The short answer
The United States National Grid (USNG) is built on the Military Grid Reference System (MGRS). On NAD83 or WGS84, a USNG grid and an MGRS grid for the same place have the same letters and numbers: 17R MM 63501 57423 is valid in both.
| USNG | MGRS | |
|---|---|---|
| Defined by | US Federal Geographic Data Committee standard FGDC-STD-011-2001 | US military and NATO mapping |
| Coverage | The United States and its territories | Worldwide, including the Norway exceptions and polar regions |
| Default datum | NAD83, with WGS84 acceptable | WGS84 by default, and any datum in practice |
| NAD27 grids | Letters don't shift; the datum must be stated, such as (NAD 27) | The square's second letter shifts by ten letters; there's no formal way to write the datum |
| Shortest grid | Suggests at least two digits, so a 10 km square | Letters alone can name a whole 100 km square |
| Typical users | Emergency response and other civilian uses | Military and land navigation |
What this means in practice
- If your map and device use WGS84 or NAD83, treat them as the same.
- If your map uses NAD27, check the datum before sharing a grid. The numbers differ, and with MGRS the letters can too.
- Say the whole reference, including zone, band and square, when people may be working from different areas.
Convert or learn more
Use the free converter, or see how to read an MGRS grid. Gridmark shows both formats offline on iPhone, iPad and Apple Watch.
Sources: MapTools, Differences Between MGRS and USNG; Federal Register notice, April 20, 2001; Wikipedia, United States National Grid.