Topographic map

From Free net encyclopedia

Image:Topographic map example.png 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 (isohypse). In other words, every point on the marked line of 100 m elevation is 100 m above mean sea level.

Image:Topographic-Relief-perspective-sample.jpgThere 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 is a consequence of erosion.
The rule of Os: closed loops are normally uphill on the inside and downhill on the outside, and the innermost loop is the highest area. If a loop instead represents a depression, some maps note this by short lines radiating from the inside of the loop, 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.

1 Sources of topographic maps
2 Global 1-kilometer map
3 See also
4 Sources of topographic maps
5 External links

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Sources of topographic maps

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United States

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 with a scale of 1:24000. 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". USGS topographic quadrangle maps are marked to make it possible to identify locations according to the Public Land Survey System in addition to latitude and longitude and UTM grids. 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.

A number of Internet sites have made these maps available on the web for affordable commercial and professional use. Because works of the U.S. Government are in the public domain, it is also possible to find many of these maps for free at various locations on the Internet, as well as DEM datasets that digitally describe topology.

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United Kingdom

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.

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France

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

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Switzerland

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

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Global 1-kilometer map

This map is derived from GTOPO30 data that describes the elevation of Earth's terrain at intervals of 30 arcseconds (approximately 1 km). It uses color and shading instead of contour lines to indicate elevation.

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Image:Topographic30deg S0W150.png	Image:Topographic30deg S0W120.png	Image:Topographic30deg S0W90.png	Image:Topographic30deg S0W60.png	Image:Topographic30deg S0W30.png	Image:Topographic30deg S0W0.png	Image:Topographic30deg S0E0.png	Image:Topographic30deg S0E30.png	Image:Topographic30deg S0E60.png	Image:Topographic30deg S0E90.png	Image:Topographic30deg S0E120.png	Image:Topographic30deg S0E150.png
Image:Topographic30deg S30W150.png	Image:Topographic30deg S30W120.png	Image:Topographic30deg S30W90.png	Image:Topographic30deg S30W60.png	Image:Topographic30deg S30W30.png	Image:Topographic30deg S30W0.png	Image:Topographic30deg S30E0.png	Image:Topographic30deg S30E30.png	Image:Topographic30deg S30E60.png	Image:Topographic30deg S30E90.png	Image:Topographic30deg S30E120.png	Image:Topographic30deg S30E150.png
Image:Topographic30deg S60W150.png	Image:Topographic30deg S60W120.png	Image:Topographic30deg S60W90.png	Image:Topographic30deg S60W60.png	Image:Topographic30deg S60W30.png	Image:Topographic30deg S60W0.png	Image:Topographic30deg S60E0.png	Image:Topographic30deg S60E30.png	Image:Topographic30deg S60E60.png	Image:Topographic30deg S60E90.png	Image:Topographic30deg S60E120.png	Image:Topographic30deg S60E150.png
1800x1800, 1MB. 60px=1min. 1px=1sec.