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The purpose of the USNG according to the FGDC, is to "create a more interoperable environment for developing location-based services within the United States and to increase the interoperability of location services appliances with printed map products by establishing a nationally consistent grid reference system as the preferred grid for NSDI applications. The U.S. National Grid is based on universally-defined coordinate and grid systems and can, therefore, be easily extended for use worldwide as a universal grid reference system."
In addition, it has the advantage of being easily plotted on USGS topographic and other properly gridded maps by using a simple "read right and then up" convention where the user measures to the East and then North in linear increments. The coordinates are easily translated to distance as they are actually in meters rather than the more complex degree based increments of latitude and longitude. Thus the distance between two coordinates can quickly be determined in the field.
USNG 1 km2 grid statewide polygons for select states can be downloaded from http://usngcenter.org. The website also provides more detailed information about the US National Grid.
USNG allows a common system based on the US Military Grid system using landmarks like hills or streams, anything permanent. It allows these maps to be printed anywhere by responders totally unfamiliar with the local area. The degree of precision is determined by the number of digits in the coordinates, 23 67 = 1000 meters, 23451 67345 = 1 meter.