UTM Grid System Explained

UTM, the Universal Transverse Mercator system, is a way of describing positions in metres on a flat grid. It divides the world into 60 vertical zones and gives each position as an easting and a northing within the zone. Surveyors, mapping agencies, hikers and emergency services use it because metres are easier to measure with than degrees.

The zones

The world between 80 degrees south and 84 degrees north is split into 60 zones, each 6 degrees of longitude wide. They are numbered from 1 to 60 eastwards, starting at the 180 degree meridian. Within each zone, a separate transverse Mercator projection is used, centred on the middle of the zone. Because the zone is narrow, distortion stays small.

Zones are further divided into latitude bands labelled with letters, in 8 degree strips (the northernmost band is 12 degrees), which help to locate a place more quickly. Many sources simply say whether a position is north or south of the equator instead.

Reading a UTM coordinate

A UTM position has three parts:

• Zone: a number and a hemisphere or band, such as 33N.
• Easting: metres east of a reference line. The central meridian of each zone is given the value 500,000 so that numbers never go negative.
• Northing: metres north of the equator in the northern hemisphere. In the southern hemisphere, a false value of 10,000,000 is used at the equator so numbers also stay positive.

An example would look like 33N 500000 4649776. The first number says where the position lies east to west within the zone, and the second says how far north.

Advantages

• Metric units: distances between two points in the same zone are found with simple arithmetic.
• Locally accurate: scale error is small within a zone.
• Concise: coordinates are short numbers without hemisphere signs.

Drawbacks

• Zones: an area that spans a zone boundary needs coordinates in two zones, or a special handling.
• Not global in a single frame: you must know the zone to interpret the numbers.
• Not intuitive: most people are not used to reading eastings and northings.

Relationship to other systems

UTM is the basis for the military grid reference system, described in MGRS explained. It is a projected coordinate system, covered in coordinate reference systems explained, and uses WGS 84 as its datum in most modern uses.

Most web services, including UPCS, exchange positions as latitude and longitude, so UTM values are typically converted before use.