68°F). In many regions these
distinct temperatures might not exist; that is, the temperature
at one weather station might be 7°C (44.6°F), whereas at the
adjacent station it could be 12°C (53.6°F). Where this kind of
pattern exists, you would draw the line between the two weather
stations in a process called interpolation because you could as-
sume that the 10°C line occurs somewhere midway between
7°C (44.6°F) and 12°C (53.6°F). Figure 2.19 shows this process
more closely with some temperature data in the upper Midwest.
In the context of isolines, several different kinds are used
when mapping specific types of geographic phenomena. The
various kinds of isolines are:
1. Isobars —connect points of equal atmospheric pressure.
2. Isotherms —connect points of equal air temperature.
3. Isohyets —connect points of equal amounts of precipitation.
4. Isopachs —connect points of equal sedimentary thickness.
5. Contours —connect points of equal elevation.
Contour lines are particularly significant because they il-
lustrate the configuration of the three-dimensional landscape,
or the topography , on a topographic map . Topographic maps
contain a great deal of information, especially at a 1:24,000
scale (Figure 2.20). Here are some simple rules to follow when
reading these maps:
(a)
1. The closer the spacing of contour lines, the steeper the
slope. Conversely, contours that have wide spacing repre-
sent terrain that is relatively flat.
2. Contours that form closed circles indicate hills. Closed
circles with hatch marks indicate closed depressions.
3. Where contours cross a stream, they form V's pointing
upstream (which is also uphill).
Isolines that connect points of equal atmospheric
Isobars
pressure.
Isotherms Isolines that connect points of equal temperature.
Isohyets Isolines that connect points of equal precipitation.
Isopachs Isolines that connect points of equal sediment or
rock thickness.
Contours Isolines that connect points of equal elevation.
Topography The shape and configuration of the Earth's surface.
Topographic map A map that displays elevation data regard-
ing the Earth's surface.
(b)
Figure 2.20 A typical topographic map. Topographic maps
show elevation patterns of a given geographic area. (a) This
topographic map focuses on the Maroon Bells area near Aspen,
Colorado. (b) Photograph of the Maroon Bells from Maroon
Lake. The arrow in the accompanying topographic map shows
the approximate line of sight from Maroon Lake toward the
mountain peaks.
 
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