UTM/MGRS introduction

I got a lot of positive feedback when I posted this in another thread (thanks – appreciated!). The kind words made me feel obliged to improve upon it, so I decided to add some graphics, clean it up and make it a thread of its own in order to make it easier to find and understand.

Background reading

I’d like to begin by pointing you to Military Map Reading 201 by the NGA. It is well worth a quick read. Fifteen minutes, and it will all come clear to you. If it doesn't, I'll instruct over MSN if need be - that's how confident I am in you really getting it. :)

In the beginning, there was UTM

The world is divided into large grid zones. They're given a number from 1 to 60 based on their east-west position (longitude) and a character based on their north-south position (latitude).

You can then give any coordinate as a combination of a grid zone and the distance east and north from the southwestern corner of that grid zone (it's really a bit more involved, but that's the general idea). 38T and two longish figures, for example. This is the UTM system.

MGRS grid squares

However, UTM is rather unwieldy to work with, especially if you are out in the field trying to call in arty support over the radio, or in an aircraft trying to communicate the position of a target or friendlies on the ground. To make it easier, the MGRS was introduced. Each grid zone is divided into a large number of 100.000 by 100.000 meter grid squares. These grid squares are given designators based on their north-south and east-west position within the grid zone, using one character for north-south and one character for east-west. AM, MM, DL etc.

Now you can point out an arbitrary 100.000 by 100.000 meter square anywhere on the planet, using a designator such as 38T ME.

Coordinates within the squares - eastings and northings

From there, we only need to be able to designate a certain smaller square within that square, in order to point out targets. You do this just in the same way as you did within the grid zone using UTM, by measuring the distance east and then north of the southwestern corner of the grid square. The two distances describe the position within the square and are known as the easting and the northing.

As the square is 100 km across, you need two figures per coordinate (east/north) to give 1 km precision (0-99 km east/north of SW corner). If 10 km precision is enough (unlikely ;)), you only need one figure. The first ten km east/north of the SW corner is then 0, the next ten 1 etc.

For pointing out targets, you will generally need to get down to ten meter squares. As there are 100.000/10 ten-meter squares to each side of the 100.000 m two-character grid squares, you then need four digits (0-9999) for the easting, and another four for the northing.

For one meter precision, you need five digits.

Putting it all together

A full MGRS coordinate consists of the grid zone, grid square and finally the easting and the northing written together as one long string of digits, with the easting first (XY, not YX). Frequently, the grid zone is omitted once it is assured that everyone else is using the same grid zone.

38T ME04586742

would mean a ten meter square (four digits for easting/northing respectively) within grid square ME in grid zone 38T.

To find the position, you'd divide square ME into ten meter squares and find column 458 from the western side. Then you'd find row 6742 from the southern edge of square ME. In reality, there are supporting lines in the maps making finding these coordinates easy. You're really finding a point 4580 m to the east and 67420 meters to the north, and then you know the object for which the coordinate is given to be in the 10 by 10 m square to the NE of this point.

Examples

I've attached two map images. In both maps, the two fixed points A and B are plotted. Also notice how you have four different grid squares in the map images, each with a different two letter designator.

In one picture the coordinates are given using two figures for easting and northing, respectively. Point A is in this case at grid YH2704 – easting 27, northing 04 in grid square YH.

In the other picture, the coordinates are given using three figures for easting and northing. Point A is in this case at grid YH278043. It is still in exactly the same position as in the previous example, but the position is given with a higher degree of accuracy.

Note how the easting is truncated instead of rounded, i e 278 becomes 27 rather than 28 when going to a lower precision. This is due to the position always being in the appropriately sized square NE of the designated point, as previously described.

The coordinates of point B are left as an exercise for the reader.

(Ignore the negative coordinates on the axes in the pictures. MGRS coordinates are never negative, I just couldn’t be arsed to edit them out after clipping the pictures from my drawing program of choice Excel . Yes, I’m a worthless artist.)

Clear as mud? Didn't think so. Go read the PDF and look at the graphics. Told you it's worth it!

DCS caveats (obsolete due to bug having been fixed)

Visualization

If you want to visualize the MGRS grid system in Google Earth, you can download a KML file which does this.

Anecdotes

"There were ten of us, living in a shoebox in the middle of the street..."

If your head is spinning at this point, consider this: I fly and work using WGS84 (lat/lon). I initially learned one coordinate system in the army, only to later transition to MGRS which I still use for weekend warrior duty. Now, I also fly fire watch missions. When coordinating with the fire services on the ground, we use the first coordinate system I learned as they haven't switched to MGRS. Coincidentally, that coordinate system gives the northing before the easting, but calls the northing X and the easting Y. Now, try to get all that sorted out while circling above a fire in a very warm cockpit being jostled by turbulence and trying to find forest roads to guide the fire trucks in along using a radio with very poor reception... :pilotfly:

Please let me know if you have any suggestions for clarifications or improvements, or if my Airman's Prayer ("Please don't let me screw this up!") didn't pay off. ;)

Cheers,

Fred

There are some USNG training materials at http://publicintelligence.net/us-national-grid-training-information-and-maps/

The grid zone and square designations are different between the two, I think, but follow similar patterns. The grid coordinates should be (functionally) the same, as they're both based on UTM. Learning to read one ought to be useful in learning to read the other.

Thanks fellas, great info

awesome, thanks effte. +1 inbound.

bartleby,

good find. While there are differences (the first and foremost being the USNG only covering the US of A, but for our purposes that's a technicality), the basic principles are the same so the teaching material there applies.

Be warned that if you learn from USNG and start putting spaces between your grid squares, eastings and northings (as I did above after 38T - we're doing it non-standard I guess ;)) people will cringe in military contexts. Allowed in USNG, not allowed in MGRS.

Practical condensed UTM explanation:

Think of it as if you're looking at a paper mapsco book.

(This isn't literal, but makes it practical)

MM 12345 67891

MM = page

123 = Squares down the page

45 = meters down the '123' square

678 = Squares across the page

91 = meters across the '678' square

They may give you MM123678. This gives you the grid but not the exact point within that grid, which is fine because it's only 99 meters.

If the coords were MM 12345 06789 they may give you MM12367 because they drop the leading zero from 067 (on the bug list). You must still enter the 0 before 67 when entering into the CDU.

You can enter UTM into your CDU by switching from L/L (lat/long) mode to UTM mode with LSK R9 (bottom right key). Then by entering the UTM coords in ten number, six number, or less, and selecting LSK L9 (bottom left key) to save them to the waypoint currently showing on the CDU.

For more info see Military Map Reading 201 at the Geo-spacial Intelligence Website.

bartleby,

good find. While there are differences (the first and foremost being the USNG only covering the US of A, but for our purposes that's a technicality), the basic principles are the same so the teaching material there applies.

Well, besides that little difference... :)

I thought I'd add the link to the Book on MGRS, for those who want to go to the source.

DMA Technical Manual; Datums, Ellipsoids, Grids, and Grid Reference Systems. NGA, 1996.

effte. Thanks for your explanations. One mission was getting me CRAZY !!!

There is a possibility that it will also drop zeroes from the northings, ie AB234012 will be given as AB23412. This is to the best of my knowledge unconfirmed, but if you get coordinates which are simply way out into nowhere, please jot them down along with other coordinates given and report.

Well, now it is confirmed... fortunately I found this thread, and was able to see where the coords were screwed..

see my track. I have to add a 0 to the northings, that means before the last two digit of the JTAC coordinates, to get them right. Hope ED can fix this.

regards,

Patrick

unrar this file for the track (JTACrarebug.trk)

http://www.megaupload.com/?d=HRRTDJIH

Immel,

great job confirming our suspicion. I can't download the track though - Megaupload says it is unavailable? Can't you attach it through the forum? It would be interesting to see.

Still haven't seen this acknowledged by ED anywhere in the forums though. Have I missed it, or should we try to bring it to their attention? Both the dropped leading zeroes and the missing grid zones.

Cheers,

Fred

Immel,

great job confirming our suspicion. I can't download the track though - Megaupload says it is unavailable? Can't you attach it through the forum? It would be interesting to see.

 

Still haven't seen this acknowledged by ED anywhere in the forums though. Have I missed it, or should we try to bring it to their attention? Both the dropped leading zeroes and the missing grid zones.

 

Cheers,

Fred

here it is, attached to the forum !

Have a look at it. I was getting crazy trying to add 0 at beginning, and at the end, before finding your thread. It's actually missiong the 0 to the Northings, before the last 2 digits.

JTACrarebug.rar

bump

my FAC gives me LN46899. I cant put this in my cdu. After reading this, it must mean I have to put LN046899. Good info thanks guys

Or LN468099

try both. see what works.

Thanks for posting this! It was very helpful

If both work, the closer is the right one, the other will be very far most of the time.

So if JTAC gives me MN35567 - what do I have to enter in the CDU?

Look for your carrent MGRS position on TAD. I mean JTAC and designated target are relatively near you.

So if your carrent MGRS position is MN03305430 on TAD, and you get MN35567 (it's a bag that omitts zeros) from JTAC, then obviously you must enter MN03505670 in CDU

Basically we all expect to be corrected with a patch

So if JTAC gives me MN35567 - what do I have to enter in the CDU?

MN035567 or MN355067 - whichever is closest to your present position. It is two groups of three, and a leading zero has been dropped from one group.

So is it droping only leaqding zeros ?

That's my understanding, just the leading zero of either the Northing or the Easting group.

Added the Google Earth visualization of the MGRS found by Warthogger and linked in another thread to the original post.

Cheers,

Fred