What a topographic map shows

On a USGS topographic map, brown lines are heights, blue is water and green is woodland, and the colours mean the same thing on every sheet.

A map that shows the shape of the ground

A road map tells you how to get from one town to the next. A topographic map does more: it shows the shape of the land itself, where it rises into hills, where it drops into valleys, and where water runs. The word comes from the Greek topos, a place, and graphein, to write or draw. Hikers, firefighters, engineers and soldiers have used these maps for well over a century, because they answer questions a road map cannot. How steep is this path? Which side of the ridge will the rain run off? Can I see the village from the top of that hill?

The model above is a small piece of invented land, 4 kilometres on each side. On the left, or below on a phone, is its contour map. On the right is the same land as a 3D model, with the map printed on its surface. Turn the model and you can see that the brown lines on the map are not decoration. Each one follows the ground at a single height, like a bathtub ring left by water that once stood at that level.

Colours that mean the same everywhere

The USGS, which has made the standard maps of the United States since 1879, uses colour as a code. Brown lines are topographic contours. Blue is water: lakes, streams, irrigation ditches and other hydrographic features. Green areas are vegetation such as woods. Red marks important roads and the land survey grid, and black shows other roads and trails, railroads, boundaries and buildings. Densely built-up areas are tinted gray or red. Learn these six colours once and you can read most of a sheet before you even look at the legend.

Features appear as points, lines or areas depending on their size. A single house may be a small black square, while a large building is drawn in its real shape. A well or a spring is a point symbol, a stream is a line, a lake is an area. Many features are also labelled in a colour that matches their class, so a stream name is printed in blue and a contour height in brown. Korea’s National Geographic Information Institute (NGII) sorts the features on its digital maps in a similar way, into transport, buildings, facilities, vegetation, water, landforms, boundaries, and annotation such as place names and grid lines.

The legend, the margin and the spot heights

Everything you need to read a sheet is printed in its margin: the name of the sheet, the scale, the contour interval, the direction of north and the date of the survey. The legend lists the symbols. Small black triangles or dots with a number beside them are spot heights, measured points on summits and road junctions. On the map above, the triangles mark the highest points of each hill. Click on top of one and compare the height of A with the number printed beside the triangle. It should match to within a few metres.

How accurate is a printed map?

Maps are measured, not guessed, and the USGS publishes how well its old 7.5-minute maps had to agree with the ground. At least 90 percent of the points tested had to be within one-fiftieth of an inch on the map, which is about 40 feet on the ground. For height, at least 90 percent of the points tested had to be within half the contour interval. That second rule is worth remembering: do not expect a height read from such a map to be better than about half the contour interval.