USGS Topographic Map Symbols: Contours

Contours

Contour lines are the principal means used to show the shape and elevation of the land surface. Each line represents an area of equal elevation. The unit of measure for elevation, and the contour interval (the elevation difference from one line to the next) is specified at the bottom of USGS topographic maps.

Bathymetric contour intervals indicate the land surface of the ocean floor (not to be confused with depth soundings).

Topographic
		Index Contour Interval. These contour lines have a number written on them. This number indicates the elevation of all points along the line. The unit of measurement (feet, meters) depends on the map.

		Intermediate Contour Interval. These lines have no numbers. They appear between index contour lines (see above).

		Supplementary Contour Interval. These dotted lines are placed in areas where elevation change is minimal. If there is a lot of space between Index and Intermetiate Contours (as happens where the land is relatively flat), these lines are added to indicate that there are elevation measurements, even if they are few and far between.

		Depression. A depression is a decline in elevation inside inside a rise in elevation. For example: a crater, whose sides rise but whose center is lower.

		Cut; Fill. Cuts and fills occur where a roadway has been blasted through land, drastically lowering (cut) or raising (fill) the elevation in a distinct and small area.


Bathymetric
		Index Contour Interval.

		Intermediate Contour Interval.

		Primary Contour Interval.

		Index Primary Contour Interval..

		Supplementary Contour Interval. These dotted lines are placed in areas where elevation change is minimal. If there is a lot of space between Index and Intermetiate Contours (as happens where the land is relatively flat), these lines are added to indicate that there are elevation measurements, even if they are few and far between.