Universal Transverse Mercator coordinate system
From Freepedia
The Universal Transverse Mercator (UTM) projection system is a grid-based method of specifying locations on the surface of the Earth. It is used to identify locations on the earth, but differs from the traditional method of latitude and longitude in several respects. The UTM system is not a map projection, but rather employs a series of zones based on specifically defined Transverse Mercator projections.
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History
The Universal Transverse Mercator coordinate system was developed by the United States Army in 1947. The system was based on an ellipsoidal model of the Earth. For areas within the conterminous United States, the Clarke 1866 ellipsoid was used. For the remaining areas of the Earth, including Hawaii, the International Ellipsoid was used. Currently, the WGS84 ellipsoid is used as the underlying model of the Earth in the UTM coordinate system.
Prior to the development of the Universal Transverse Mercator coordinate system, several European nations demonstrated the utility of grid-based conformal maps by mapping their territory during the interwar period. Calculating the distance between two points on these maps could be performed more easily in the field (using the Pythagorean theorem) than was otherwise possible using the trigonometric formulas required under the graticule-based system of latitude and longitude. In the post-war years, these concepts were extended into the Universal Transverse Mercator / Universal Polar Stereographic (UTM/UPS) coordinate system, which is a global (or Universal) sytem of grid-based maps.
Definition of a UTM zone
The UTM system divides the surface of the Earth between 80° S latitude and 84° N latitude into 60 zones, each 6° of longitude in width and centered over a meridian of longitude. Zones are numbered from 1 to 60. Zone 1 is bounded by longitude 180° to 174° W and is centered on the 177th West meridian. Zone numbering increases in an easterly direction.
Each of the 60 zones in the UTM system is based on a specifically defined Transverse Mercator projection, which is capable of mapping a region of greater north-west extent with a low amount of distortion. By using narrow zones of 6° in width, and reducing the scale factor along the central meridian to 0.9996, (a reduction of 1:2500) the amount of distortion is held below 1 part in 1,000 inside each zone. Distortion of scale increases to 1.0010 at the outer zone boundaries along the equator.
The reduction in the scale factor along the central meridian creates two lines of true scale located approximately 180 km on either side of, and approximately parallel to, the central meridian. The scale factor is too small inside these lines and too large outside of these lines, but the overall distortion scale inside the entire zone is minimized.
Locating a position using UTM coordinates
Positions are located by measuring the distance along the projected surface from the central meridian and the equator. In order to avoid dealing with negative numbers, the central meridian of each zone is given a "false easting" value of 500,000 meters. In the northern hemisphere, positions are measured northward from the equator, which has an initial "northing" value of 0 meters. In the southern hemisphere, northings are measured southward from the equator, which is given a "false northing" of 10,000,000 meters so that no point within the zone has a negative northing or easting value.
Overlapping Grids
Distortion of scale increases in each UTM zone as the boundaries between the zones are approached. However, it is often convenient or necessary to measure a series of locations on a single grid when some are located in two adjacent zones. Around the boundaries of large scale maps (1:100,000 or larger) coordinates for both adjoining UTM zones are usually printed within a minimum distance of 40 km on either side of a zone boundary. Ideally, the coordinates of each position should be measured on the grid for the zone in which they are located, but because the scale factor is still relatively small near zone boundaries, it is possible to overlap measurements into an adjoining zone for some distance when necessary.
External links
- U.S. Geological Survey UTM Grid Fact Sheet
- National Geodetic Survey (U.S.) UTM Utilities
- [1] TM8358.2: Defense Mapping Agency Technical Manual 8358.2 The Universal Grids: Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS)]
- Converting Latitude/Longitude to Universal Transverse Mercator (UTM)
- UTM Zones
- UTM conversion library written in Perl
References
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