Contour lines

How to read it

The USGS’s symbol sheet says each contour is a line of equal elevation, so contours never cross, and together they show the shape of the land. Index contours are drawn wider and carry elevation numbers at intervals along them; the narrower intermediate contours between them add detail. The difference in height between neighbouring contours is the contour interval, chosen to suit the land and printed in the margin of every USGS map: 10 feet or less for flat country, 100 feet or more for mountains. Close lines mean a steep slope, and widely spaced lines, or none, mean fairly level ground.

Natural Resources Canada and the Ordnance Survey describe contours the same way, and both note that the elevation numbers read uphill.

Also searched as: brown lines on a map, contour interval, contour line, index contour, intermediate contour, topographic lines.

Differences by country

USGS maps mark index contours with heights in feet or metres. Ordnance Survey maps draw contours in thin orange or brown lines, normally 10 metres apart and 5 metres in flatter areas; the 1:25 000 OS Explorer maps use 5 m or 10 m. Natural Resources Canada’s legend has the same index and intermediate pair.

Look-alikes

Contours with short ticks on one side are Depression contours, round a hollow. On a weather map, lines that look like contours are isobars, joining places of equal air pressure. A single height marked with a small cross is a Spot elevation.

Older maps

The USGS says most of the symbols on its 1947 to 2009 maps are still in use. The same booklet notes that symbols are refined over time, so maps in one series can show the same feature slightly differently.