MGRS?

[Doug Rinckes]

In the last couple of days, I've been asked why a new location system is needed when Military Grid Reference System exists. I'm interested in what you think. (There's a site, http://dominoc925-pages.appspot.com/mapplets/cs_mgrs.html, you can use to pick and enter MGRS codes.)

What was it's aim?

Would you use it to provide addresses (like homes or businesses)? What about arbitrary locations (fountain, meeting place, wells, disaster relief situations)?

What are it's good points? Does it have any bad points?

Dubai (see http://www.mylocation.ae) has adopted it to provide building IDs. What do you think of that?

Doug

[Doug Rinckes]

OK, that's nobody then. :-)

Here are my thoughts based on a reading of MGRS on wikipedia, a couple of other docs, and spending some time playing around with it. This is the alternative I get asked most about, second only to w3w.

I'd be really interested if I've made any mistakes, so please chime in.

MGRS was developed by the US military, sometime before 1956. The aim was to allow soldiers to identify their location using just a paper map and a protractor. MGRS was almost exclusively used by the military, but from 2008 the US Geological Survey started including MGRS coordinates on US domestic maps.

MGRS is good if you want to get a reference from a paper map, give it to someone, and allow them to find that on a paper map.

I think that MGRS has some disadvantages:

1. The same point can have multiple MGRS codes
   
2. MGRS codes that are physically close can be quite different
   
3. You can't remove characters from the end of MGRS codes - you need to understand the structure

MGRS has the most complex structure of the location systems I've looked at so it's worth taking a bit of time to outline it. An example MGRS code for 10 meter x 10 meter area is 40R CN 1234 5678:

1. 40R identifies a 6x8 degree area (called a grid zone)
2. CN is a 100x100km square (termed a 100,000 meter square) within that area
3. 1234 5678 is a 10x10 meter area 1234 meters east and 5678 meters north of the south west corner of the square.  The number of digits defines the size of the area - 123 456 would mean a 100x100 meter area, 12345 67890 means a 1x1 meter area.
   

If we know that we're talking about 40R CN, we can drop that, and just have the eight digits. This is short but not as short as OLC can do.

MGRS codes combine degrees with meters, but a degree is not a constant meter width. This means that at the grid zone boundaries, the 100,000 meter squares are going to overlap. This is where the fun is.

Below is a an illustration of where the grid zones 15S, 15T, 16S and 16T intersect. (This is roughly in the middle of Chicago and St Louis.)

The diagram shows the 100,000 meter squares XE, YE, BK, YD and BJ.

• XE extends over the boundary of 15S and 15T
  
• YE (outlined in green) and BK (outlined in purple) fall into all four grid zones
  
• YD is only included in 15S but it extends into 16S.
  
• BJ is only included in 16S but it extends into 15S.
  

All this overlapping means that individual locations have more than one MGRS code. Software libraries that do the conversion should figure this out and do the right thing, but

A specific aim of Open Location Code was to allow two people to visually compare codes and tell if they are close together. But at the borders of MGRS grid zones, this doesn't work. For example, 15SYD500500 is right next to 16SBJ588500.

If you remove a single digit from the end of an MGRS code (990115), it's no longer valid, since if there are an odd number of digits, it doesn't know how many are easting and how many are northing. If you remove multiple digits, say two, you will get a valid code, but in a different place. For example, 123456 (12.3 km east, 45.6 km north) becomes 1234 (12 km east, 34 km north).

Lastly, yes, I know Dubai is using MGRS easting and northing for building references. It works for them because the entire city is within the reference 40RCN. That's great until you get to Sharjah. It's city is connected to Dubai, but in 40RCP, so the system can't be extended to them.

They could have done better. I think OLC is a way to solve the problem in a better way.

[Doug Rinckes]

Oops - I copied in the image from a document and it's only done it as a link.

Here it is so everyone can see it.

[joseph...@gmail.com]

I just learned about Dubai and MGRS and in poking around stumbled over this thread.

In the USA, FEMA had adopted the U.S. National Grid (USNG) as the standard coordinate system for ground-based search-and-rescue (SAR).  USNG has also been adopted as a standard by Florida, Minnesota and several other states.

USNG = MGRS except USNG is written with spaces for easier readability.

Downtown Seattle
10T ET 5007 7276  USNG
10TET50077276  MGRS

For more on USNG see http://usngcenter.org/

I am a software developer and have produced an enhanced Google map viewer called Gmap4 that, in addition to other features, can display a USNG grid down to 100m x 100m.  The following link starts Gmap4, opens centered on Seattle and displays a 10km USNG grid.  Keep zooming in and the grid will change.
http://www.mappingsupport.com/p/gmap4.php?usng=10T_ET_4721_7207&tilt=off&z=11&t=h

Joseph

[Doug Rinckes]

Wow Joseph, that's really nice! It surprised me when researching the different systems that implementations showing MGRS or USNG coordinates were so thin on the ground. Nice one!

Doug Rinckes

Technical Program Manager

Google Switzerland

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