There are many ways of representing locations. One of the most common uses angular latitude and longitude coordinates to specify the location of a point. These Geographic coordinates may be expressed in several different notations, including Decimal Degrees (DD), Degrees Decimal Minutes (DDM), and Degrees Minutes and Seconds (DMS). The formula for converting Degree Minutes and Seconds to Decimal Degrees is discussed in the ArcGIS 10 Help. Another common method is to use a projected coordinate system and X and Y coordinate pairs, that is, two sets of numbers in a planar Cartesian system.
Gridded systems, like Universal Transverse Mercator (UTM), United States National Grid (USNG), Military Grid Reference System (MGRS) divide the world into zones which are projected and then overlaid with one or more grids. Within the grids, point locations are specified by their position East and North the zone origin or the southeast corner of the cell. For USNG and MGRS the numeric values of the northing and easting can vary from 1 to 5 digits, depending on the precision of the location (they must both use the same number of digits) and then the values are concatenated.
Two less frequently used systems, the Global Area Reference System (GARS) and Geographic Reference System (GEOREF), divide the world into polygonal areas based on different sets of nested grids and encode locations as a string of grid ID values.
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