Contour Lines of a Topographic Map

Contours are the thin brown lines that snake across a topographic (topo) map.  Contour lines connect equal points of elevation such that every point on a specific line will be at that elevation above sea level. 

 

Contour lines are distinct and separate from lines for roads, coordinate systems (e.g., latitude and longitude) and trails.  Man made features like roads and buildings are black.  Contour lines are brown.  By adding elevation data to the map contour lines provide a three dimensional view of the terrain.  These lines provide shape and a sense of texture. 

The graphic above uses shading and electronic editing in an attempt to gain a three dimensional view of the terrain.  Compare the two maps.

Here are a few “keys to the kingdom” of reading these the brown contour lines.

There are two primary types of contour lines, index and intermediate lines. 

The dark brown, wider lines (below) are “index lines.”  The numbers adjacent correspond to altitude along the line.

For example, if the hiker locates his position on the topo and its right on top of the dark brown line with 5200 printed on the line, the altitude at that point is 5200 feet.  Consider the altitude to be the height above sea level.

The faint brown lines between the index lines are “intermediate contour” lines (see graphic above.)  Critical to intermediate lines is the specific elevation change between the lines.  This is known as the “contour interval.” The contour interval could be 10 feet, 20 feet or 200 feet; it just depends on the scale of the map and terrain.  To find the contour interval on a topo go to the bottom of the map or to the map key/index.  On a United States Geologic Survey (USGS) 7.5 minute topo map it will be at the bottom center of the map.  In the graphic below, the contour interval is circled in red.

In the small map above, look at Browns Mountain.  The spacing between the intermediate contour lines represents an elevation change of 20 feet.  Notice that the index lines are spaced five contour intervals apart or 100 feet between index lines.

Contour lines (index and intermediate) can provide a view of slope and pitch, depressions, ridge lines and level ground; the highs and lows of the earth’s surface.

The contour lines at Browns Mountain are close together and represent a steep increase in elevation.  Lines close together can indicate a peak, hill, ridge line or a cliff.

The contours of the land area to the left or west of Browns Mountain are spaced farther apart.  Such lines indicate flat ground like a meadow or plain.  Lines far apart make for gentle slopes and flat ground.

Ridges, valleys, and streams are represented by contour lines too.  A line’s shape identifies these land features.  For example, a valley’s shape is formed by a collection of “v’s.”  Tips of the v's point toward higher elevations.  Look at Alder Creek on the map below.  Note that the creek bed is in the bottom of a valley and water flow is from higher elevation to lower.

The contour lines that shape Alder Creek have v’s that point to higher altitude.  Look closely at the index lines and try to determine the elevation change as the creek flows north.

Contours shaped like a v or u, pointing toward lower elevation denotes ridges.  Notice the shape of the contour lines to the right and east of Alder Creek.  The v’s tips point toward lower elevation.  In fact, the v’s have become more like expanded “u’s.”

For more information about contour lines visit www.landnavigation.org or search the internet for the “World of Teaching – Topographic maps” (a very fine power point presentation.)