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Universal Transverse Mercator Projections The most widely used method used to identify points on the curved surface of the Earth is with a system of reference lines called parallels of latitude and meridians of longitude. Difficulties arise in transferring positions and the reference lines of longitude and latitude to a flat surface. On some maps, the meridians and parallels appear as straight lines and on others the meridians and parallels appear as curved lines. These differences are due to the mathematical treatment required to portray a curved surface on a flat surface so that important properties of the map (such as the portrayal of distance and relative position) are shown with minimum distortion. The system used to portray a part of the round Earth on a flat surface is called a projection. To simplify the use of maps and to avoid the inconvenience of pinpointing locations on curved reference lines, cartographers superimpose on the map a rectangular grid consisting of two sets of straight, parallel lines, uniformly spaced, each set perpendicular to the other. This grid is designed so that any point on the map can be designated by its latitude and longitude or by its grid coordinates, and a reference in one system can be converted into a reference in another system. Such grids are usually identified by the name of the particular projection for which they are designed. The Universal Transverse Mercator projection is designed to provide a single grid system that can be applied to the surface of the earth. In this projection, the world is divided into 60 north-south zones, each covering a strip 6° wide in longitude. These zones are numbered consecutively beginning with Zone 1, between 180° and 174° west longitude, and progressing eastward to Zone 60, between 174° and 180° east longitude. Each zone is divided into horizontal bands spanning 8 degrees of latitude. These bands are lettered, south to north, beginning at 80° S with the letter C and ending with the letter X at 84° N. The letters I and O are skipped to avoid confusion with the numbers one and zero. The band lettered X spans 12° of latitude. A square grid is superimposed on each zone. It is aligned so that vertical grid lines are parallel to the centre of the zone, called the central meridian. In each zone, coordinates are measured north and east in meters. The northing values are measured continuously from zero at the Equator, in a northerly direction. To avoid negative numbers for locations south of the Equator, the Equator has been assigned an arbitrary northing value of 10,000,000 meters. A central meridian through the middle of each 6° zone is assigned an easting value of 500,000 meters. Grid values to the west of this central meridian are less than 500,000; to the east, more than 500,000. An easting of zero will never occur, since a 6° wide zone is never more than 674,000 meters wide. Minimum and maximum easting values are: 160,000 m E and 834,000 m E at the equator and 465,000 m E and 515,000 m E at 84° N.
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UTM is therefore a single grid co-ordinate system that applies to 60 separate projections that in total can be used to map almost all of the surface of the Earth. If you are interested in the mathematics required to translate positions between one co-ordinate system and another then start at http://www.posc.org/Epicentre.2_2/DataModel/ExamplesofUsage/eu_cs34.html |
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