UTM vs latitude and longitude: what is the difference?

Latitude and longitude in short

Latitude is the angle north or south of the equator, from minus 90 to plus 90 degrees. Longitude is the angle east or west of the Greenwich meridian, from minus 180 to plus 180 degrees. Together they describe any position on Earth with one pair of numbers, for example 48.85837, 2.29448 for the Eiffel Tower. GPS receivers, phones and web maps all use them, on the WGS84 datum.

The same position can be written in decimal degrees, as above, or in degrees, minutes and seconds, 48° 51' 30.1" N, 2° 17' 40.1" E. The downside of degrees is that they are not a fixed length: one degree of latitude is about 111 km everywhere, but one degree of longitude is 111 km at the equator, 56 km at 60 degrees north and zero at the poles.

How UTM works

Universal Transverse Mercator splits the Earth between 80 degrees south and 84 degrees north into 60 zones, each 6 degrees of longitude wide, numbered eastwards from 180 degrees west. Each zone has its own map projection, so inside the zone positions are given in metres: an easting, measured from a line 500,000 metres west of the zone's central meridian, and a northing, measured from the equator (plus 10,000,000 metres in the southern hemisphere, so numbers stay positive).

The Eiffel Tower is 31U 448251 E 5411952 N: zone 31, latitude band U, about 51.7 km west of the central meridian at 3 degrees east and 5,412 km north of the equator. Because the unit is the metre, the distance between two points in the same zone is almost simply the difference in eastings and northings, which is why UTM is popular for surveys, hiking maps, search and rescue and GIS projects.

1. The zone number comes from the longitude. Zone 1 covers 180 to 174 degrees west, zone 31 covers 0 to 6 degrees east.
2. The latitude band letter (C to X, without I and O) shows which 8 degree strip the point is in; it is not part of the projection itself.
3. The easting is between about 166,000 and 834,000 metres at the equator, and the northing is a distance from the equator.

Accuracy and distortion in UTM

Every map projection distorts something. In UTM the scale is 0.9996 on the central meridian, exactly 1 on two lines about 180 km east and west of it, and grows towards the zone edges, to about 1.001 at the equator. A distance read straight from UTM coordinates is therefore off by at most about one metre per kilometre, and much less in most places. That is good enough for most field work but not for precise surveying, which applies the scale factor.

The conversion between latitude, longitude and UTM is exact mathematics, not an approximation. GeoDrafter uses the Krüger series, which agrees with the standard PROJ library to within a micrometre. Errors only come in when the wrong zone, hemisphere or datum is used.

Common mistakes

Coordinates in two different zones cannot be compared directly; a point near a zone edge has very different eastings in its own zone and the next. Norway and Svalbard have special zones, so southwest Norway uses zone 32V, which is 9 degrees wide. A southern-hemisphere northing read as northern puts the point 10,000 km away. And UTM on an older datum, such as ED50 in Europe or NAD27 in North America, differs from WGS84 UTM by up to around 200 metres, so always check the datum or EPSG code.

MGRS, the military grid reference system used by NATO and many rescue services, is UTM with the zone, band and a two-letter 100 km square in front, for example 31U DQ 48250 11951.

When to use which

Use latitude and longitude to share a position with anyone, to search in a map app or to work across large areas and many countries. Use UTM when you work inside one region and want metres: for field measurements, local GIS projects, paper topographic maps with a grid, or reports that ask for eastings and northings. National systems such as the British National Grid or State Plane in the United States follow the same idea with a projection tuned to one country or state.