Contour lines are the brown lines that give a topographic map its name. Each one connects points of equal elevation, so walking along a contour line keeps you at the same height. Lines packed close together mean steep ground, and lines spread far apart mean gentle or flat ground. The elevation difference between neighboring lines, called the contour interval, is printed in the bottom margin of every USGS map.
What do contour lines show on a topo map?
Each brown contour line connects points of the same elevation. Close-together lines mean a steep slope and widely spaced lines mean gentle terrain. Every fifth line is a thicker index contour labeled with its elevation.
IN THIS GUIDE
The main contour lines · Depressions, cuts, and fills · Glaciers and permanent snowfields · Underwater (bathymetric) contours · Reading terrain from contours · FAQ
The main contour lines
Start with the thick, labeled index contours to get your bearings, then count the thin lines between them.
Index
A thicker brown line labeled with its elevation, here 6,000 feet. Usually every fifth contour. Find the nearest index contour, then count lines up or down to work out any elevation.
Intermediate
The thinner brown lines between index contours, each one contour interval apart. Multiply the number of lines you cross by the interval to get your elevation gain.
Supplementary
A fainter contour drawn at a fraction of the normal interval, often half. It's added in flat country where the regular lines would miss low ridges, terraces, or shallow basins.
Index – approximate or indefinite
A dashed index contour. Used where the mapmakers couldn't place the line to the map's accuracy standard, for example under dense forest canopy.
Intermediate – approximate or indefinite
A dashed intermediate contour, for the same reason. Treat the shape as reliable but the exact position as approximate.
Depressions, cuts, and fills
A closed loop of contours is usually a hilltop. These symbols cover the exceptions, and the places where people have reshaped the land.
Depression
Closed contours with short ticks pointing inward, or downhill. Marks a sinkhole, crater, pit, or basin with no outlet. Without the ticks, the same loops would be a hill.
Cut
Contours bent around a cut, where a road or railroad was dug through a hill. Expect steep banks on both sides of the roadway.
Fill
Contours bulging around an embankment built up to carry a road or railroad across a low spot. Notice the stream passing through a culvert underneath.
Continental divide
A heavy brown dashed line along a ridge. Rain falling on one side flows to the Pacific, and on the other side to the Atlantic or Gulf of Mexico.
Glaciers and permanent snowfields
On ice and permanent snow, contours change from brown to blue. You'll see these on maps of Alaska, the Cascades, the northern Rockies, and other glaciated ranges.
Contours and limits
Blue contour lines inside a dashed blue outline. The contours show the shape of the ice surface, and the dashed line marks the edge of the glacier or snowfield.
Formlines
Short blue dashes that show the general shape of the ice where exact contours weren't surveyed. Useful for the overall slope, not for precise elevations.
Glacial advance
Used on revised maps where a glacier has grown since an earlier edition. The new ice is drawn over the old brown land contours.
Glacial retreat
Used where a glacier has shrunk since an earlier edition. The newly exposed ground is shown inside the old ice outline. Most glaciers have retreated since their maps were made, so expect more bare rock and moraine than the map shows.
Underwater (bathymetric) contours
Combined topographic-bathymetric maps carry contour lines below the water too, showing the shape of lake and ocean floors. They're drawn in blue or black depending on location.
Index Topo-bathymetric maps
A thicker blue depth contour, labeled with its depth. It works just like an index contour on land.
Intermediate Topo-bathymetric maps
Thinner blue depth contours between the index lines.
Index primary Topo-bathymetric maps
A thick black depth contour, labeled with its depth. Black is used for the primary depth contours in some areas.
Primary Topo-bathymetric maps
A thinner black depth contour between primary index lines.
Supplementary Topo-bathymetric maps
A light contour added where the bottom is nearly flat, at a fraction of the normal depth interval.
WATCH
Topo Map Reading, part 3: Contour Line Basics
A one-minute lesson from our Topo Map Reading series.
Also in the series: Part 4: Determining Contour Direction · Part 5: Terrain Features
Watch the full Topo Map Reading series →Reading terrain from contours
- Check the contour interval first. It's printed in the bottom margin. A 40-foot interval in the mountains and a 10-foot interval on the plains make the same spacing mean very different slopes.
- V's point uphill along streams. Where contours cross a valley, they bend into a V or U that points upstream. Along a ridge, the V's point downhill.
- Spacing is steepness. Evenly spaced lines are a steady grade. Lines that bunch together and then spread apart mean a cliff band or a bench.
- Find the saddles. An hourglass shape between two closed loops is a saddle or pass, and it's often the easiest way across a ridge.
- Contours never cross. At a vertical cliff they can merge into one line, but they never cross.
PUT IT TO USE
See these symbols on a real map
Frequently asked questions
What is a contour interval?
The contour interval is the difference in elevation between neighboring contour lines. It's printed in the map margin, for example "Contour interval 40 feet". Flat areas use small intervals, and mountainous areas use large ones.
How do you tell a hill from a depression on a topo map?
Both are closed loops of contours. A depression has short tick marks on the inside of the lines, pointing downhill. A hill doesn't, and often has a spot elevation at the top.
What is an index contour?
An index contour is a thicker contour line, usually every fifth one, labeled with its elevation. It makes it easy to read elevations without counting every line.
Why are some contour lines blue?
Blue contours show the surface of glaciers and permanent snowfields, or water depth on topographic-bathymetric maps. Brown contours are always land.
MORE SYMBOL GUIDES
Benchmarks & Spot Elevations: What BM Means on a Topo Map · Water Symbols on Topo Maps: Streams, Lakes, Springs & Wetlands · Map Grids on Topo Maps: Latitude, Longitude, UTM & State Plane
← Back to all USGS map symbols
Symbol artwork from the U.S. Geological Survey Topographic Map Symbols brochure (public domain).