MGRS – Truncate vs Round?

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)

Tags: Ground Guidance, MGRS, Military Grid Reference System, Military Map Reading, Primordial