Topographic Maps, Publications and Digital Products
A topographic map is a type of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. The various features shown on the map are represented by conventional signs or symbols. For example, colors can be used to indicate a classification of roads. These signs are usually explained in the margin of the map, or on a separately published characteristic sheet. Topographic maps are also commonly called contour maps or topo maps. In the United States, where the primary national series is organized by a strict 7.5 minute grid, they are often called topo quads or quadrangles. Topographic maps conventionally show topography, or land contours, by means of contour lines. Contour lines are curves that connect contiguous points of the same altitude(isohypse). In other words, every point on the marked line of 100 m elevation is 100 m above mean sea level.
There are several rules to note when viewing topographic maps:
- The rule of V's: sharp-pointed vees usually are in stream valleys, with the drainage channel passing through the point of the vee, with the vee pointing upstream. This is a consequence of erosion.
- Spacing of contours: close contours indicate a steep slope; distant contours a shallow slope. Two or more contour lines merging indicates a cliff.
Inset of USGS topographic map of Stowe, Vermont, USA
Of course, to determine differences in elevation between two points, the contour interval, or distance in altitude between two adjacent contour lines, must be known, and this is given at the bottom of the map. In most cases, contour intervals are consistent throughout a map. Sometimes dashed contour lines are present; these represent half the noted contour interval.
These maps usually show not only the contours, but also any significant streams or other bodies of water, forest cover, built-up areas or individual buildings (depending on scale), and other features and points of interest. Today, topographic maps are prepared using photogrammetric interpretation of aerial photography. Older topographic maps were prepared using traditional surveying instruments.
The Department of the Interior, U.S. Geological Survey (USGS) is the distributor of public sale NGA topographic maps, publications and digital products.
U.S. Military Installations Map (CONUS). This is a map of the major U.S. military installations in the continental United States. It is used by military and government contacts to assist in Federal communication to state and local.