I previously posted a primer on how to read MGRS coordinates. One of the readers posed an interesting question regarding if we round or truncate MGRS coordinates. The short answer is that we always truncate MGRS coordinates.

Why? Continue reading after the break to find out!

Rationale:

In the primer I explained that we need to think of the MGRS coordinate system as a square region based on the precision given.

Let’s say a coordinate comes in with 1m precision, 1367507896. To report this, we need to convert it to 10m precision. Here’s how it looks if we round vs. truncate:

• Rounded: 13680790
• Truncated: 13670789

Note that our objective originally reported isn’t contained by the rounded search area coordinates. This isn’t a very compelling argument, since the objective isn’t much outside the search area. Let’s start looking at more compelling arguments.

The incoming coordinate is now 13670789. The reporter says he isn’t sure of the 10m precision, so the commander orders a search of the 100m area. Rounded vs Truncated:

• Rounded: 137079
• Truncated: 136078

Now we see how much rounding can hurt a search for an objective. But you might not still be convinced that we should be truncating vs. rounding. So I’ll throw my ace in the hole out there to you.

We receive a report that our objective is somewhere in the vicinity of 136078, but our reporter doesn’t have an accurate GPS signal. Our commander sends out his forces to scour the entire 1 km grid square. Rounded vs. Truncated:

• Rounded: 1408
• Truncated: 1307

So now we have an angry unit and commander cause they just spent hours wasting their search efforts in the entire wrong spot.

I welcome all comments/feedback!

Ben (benjamin.post@primordial.com)

Purpose: To provide Primordial employees (and web surfers) a quick introduction and familiarization to the Military Grid Reference System (MGRS).

Direction: Read and understand this document and if needed, direct questions to Benjamin Post. (benjamin.post@primordial.com)

Motivation: Given Primordial’s product line and customer base, it is paramount that our employees understand how our customers reference locations in throughout the world. MGRS is the de facto standard for reporting coordinates in the United States military, and has also been adopted by NATO.

Background

MGRS is the geocoordinate standard for all NATO militaries. Barring naming conventions, the actual grid between 80˚ south and 84˚ north is identical to Universal Transverse Mercator (UTM). So, if you are familiar with UTM coordinates, then really you just need to learn the language of MGRS to interpret locations. The grid for the polar regions is based on the Universal Polar Stereographic system, but with the low probability of ongoing military operations in these regions we will ignore these regions for now. Modern MGRS is based on the WGS84 geodetic datum.

MGRS Coordinates

Now that we are familiar with the background of the MGRS, let’s dive into breaking apart an actual MGRS Coordinate.

15T VK 86049 79921 – Places us at the cursor location seen below, which FYI is right outside our office in the parking lot. (Notice the lower right corner of the pic?)

15T represents the Grid Zone designation, which represents an area that is approximately 6˚ wide and 8˚ tall relative to Latitude and Longitude. (Pretty big.)

VK represents the 100,000 meter square identifier. Pretty simple, it covers a 100,000 meter square. It is important to note that these regions are not perfectly square, nor are they always 100,000 meter squares.

86049 is the Easting value within VK. This is the horizontal position within VK, meaning that we are 86049 meters east from the western edge of VK.

79921 is the Northing value within VK. This is the vertical position within VK, meaning that we are 79921 meters north from the southern edge of VK.

Here’s how we read the MGRS Coordinate:

1. Locate the Grid Zone designator. (Hint: 1 is the International Date Line heading East and ends with 60 back at the International Date Line, and letters go up from 80˚S starting with C.)
2. Locate the 100,000 meter square designator. This uses the AA format, meaning that the indentifiers will cycle through letters of the alphabet. (O and I are omitted, consider why for a moment.) This is far too difficult to explain, so here is a picture to help. Notice that A, H, J, R, S, and Z are used for oddball identifiers along the east and west edges…Squint harder. [Update: Thanks to Mikael Rittri, who made the MGRS grid in SpatialAce of Carmenta, with a background map from Natural Earth by  Tom Patterson, US National Park Service.]
3. Now we get to the Easting and Northing. In the military we read this as “left to right, then up.” I have also heard to it referred to as “in the house and up the stairs.” Either way, here’s how it is done using our string from before. Did you forget it already? 15T VK 86049 79921
   1. 86049 79921 – 86,79 represent the gridlines we want to find. So we locate the intersection of these two lines. (Remember-left to right, then up.) Going horizontally we locate 86, then following that gridline up vertically we locate 79.
   2. 86049 79921 – Now here we want to go 049 meters horizontally, followed by 921 meters vertically.
4. Congrats…

MGRS Precision

Not much can be done about the Grid Zone designation and very little will affect the 100,000 meter square identifier, but the number of digits used in the easting and northing will vastly affect precision. Each digit represents a magnifier of 10 in determining the precision of a given coordinate. Let’s break it down, shall we?

Easting and Northing can come in 2, 4, 6, 8, and 10 digit strings, and more digits equal greater precision. Here it is best to picture the coordinate as providing a square area versus an actual point.

2 digits represent a 10km X 10km square. While it is highly unlikely that we will ever see this in use, we must be aware that it is possible to report this way.

4 digits represent a 1km X 1km square. This is significant in that it actually identifies a grid square. If we are given the following string 15T VK 8679 as coordinates for an enemy compound, then we must assume that the compound is located anywhere within the grid square designated by 86,79. While this may be acceptable for easily identifiable features, i.e. mountains, lakes, villages, etc. It is inadequate for actual point locations.

6 digits represent a 100m X 100m square. While this is not ideal if you absolutely want to be located, or to locate a specific person of interest, it is good if you are in a pinch or cannot accurately determine your exact location. If we are given a coordinate string of 15T VK 860799, then the location of interest is located in the following area.

8 digits represent a 10m X 10m square. This is the standard for reporting point locations in the military, so get used to seeing this. Here’s what it looks like if we are given the following string. 15T VK 8605 7992 – Notice rounding occurred in the easting value. If you can’t find what you are looking for in this location…God help you.

10 digit values represent 1m X 1m, which is about as close to a point as you can get. This is not widely used in the military due to inaccuracies in GPS reporting devices. Maybe they should upgrade to farm grade GPS receivers, those things are crazy accurate. No need for a picture as this is the point of the cursor.

The National Geospatial Agency provides a compelling argument as to why military coordinates should be limited to 6 and 8 digit easting and northings. Here is what they have to say from their Military Map Reading 201 guide.

“An often-confusing point is that most NIMA 1:50,000 scale maps are made to an accuracy of 50m at the 90% confidence interval. This means that 90% of all well-defined points on a map will fall within a 50 meter radius of their actual position on the earth’s surface. The confusion comes in when soldiers try to use a map to get a 10-digit grid coordinate, which equates to a 1 meter precision. A 1:50,000 scale map is only accurate to 50m 90% of the time so a 6 digit (100m precision) or an 8 digit (10m precision) are more appropriate.”

Conclusion

Hopefully this primer has you ready to dive into the world of MGRS coordinates and military map reading. It might seem a little intimidating at first, but after a little studying of the basics, you will find that it makes sense. I hope to expand on this primer by offering a class or perhaps authoring another reading on how to use a military protractor to determine locations on military maps and other land navigation practices.

References:

http://en.wikipedia.org/wiki/Military_grid_reference_system

National Geospatial Agency’s Military Map Reading 201

Of course the legendary Army Land Navigation Field Manual (FM 3-25.26)

My Brain…Access is limited, but space is available!

- Benjamin Post