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THE RULES OF CONTOUR LINES

When reading contour maps, or trying to determine what the elevations of
contour lines are, one must apply a few basic rules of contour lines. There are no
exceptions to these rules!

1. Closed contour lines on a map indicate either a hill (peak, mountain, etc.)
or a hole (depression, etc.). Closed contours that indicate that the land slopes
down into a hole are marked by hachured lines to distinguish them from closed
contours that indicate that the land slopes up over a hill (Figure 4.2).

Profile view

Map view

hatchure
lines

Depression Hill

Fig. 4.2

2. A single contour line represents a single elevation along its entire length.

In other words, the elevations of all points along a contour line are the same.
3. Contour lines never split, cross, or intersect. At a vertical cliff they do,
however, come together and touch (Figure 4.3).

ocean

Fig. 4.3

4. The elevation of a contour line is always a simple multiple of the contour
interval. For ease of reading, by convention, each fifth consecutive contour line is
an index contour (drawn as a thicker line than adjacent contours and also

numbered somewhere along the trace of the contour line). Commonly used
intervals are 5, 10, 20, 40, and 80 feet or 25, 50, 75, 100 meters.
5. Widely spaced contour lines indicate a gentle slope. Closely spaced

contours indicate a steep slope.
6. Every contour line eventually closes on itself. However, any one map will
not be large enough to show the full extent of all contour lines, and some will

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