Contour lines on a map show topography or changes in elevation. They reveal the location of slopes, depressions, ridges, cliffs, height of mountains and hills, and other topographical features. A contour line is a brown line on a map that connects all points of the same elevation. They tend to parallel each other, each approximately the shape of the one above it and the one below it. In the image below, compare the topographic map with the landscape perspective.
- Contours have general characteristics; some of which are illustrated in the images below.
- Concentric circles of contour lines indicate a hill.
- Evenly spaced contours indicate uniform slope.
- Widely spaced contours indicate a gentle slope.
- Widely spaced contours at the top of a hill indicate flat hilltop.
- Close together contours indicate steep slope, wall, or cliff.
- Close together contours at the top of a hill indicate a pointed hilltop.
- Crossing or touching contours indicate overhanging cliff.
- Jagged, rough contours indicate large outcrops of rocks, cliffs, and fractured areas.
- “V” shape contours indicate stream beds and narrow valleys with the point of the “V” pointing uphill or upstream.
- “U” shape contours indicate ridges with the bottom of the “U” pointing down the ridge. A saddle is a ridge between two hills or summits.
- “M” or “W” shape contours indicate upstream from stream junctions.
- Circles with hachures or hatch lines (short lines extending from the contour line at right angles) indicate a depression, pit, or sinkhole.
- Spot elevations (height of identifiable features) such as mountain summits, road intersections, and surfaces of lakes may also be shown on the map.
Contour interval is the difference in elevation between two adjacent contour lines. On USGS maps, contour intervals are usually 1, 5, 10, 20, 40, and 80 feet. Occasionally you will find a map with a 25 foot contour interval or metric units, but not often. To make the contours easier to read, every fifth one is the index contour which is printed darker and has the elevation in feet from mean sea level marked on the line. The thinner or lighter colored contour lines are called intermediate contours.
**Information adapted from the National Interagency
Incident Management System Basic Land Navigation
Manual, PMS 475 dated June 2007.