Laboratory Manual for Introductory Geology 4e

9.3.2 Contour Lines and Elevation
Topographic maps provide several key features to help determine elevations. Two of
the easiest to use are benchmarks and index contour lines. A benchmark is an accurately
surveyed point marked on the ground by a brass plaque cemented in place.
It records the latitude, longitude, and elevation of that spot. On topographic maps,
elevations of selected points (such as hilltops, lake surfaces, and highway intersections)
are indicated with a symbol and a number, such as X 1438”
or D 561
. The latter
symbol represents the location of a benchmark.
To find the elevations of other points on the map, however, we must use the
contour lines. Take another look at Figure 9.4, but first, remember to check
for the contour interval at the bottom of the map for the difference in elevation
between adjacent contour lines. On Figure 9.4, as on most topographic
maps, every fifth contour line is darker than those around it and has its elevation
labeled. These lines are index contours, which represent elevation intervals that
are five times the contour interval (multiples of 5 3 20 in Fig. 9.4—300 feet,
400 feet, and so on). To determine the elevation of an unlabeled contour line,
determine its position relative to an index contour. For example, a contour line
immediately adjacent to the 500-foot index contour must be 480 or 520 feet—20
feet lower or higher than the index contour on a map with a contour interval
of 20 feet.
What about the elevation of points between two contour lines? All points on
one side of a contour line are at higher elevations than the line itself, and those
on the opposite side are at lower elevations—but how much higher or lower? The
contour interval in FIGURE 9.6 is 50 feet, so a point between two adjacent contour
lines must be less than 50 feet higher or lower than those lines. Estimate the
elevation by how close a point is to either contour line: a point midway between
the 500- and 550-foot contour lines would be estimated as 525 feet above sea level;
a point ¾ of the way to the 550-foot contour would be about 538 feet above sea
level, and so forth.
Point C on Figure 9.6 lies between two 500-foot contour lines, and the adjacent
contours indicate that C is near the crest of a hill. What can we say about the elevation
of the highest point on the hill? First, that it is over 500 feet, and second, that
it must be less than 550 feet, because there isn’t a 550-foot contour line. There is no
further information that would allow us to be more precise.
The elevation of Point A is between 400 and 450 feet. Because the slope of the
hill is constant between those contour lines, and A is halfway between them, the
estimated elevation of A is 425 feet above sea level.
Point B lies between 350 and 400 feet. Here, however,
the spacing of the contour lines indicates that FIGURE 9.6 Reading elevations of hills and valleys from
the slope is gentler near the 350-foot line than near
topographic maps.
the 400-foot line. The estimated elevation of B is 355
Map view
feet above sea level.
E
D
A C
Concentric contour lines may indicate either a hill (as
E
in Exercise 9.1b) or a depression. To avoid confusion, it
must be clear that concentric contour lines outlining a
B
depression indicate progressively lower elevations toward
C
A
the center rather than the higher elevations that would
E
D
indicate a hill. To show this, small marks called hachures
500
E
B 400
are added to the line, pointing toward the lower elevation
(see the innermost two lines in FIG. 9.7).
Repeat
300
200
The feature in Figure 9.7 could be a volcano with a
100
Repeat
crater at its summit. Note that the nested contour lines 0
350
300
250
200
200
250
300
350
350
400
450
500
500
450
400
350
300
9.3 READING TOPOGRAPHIC MAPS
223