Reading Topographic Maps
Interpreting
the colored lines, areas, and other symbols is the first step in using topographic
maps. Features are shown as points, lines, or areas, depending on their size
and extent. For example, individual houses may be shown as small black squares.
For larger buildings, the actual shapes are mapped. In densely built-up areas,
most individual buildings are omitted and an area tint is shown. On some maps,
post offices, churches, city halls, and other landmark buildings are shown
within the tinted area.
The first features usually noticed on a topographic
map are the area features, such as vegetation (green), water (blue), and
densely built-up areas (gray or red).
Many features are shown by lines that may be straight,
curved, solid, dashed, dotted, or in any combination. The colors of the lines
usually indicate similar classes of information: topographic contours (brown);
lakes, streams, irrigation ditches, and other hydrographic features (blue);
land grids and important roads (red); and other roads and trails, railroads,
boundaries, and other cultural features (black). At one time, purple was used
as a revision color to show all feature changes. Currently, purple is not used
in our revision program, but purple features are still present on many existing
maps.
Various point symbols are used to depict features such
as buildings, campgrounds, springs, water tanks, mines, survey control points,
and wells. Names of places and features are shown in a color corresponding to
the type of feature. Many features are identified by labels, such as
“Substation” or “Golf Course.”
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. 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. A map of a relatively flat area may have a
contour interval of 10 feet or less. Maps in mountainous areas may have contour
intervals of 100 feet or more. The contour interval is printed in the margin of
each U.S. Geological Survey (USGS) map.
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.