Contour lines

A map is flat, but the land has hills.

So how can a flat map show a hill? Mapmakers draw lines around it. Each line is at one height.

A map is flat, but real land goes up and down. To show hills and valleys, mapmakers use contour lines. Ordnance Survey, Britain's national mapping agency, describes a contour as a line drawn on a map that joins points of equal height above sea level.

Imagine walking along a contour line painted on the side of a real hill. A U.S. Geological Survey (USGS) classroom activity explains that you would neither go up nor down: you would stay level all the way around.

The USGS defines elevation contours as imaginary lines connecting points with the same elevation, above or below a reference surface that is usually mean sea level. Ordnance Survey puts it simply: a contour joins points of equal height above sea level. The USGS calls contours the distinctive characteristic of a topographic map.

A USGS classroom activity offers a useful way to picture one: walk along a contour painted on the ground around a hill and you would neither go up nor down. Contours make it possible to show the height and shape of mountains, the steepness of slopes and even the depths of the ocean bottom.

Look at how close the lines are.

Lines close together mean a steep hill. Lines far apart mean the ground is nearly flat.

The spacing of contour lines tells you how steep the ground is. Ordnance Survey's rule: the closer the contour lines are together, the steeper the slope. Lines that are far apart mean a gentle slope, and the USGS adds that very wide spacing, or no contours at all, means the ground is fairly level.

Contour lines never cross each other, because each line stands for just one height. One spot cannot be two heights at once.

Spacing encodes slope. Contours very close together represent steep slopes; widely spaced contours, or none at all, mean the ground is relatively level (USGS). Ordnance Survey teaches the same rule and asks map readers to judge from the spacing whether a slope is steep or shallow.

Because each contour is a line of a single elevation, the USGS notes, contours never cross. The logic is simple: a point on the ground has one elevation, so it cannot lie on two different contours. The lines also tell you which way is uphill. Ordnance Survey points out that contour numbers are printed to read uphill: the top of each number faces up the slope.

Each line is a little higher than the one below it.

On some walking maps, each line is 5 meters higher than the one below. In the mountains, it can be 10 meters.

The difference in height between one contour line and the next is called the contour interval. On USGS maps it is printed in the margin.

Ordnance Survey says the interval on its 1:25,000 walking maps is usually 5 meters, though in mountain areas it may be 10 meters. The USGS chooses the interval to suit the land: a map of a flat area might use 10 feet or less, and a map of mountains 100 feet or more.

The contour interval is the elevation difference between adjacent contours. The USGS selects it to best show the general shape of the terrain and prints it in the margin of each map: relatively flat areas may have an interval of 10 feet or less, mountainous areas 100 feet or more. Ordnance Survey's 1:25,000 maps usually use 5 meters, or 10 meters in mountainous regions.

To make heights easier to read, USGS maps draw some contours wider, as index contours, and print elevation values along them. The narrower intermediate and supplementary contours between them add detail. Choosing an interval is a trade-off: a small interval shows fine detail but crowds steep ground with lines, while a large one keeps a mountain readable but hides small bumps.

A hill looks like rings inside rings.

The smallest ring is near the top. Can you find the top of the hill?

The shapes the lines make show the shapes of the land. A hill or mountain appears as a set of rings, one inside another, with the smallest ring near the top. National Geographic Education notes that loops like these can show a hill, or, where the height goes down, a hollow in the ground.

Britannica Kids explains that contour maps can show mountains, hills, plateaus, ridges, valleys and basins. Ordnance Survey suggests thinking of contours as tide marks left by the sea as the tide goes out, a new line every 5 meters.

Contour patterns map landforms. Concentric closed loops with rising values mark a hill or summit; National Geographic Education notes that similar loops with falling values can mark a depression. Britannica Kids lists the features contour maps can show: mountains, hills, plateaus, ridges, valleys and basins.

Ordnance Survey's image is helpful: think of the contours as tide marks left on the land by a falling sea, one every 5 meters. The USGS points out that contours show the depths of the ocean too: bathymetric contours, printed in blue or black, map the shape of the seafloor. Britannica Kids notes that older maps showed highlands with sketches of molehills or snowcapped peaks, and that some topographic maps show slope with hachures, short lines drawn down the slope.

On many hiking maps, contour lines are brown.

Blue lines are often water. Check the key to be sure.

On both Ordnance Survey and USGS maps, contour lines are brown. On USGS maps, water is blue, so contours and streams are easy to tell apart.

USGS topographic maps of the United States are made at many scales. Its current main series, called US Topo, is at 1:24,000 and has been published as digital maps since 2009. Older paper maps, scanned into a historical collection, go back to 1884.

Ordnance Survey prints its contours in brown on 1:25,000 maps, and the USGS shows topographic contours in brown lines of different widths. On USGS maps water features are blue, which keeps the two easy to separate.

The USGS has been the primary civilian mapping agency of the United States since 1879. Its current topographic series, US Topo, is at 1:24,000 scale and has been published as digital documents since 2009; the Historical Topographic Map Collection holds scanned images of paper maps published between 1884 and 2006. Today US Topo maps are computer-generated from national databases rather than hand-scribed, but the conventions a reader needs, brown lines, index contours and a stated interval, are the same.

You can make a hill from clay.

A grown-up helps slice it into flat layers. Draw around each layer on paper. Now you have a contour map!

The USGS has a classroom activity called Play-Dough Topo. You build a hill or volcano out of modeling clay, then a grown-up helps cut it into flat layers of equal thickness using fishing line. Trace around each layer, one inside the other, on paper.

The outlines you draw are contour lines, and the thickness of each slice is your contour interval. Compare the steep side of your hill with the gentle side: where are your lines closest together?

The USGS activity Play-Dough Topo has students build a model volcano from modeling clay, slice it horizontally into layers of equal thickness with fishing line or clay-cutting wire, and trace each layer to produce a topographic map. It is designed for grades 5–12 and takes about 50 to 90 minutes.

Building the model makes the abstract rules concrete. The equal slice thickness is the contour interval; the traced outlines cannot cross; steep flanks give tightly packed lines; and if your volcano has a crater, it shows up as a loop where the height goes down, the kind of loop National Geographic says can mark a depression. Try it, then compare your drawing with a real topographic map of a volcano.