GPS Tip: What is USNG?
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Thu, 02 Mar 2006 20:50:01 GMT
GPS TIP OF DAY
http://www.mobilecrossing.com/support/tips.html
Wednesday - March 1, 2006
GPS Tip: What is USNG?
The USNG is the United States National Grid. It is identical to the
military MGRS, Military Grid Reference System, and is based on the UTM
grid previously discussed. Like UTM, USNG divides the earth into 60
zones and uses an offset center line for the measurements which are
also done in meters. However, the military has also divided the world
into maps that are each 100Km on a side (62.5 miles). Each map is given
a two letter designation that is systematic but arbitrary. Its origin
is at the equator on the international dateline so by the time you get
to the US borders you probably have no idea what the 2 letters for a
map for your area might be. For San Francisco the map letters are EG
and will continue to increment eastward to FG, and GG. Crossing a zone
line will cause the map sequence to jump. Within a map the grid numbers
are identical to the least significant 5 digits in UTM easting and
northing. In addition the USNG system uses the same zone numbers and
letters as UTM. A place in San Francisco might have a USNG designation
of 10SEG 4851282065. The 10S is the UTM zone while the EG is the name
of the map. The remaining numbers are divided in half with the first 5
numbers showing the easting and the second half showing the northing.
As you move north from San Francisco the maps would have a map
designation of EH and then EJ since the letters I and O are not used.
The easiest way to figure out the map letters is to use a program. The
US government provides an online service and a DOS program to convert
UTM or Lat/Lon values to USNG (link:
http://www.ngs.noaa.gov/TOOLS/usng.html), but I prefer the one from
Mentor Software, Inc. (link:
http://www.mentorsoftwareinc.com/FREEBIE/FREE0100.HTM). Note MGRS =
USNG. This program uses only an N or S in the UTM zone field. N is for
the northern hemisphere and S for the southern. Officially USNG uses
NAD83 as the datum but WGS84 as generated by a GPS is accepted as is
NAD27 but you have to list the datum in the reference for NAD27.
While 10 digits provide 1 meter precision you can reduce the precision
of USNG by chopping off pairs of digits. For example if you only wanted
10 meter accuracy you could use 48518206 in the above example for San
Francisco or even 4882 for 1 Km. You must always use an even number of
digits so it can be divided into two halves for the easting and
northing.
.
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