UTM, MGRS, plus codes: other ways to write a place

Latitude and longitude are measured in degrees, which are awkward for measuring on the ground. How many degrees is 200 metres? It depends where you are.

So surveyors, armies and hikers often use grid systems that count in metres instead. The best known is UTM, the Universal Transverse Mercator grid.

UTM slices the world into 60 zones, each 6 degrees of longitude wide, numbered eastward from 180° west. Inside a zone, a place is given as metres east (easting) and metres north (northing).

A letter after the zone number, the latitude band, says roughly how far north or south it is.

The Military Grid Reference System is UTM with the big numbers shortened. It names a 100 km square with two letters, then gives the position inside it.

NATO forces and many search-and-rescue teams use it, because a reference like 29U PV 82352 14683 is easy to read over a radio.

Plus codes, made by Google and free to use, turn a location into a short code of letters and numbers with a plus sign in it. They work where there are no street addresses.

what3words gives every 3-metre square on Earth three words. It is a private company's system, so the words only mean something inside its own app and services.