The US National GRID (USNG)
Lately I’ve been hearing more and more about the US National Grid (USNG), particularly where it relates to emergency response. We’re working on a project with a 13-county emergency response group and we’ll need to incorporate USNG. I thought I’d say a thing or two about it while it’s fresh in my mind.
First, USNG is not a projection. If I hear one more person call USNG a projection, I may kick my dog.* USNG is a notation or presentation standard. It is an alpha-numeric numbering overlay and it uses the Universal Transverse Mercator (UTM) projection. In other words, USNG is a grid painted on top of UTM. UTM is the projection. USNG is the paint.
USNG is used to describe spatial areas in a standard manner so that emergency workers are communicating in the same language during an emergency. It was made a standard at the end of 2001 (by DHS and managed by the FGDC) and in 2005 the Department of Homeland Security (DHS) recommended that any DHS grant submission reference the use of the USNG. USNG derives from the Military Grid Reference System (MGRS), it’s used in pretty much all modern GPS equipment, and I refuse to use another acronym for the rest of this discussion.
Basically, instead of reporting a location with an X and Y coordinate ala Euclid, you use an algorithm to render the location as a single string. For example, the Washington Monument would be 18SUJ2348306479.
Here’s where I’ll rant for a second. 18SUJ2348306479 doesn’t mean anything to me. I’d be surprised if it means anything to you. If you called me and told me you were stranded at 18SUJ2348306479, I’d say you were going to be there for a while. I’ve asked a number of people how this is a more intuitive representation of a location than a simple X and Y, and I have yet to hear a satisfactory answer. But I digress. The fact is during an emergency I’ll probably dive under my bed at 17RUN3938271 and won’t be of much help anyway.
Where USNG makes things a bit more complicated for GIS is in the fact that it isn’t a projection. If it were, using it would be straightforward. While figuring out the USNG representation of a location isn’t technically challenging, it isn’t straightforward either.
The US is divided into 6-degree longitudinal zones designated by a number and 8-degree latitudinal bands designated by a letter. Each area is given a unique alpha-numeric Grid Zone Designator (GZD) (i.e. 18S). Each GZD 6x8 degree area is covered by 100,000-meter squares where each square is identified by two unique letters. (i.e. 18SUJ). A point position within the 100,000-meter square is derived from the UTM grid coordinates Easting (E) and Northing (N). An equal number of digits are used for E and N where the number of digits depends on the precision desired in position referencing, with Easting first and Northern second. So, if you saw 18SUJ2102, it would be GZD 18S, 100,000 meter square UJ, 21 Easting, 02 Northing. Oh my.
The number of numbers after the letters determine precision - remember we’re technically talking grids, not points. 18SUJ20 is 10 km. 18SUJ23480647 is 10 m.
There are a number of ways to deal with USNG and different solutions for different software. Let’s talk about those next month.
*Technically I don’t have a dog. My wife and I have three cats. I think dogs are generally more likable and manly companions, however, so I refer to my cats as dogs. I don’t think our dogs are on the same page (they refuse to “fetch” and still look upon me as a glorified can opener), but it makes me happy.