RULES FOR CONTOUR LINES
- If one were to walk along a contour line, he or she would walk a
level line parallel to mean sea level.
- Contour lines connect points of equal elevation.
- If one were to walk at right angles to or cross contour lines, he
or she would be going either up slope or down slope.
- If one could trace any particular contour line far enough, perhaps
across hundreds of quadrangles, he or she would find that it eventually
closes upon itself, making a complete circle.
- Contour lines never branch.
- Contour lines never join.
- For ease in counting, every 5th contour line is heavier than the
intervening four and has its value in feet labeled. An exception is
found on sheets having a 25 ft. contour interval, in which cas every
fourth line is heavier.
- Concentric rings indicate a hill or peak unless they are hachured
to represent a pit or depression.
- The value of a hachured contour line is the same as the next unhachured
line lower down the slope.
- To find the value of hachured contour lines within hachured contour
lines, one must count downward the number of feet given as the contour
interval for each of such lines.
- On any given map, closely spaced contour lines indicate steeper slopes
whereas widely spaced contour lines indicate more gentle slopes. One
cannot apply this rule from one quadrangle sheet to another unless the
two sheets have the same contour interval and the same scale.
- Contour lines point upstream when crossing streams, rivers, ravines
- Any point on a contour line is as high above mean sea level or has
the same elevation as the value of the contour line.
- Any point between two contour lines has an elevation somewhere between
the elevations of the two contour lines.