**WHAT IS UNIVERSAL TRANSVERSE
MERCATOR?**

UTM Coordinates |
A Universal Transverse Mercator
(UTM) projection is a Transverse Mercator (TM) projection generated using a uniform
international system of longitude zones. There are 60 zones, each 6 degrees of longitude
in width. Zone numbers begin at the International Dateline (180 degrees longitude) and are
numbered east around the world from 1 to 60. Thus zone 30 and zone 31 are on the west and
east sides of the Greenwich meridian respectvely. Each of these zones has a Central Meridian (CM) equivalent to the longitude at the center of the zone. Thus a coordinate tranformation for any location within a specific UTM zone will use the CM defined by this zone system. For example, Houston, Texas (longitude 95 degrees west) will be in zone 15 which uses the CM -93.0, and Sydney, Australia (longitude 151 degrees east) will be in zone 56 and will use the CM +153.0. Although a Transverse Mercator projection needs a number of other variables defined before computations can be proceed, the basic purpose of the so-called UTM system is to define a uniform set of merdians to ensure consistency between maps produced with the TM projection. |

The Transverse Mercator (TM)
projection system is an arithmetic procedure for projecting latitude and longtiude
coordinates (spherical or polar coordinates) into X and Y coordinates (linear or Cartesian
coordinates). As noted above the UTM projection is a specific example of the TM system. A TM projection is a cylindrical projection, as opposed to a Lambert conic projection. It can be visualized by assuming that the earth resides in a cylindrical tube as illustrated in the sketch at left. The Central Meridian along which the projection is made touches the circumference of the cylinder, and all points on the surface of the earth within the projection zone are projected onto the surface of the cylinder. When the cylinder is unwrapped, the result is a map called a Transverse Mercator projection. It is "transverse" because the cylinder lies in a horizontal position (tangent to longitude) rather than a vertical or normal position (tangent to latitude). A TM projection also requires that a spheroid or 'earth shape' be specified (usually 'Clarke 1866' in the USA or 'International' for global maps) and that an origin for the coordinate system be defned. By default a UTM projection assumes that the units are metric, that the Y origin is the equator, and the X origin is 500,000 meters at the CM. There are many variations on the TM projection, including a number of dedicated projections used by the USGS in the State Plane coordinate system. The UTM is just one of the variations of the TM projection. The UTM has become a convenient standard for global mapping because it simplifies many of the complexities of making one map conform to another. Complete details and sample calculations for UTM's are available in the USA from USGS Survey Bulletin 1532. |

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