How Do I Read a Topographic Map?

Topographic maps contain a significant amount of information, both within the map and along the margins. Before investigating a map, always look in the margins, including the area below the map, for information regarding title, location, scale and authorship (Figure 2.13).

Map margin illustrating symbology of a topographic map. — **Figure 2.13**: Bottom margin of a topographic map of the Lassen Peak Quadrangle (2018) (Public Domain; USGS, modified by Chloe Branciforte).

Many features on a topo map are shown by lines that may be straight, curved, solid, dashed, dotted, or in any combination. Topographic contours are shown in brown by lines of different widths. Each contour is a line of equal elevation; therefore, contours never cross. They show the general shape of the terrain. To help the user determine elevations, index contours are wider. Elevation values are printed in several places along these lines. The narrower intermediate and supplementary contours found between the index contours help to show more details of the land surface shape (Figure 2.14).

Contours on a topographic map. — **Figure 2.14**: Contour interval, line and index contours for the Lassen Peak Quadrangle (2018) (Public Domain; USGS, modified by Chloe Branciforte).

Contours that are very close together represent steep slopes. Widely spaced contours or an absence of contours means that the ground slope is relatively level. The elevation difference between adjacent contour lines, called the contour interval, is selected to best show the general shape of the terrain. The contour interval is printed in the margin of each USGS map (Figure 2.15). Bathymetric contours are shown in blue or black, depending on their location. They show the shape and slope of the ocean bottom surface. The bathymetric contour interval may vary on each map and is explained in the map margin.

Location of scales and contour interval for the map. — **Figure 2.15**: Contour interval for the Lassen Peak Quadrangle (2018) (Public Domain; USGS, modified by Chloe Branciforte).