Figure 5-9.--Concave slope.
contour note in the map margin identifies them as
approximate contours.
Figure 5-8.--Uniform, steep slope.
In addition to the contour lines, bench marks and
spot elevations are used to indicate points of known
elevation on the map. BENCH MARKS, the more
To estimate the elevation of the bottom of a
accurate of the two, are symbolized by a black X, for
example, XBM 124. The elevation value shown in black
depression, subtract half of the contour interval from the
refers to the center of the X. SPOT ELEVATIONS,
value of the lowest contour around the depression.
shown in brown, generally are located at road junctions,
On some maps, the index and intermediate contour
on hilltops, and other prominent landforms. The symbol
lines do not show the elevation and relief in as much
^P^P designates an accurate horizontal control point.
detail as may be needed; then SUPPLEMENTARY
When a bench mark and a horizontal control point are
CONTOURS may be used. These contour lines are
located at the same point, the symbol BM^P^P^P is
dashed brown lines, usually at one half of the contour
used.
interval for the map. A note in the marginal information
The spacing of the contour lines indicates the nature
indicates the interval used. They are used exactly like
of the slope. Contour lines evenly spaced and wide apart
solid contour lines.
indicate a uniform, gentle slope (fig. 5-7). Contour lines
evenly spaced and close together indicate a uniform,
On some maps, the contour lines may not meet the
steep slope. The closer the contour lines are to each
standards of accuracy but are sufficiently accurate in
other, the steeper the slope (fig. 5-8).
both value and interval to be shown as contours rather
than as form lines. On maps of this type, the contours
Contour lines closely spaced at the top and widely
are considered as approximate and are shown with a
spaced at the bottom indicate a concave slope (fig. 5-9).
Considering relief only, an observer at the top of a
dashed symbol; elevation values are given at intervals
concave slope can observe the entire slope and the
along the heavier (index contour) dashed lines. The
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