DETERMINING CONTOUR INTERVALS

 
Most contour lines on topographic maps are not labeled with elevations.  Instead the reader of the map needs to be able to figure out the elevation by using the labeled contour lines and the contour interval (see previous page for explanation).  On most maps determining contour interval is easy, just look in the margin of the map and find where the contour interval is printed (i.e. Contour Interval 20 ft).

For the maps on this web site, however, the contour interval is not listed because we only parts of topographic maps, not the whole map which would include the margin notes.  However we usually don't need to be given the contour interval.  We can calculated from the labeled contours on the map as is done below.

 

This method works if we don't have any topographical complications, areas where the elevation is not consistently increasing or consistently decreasing..  With practice these areas can usually be easily recognized.  Also this method does not tell the units for the contour interval. In the United States most topographic maps, but not all, use feet for elevation, however it is best to check the margin of the map to be sure.    
 

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Last Updated: 01/18/2014

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