MGRS to latitude and longitude

Model: NGA MGRS (AA lettering) over UTM and UPS on WGS 84

Show your work

1. Square the reference names

   the digits give a square, not a point: half as many digits, ten times the square

   17TNE8630977770 = 1 m across

2. Its south-west corner

   the grid easting and northing the reference resolves to

   zone 17, 586,309 m E, 4,477,770 m N = 40.446107°, -79.982233°

3. Centre of the square

   the middle of it, which is the position to use

   0.5 m from the corner = 40.4461117°

The same steps an agent gets from the MCP server with explain: true.

Accuracy: Exact; the true point is anywhere in the square

When to use this: Use this when an MGRS or USNG reference arrives by radio, from a report, or off a map and you need to plot it: it returns the south-west corner and the center of the square it names, with the square's size so the precision is explicit. It is also how to check the precision of a reference you were given: the square's size is reported, so an eight-digit reference is visibly a ten-meter square rather than a point.

Limitations: The reference is a square and the true point is anywhere within it: at six digits that is a hundred-meter square, at four a kilometer. The center is offered for plotting, not as the reported position. A reference without its grid zone is ambiguous over long distances, and this reports what it assumed. Polar references use UPS rather than UTM and follow different lettering, which this handles but which is worth knowing when a reference looks unusual.

Worked example: Decode a 1 m reference. Source: add-geodesy-suite scenario: 1 m square, center offset 0.5 m. It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.

Review: Not yet independently reviewed by a geodesist.

Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.0, core 0.1.0. See this tool in the verification report.

Changes

• 2026-09-19, changed: MGRS encode and decode are now stable. They match the Washington Monument example in the FGDC US National Grid standard and 2,000 references from NGA GEOTRANS; where GEOTRANS's own projection is 1.5 cm off near the poles and in Svalbard, PROJ confirms geoprims. Results are unchanged. Changelog

Checked against: 24 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• Military Grid Reference System, NGA.STND.0037, National Geospatial-Intelligence Agency, NGA.STND.0037_2.0.0_GRIDS. MGRS lettering (AA scheme) and truncation. Read free.
• GeographicLib MGRS and UTMUPS classes, Karney, C. F. F., GeographicLib, GeographicLib 2.x. MGRS.cpp: band tolerance, square validation, UPS lettering. Read free.

Terms

MGRS: Military Grid Reference System

UTM and UPS coordinates written as a zone, a latitude band, a two-letter 100 km square, and digits, like 17T NE 86309 77770. Source: NGA.STND.0037 MGRS