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Topographic map

Example of a topographic map with contour lines
Example of a topographic map with contour lines

Topographic maps, also called contour maps, topo maps or topo quads (for quadrangles), are maps that show topography, or land contours, by means of contour lines. Contour lines are curves that connect contiguous points of the same altitude. 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 Vs: 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 a consequence of erosion.
  • The rule of Os: closed circles are normally uphill on the inside and downhill on the outside, and the innermost circle is the highest area. If a circle instead represents a depression, some maps note this by short lines radiating from the inside of the circle, called "hachures".
  • Spacing of contours: close contours indicate a steep slope; distant contours a shallow slope. Two or more contour lines merging indicates a cliff.

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.

Topographic maps are prepared using aerial photography.

Sources of topographic maps

The United States Geological Survey produces several national series of topographic maps. The oldest series is the fifteen-minute series, meaning that each map portrays an area fifteen minutes of longitude wide by fifteen minutes of latitude high; roughly . The newer standard series is the 7.5-minute series. Each 7.5-minute series map covers one quarter of the area of a fifteen-minute series map, and therefore each is called a quadrangle or "quad". There are also other series, including some county maps and maps of special interest areas, such as the Grand Canyon, and there are also smaller-scale maps showing much larger areas.

The Ordnance Survey produces topographic map series covering the United Kingdom at 1:25,000 and 1:50,000. They have a mapping database from which they can print specialist maps at any scale.

The Institut Géographique National produces topographic maps of France at 1:25,000 and 1:50,000.

The Federal Office of Topography produces topographic maps of Switzerland at seven different scales.

See also

Sources of topographic maps

Last updated: 05-07-2005 17:57:35
Last updated: 05-13-2005 07:56:04