UTM & MGRS Coordinate Converter

Converts between geographic coordinates, Universal Transverse Mercator (UTM) and the Military Grid Reference System (MGRS) on the WGS 84 ellipsoid. The projection uses Krüger's series to sixth order, as described by Karney (2011), which stays accurate to well under a millimetre across a whole zone, so the easting and northing agree with PROJ to the nanometre rather than drifting at the zone edges as the old textbook formulas do. The two standard exceptions are applied: zone 32V is widened over south-west Norway, and Svalbard uses only zones 31X, 33X, 35X and 37X. UTM covers 80°S to 84°N; the polar caps use the Universal Polar Stereographic grid, which this tool does not compute, and it says so instead of returning a figure from the wrong projection. An MGRS reference does not name a point but a square: 18TWL8562811322 is a 1 m square and 18TWL81 a 10 km one, and MGRS truncates rather than rounds, so the coordinates shown for an MGRS input are the square's south-west corner. The 100 km square letters repeat every 2,000 km of northing, which is why the band letter matters when reading one back. If you enter UTM in a zone that is not the point's standard one — common near zone boundaries — the result is re-expressed in the standard zone and a note says so. Everything assumes WGS 84: coordinates from an older datum such as NAD27, ED50 or the Tokyo datum can be off by tens to hundreds of metres, and no datum shift is applied.