Introducing the United States National Grid
Waypoint 5Go to the FGDC USNG page (www.fgdc.gov/usng) and click on the first link to view the USNG standard (FGDC-STD-011-2001). This long document (43 pages) presents the objective, scope, applicability, and structure of USNG and detailed instructions for its use. Because this is a standards document, it mandates implementation rules. These rules are summarized below. Although dry, these rules are important, particularly Rule 4-Spatial Reference. An understanding of datums is necessary to properly apply USNG.
FGDC USNG StandardsThe Rules
(Adapted from the United States National Grid Standards Working Group Federal Geographic Data Committee, December 2001, FGDC-STD-011-2001)
Rule 1Conformance, MGRS
USNG coordinates shall be identical to the MGRS numbering scheme over all areas of the United States including outlying territories and possessions.
Rule 2Conformance, UTM
USNG basic coordinate values and numbering are identical to UTM coordinate values over all areas of the United States including outlying territories and possessions.
Numbering scheme shall be alphanumeric as follows:
- Grid Zone Designation (GZD)The United States geographic area shall be divided into six-degree longitudinal zones designated by a number and eight-degree latitudinal bands designated by a letter. Thus, each area is given a unique alphanumeric GZD.
- 100,000-Meter-Square IdentificationEach six-by-eight-degree GZD area shall be covered by a specific 100,000-meter square identified by a two-letter pair.
- Grid CoordinatesA point position within the 100,000-meter square shall be given by the UTM grid coordinates in terms of its easting and northing. For specific requirements or applications, the number of digits will depend on the precision desired in position referencing. In this convention, reading shall be from left with easting first, then northing. An equal number of digits shall always be used for easting and northing.
Rule 4Spatial Reference
The standard datum for USNG coordinates shall be the North American Datum 1983 (NAD 83) or its international equivalent, the World Geodetic System 1984 (WGS 84).
Paper maps using the USNG grid shall conform to the National Map Accuracy Standards.
For general field applications, a precision of 100 (or 10 meters) will be typical. For general applications, precision of up to one meter may be used. For special applications, USNG can provide precision greater than one meter.
Waypoint 6Visit the National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey (NGS) Web site at www.ngs.noaa.gov. At this site, click on Download Software and choose USNG from the list of utilities. Download the documentation as well as the utility.
This free DOS utility converts geodetic latitude and longitude or UTM to USNG and vice versa. When the utility is run, it prompts for direction of conversion, input datum, input coordinates, and other information. The program processes single points at the command line, and it converts properly formatted file data to create a second output file. Read the documentation carefully.
Waypoint 7Visit the National Information and Support Center for Geoaddressing (usgrid.gmu.edu) at George Mason University in Fairfax, Virginia. Bookmark this page and visit it periodically, especially the Resources area, to learn about new applications for USNG and stay in touch with others deploying and using USNG.
Watch for opportunities to use USNG coordinates and visit the sites listed in the article for updates, utilities, and new information. Visit Rescue Beyond the Rocks to download an exercise and the sample dataset that includes a polygon shapefile of GZDs that can be used with data for the continental United States in an ArcMap document to determine the USNG coordinates for a location.