What is the Universal Transverse Mercator (UTM) Coordinate System?

Definition

The Universal Transverse Mercator (UTM) is a coordinate grid system that divides the Earth between 84° N and 80° S into 60 zones. Each zone is 6° of longitude wide and has its own Transverse Mercator projection. UTM gives a position as an easting and a northing in metres.

NGA counts 120 separate Transverse Mercator projections, because each zone has a northern and a southern part with its own false northing. North of 84° N and south of 80° S, the standard grid is the Universal Polar Stereographic (UPS) grid.

The U.S. Army adopted the UTM projection and grid in 1947 for large-scale military maps of the whole world. Snyder describes UTM as the ellipsoidal Transverse Mercator with specific parameters, such as central meridians. A UTM coordinate is therefore a projected form of a latitude and longitude. The two describe the same point.

Why use UTM

Latitude and longitude are angles. NGA states that this non-rectangular system is often unsuitable for work that needs distance calculations. A grid gives linear units, which simplifies distance measurements and other calculations.

UTM is not more accurate than latitude and longitude. The USGS states that the two are different ways to position the same point. The benefit is the unit. An easting or a northing is a distance in metres on a plane.

Zones and bands

Zone 1 extends from 180° W to 174° W. The zones count eastward to zone 60, which extends from 174° E to 180° E. Each zone extends 3° in each direction from its central meridian. At the equator, the zone of 6° is about 668 km wide. The easting limits of zone 33N are 166,021 m and 833,979 m.

There are two exceptions. Between 56° N and 64° N, zone 32 is 9° wide (3° E to 12° E) and zone 31 is 3° wide. Between 72° N and 84° N, zones 32, 34 and 36 do not exist, and zones 31, 33, 35 and 37 are wider.

For the grid zone designation, each zone is also divided into latitude bands. Beginning at 80° S, twenty bands carry the letters C to X, and the letters I and O are not used. The bands are 8° tall. Band X is 12° tall. Washington, D.C. is in grid zone 18S, which covers 72° to 78° W and 32° to 40° N.

Take care with the letter N. In a name such as “UTM zone 33N”, the letter means the northern hemisphere. In a grid zone designation, band N is the latitude band from 0° to 8° N, because the bands count from C at 80° S.

How to find the zone

The NGA tables give two rules:

• The zone number is the whole-number part of (longitude + 180) / 6, plus 1. West longitudes are negative.
• The central meridian is 6 × zone − 183, in degrees. Zone 33 gives 15° E.

The rule holds outside the exceptions above. Singapore is in zone 48. EPSG lists Singapore in the area of use of WGS 84 / UTM zone 48N, which covers 102° E to 108° E.

Easting and northing

Each UTM projection uses these parameters:

• Scale factor on the central meridian: 0.9996.
• False easting: 500,000 m.
• False northing: 0 m in the northern hemisphere and 10,000,000 m in the southern hemisphere.
• Origin latitude: 0°, the equator.

The false easting gives every point in a zone a positive easting. At the equator, the zone edges are at about 166,000 m and 834,000 m. A northing counts metres north from the equator in the northern hemisphere. In the southern hemisphere, it is 10,000,000 m less the distance south of the equator.

For example, take the point where the central meridian of zone 33 crosses the equator. In the northern hemisphere, it has easting 500,000 m and northing 0 m. A point 100 km east of that meridian, on the grid, has easting 600,000 m.

How to write a UTM coordinate

Snyder states that the zone number and the x and y coordinates define a point when you know the ellipsoid and the hemisphere. For its civil mapping, the USGS uses only the zone number and the coordinates. The 100,000 m square identification of the military grid is not essential. A complete UTM coordinate has these parts:

• The zone number, for example 33.
• The hemisphere, N or S, or the latitude band letter of the grid zone designation.
• The easting in metres.
• The northing in metres.
• The datum, for example WGS 84.

EPSG names the codes the same way. EPSG:32633 is WGS 84 / UTM zone 33N, and EPSG:32733 is WGS 84 / UTM zone 33S. The 33S coordinate system has the same central meridian and a false northing of 10,000,000 m.

Scale and grid north

A grid distance is the distance on the ellipsoid multiplied by the scale factor. On the central meridian, 100 m on the ellipsoid measures 99.96 m on the grid. The scale factor is 1 along two lines about 180 km east and west of the central meridian. Between these lines the grid is smaller than the ground. Beyond them it is larger. Within a zone, the difference can reach 1 part in 1,000. Snyder states that the 0.9996 reduction keeps the variation in a zone small.

The scale grows with the square of the distance from the central meridian (Snyder, equation 8-16). Table 11 of Snyder gives the distance at which the scale factor reaches each value, on the Clarke 1866 ellipsoid:

The angle between grid north and true north is the grid convergence. A bearing on the grid differs from a true bearing by this angle.

UTM and UPS

Beyond 84° N and 80° S, NGA specifies the UPS grid. UPS has a northern and a southern projection. Each has a scale factor of 0.994 at the pole, a false easting of 2,000,000 m and a false northing of 2,000,000 m.

Datum

UTM is a projection, so a UTM coordinate needs a datum. NGA states that without a horizontal datum, coordinates remain imprecise or ambiguous. For the same zone, WGS 84 / UTM 33N (EPSG:32633) and ETRS89 / UTM 33N (EPSG:25833) are different coordinate reference systems. EPSG states that ETRS89 and WGS 84 coincide within 1 m.

NGA states that the WGS 84 ellipsoid is the standard reference ellipsoid for DoD mapping. Snyder notes that older UTM maps used regional ellipsoids, such as Clarke 1866 for land under United States jurisdiction. Check which ellipsoid a UTM coordinate uses before you merge it with a WGS 84 data set. See World Geodetic System for the datum.

Use in airfield work

For air navigation, ICAO Annex 14 and EASA require WGS-84 as the horizontal reference system (ADR.OPS.A.020). Annex 14 states that reported aeronautical geographical coordinates are latitude and longitude in the WGS-84 datum. A UTM grid is a projected form of the same WGS 84 positions.

On a UTM grid, the distance between two points in one zone is the plane distance. The area of a polygon follows from plane geometry, in square metres. Write the zone, the hemisphere, the easting, the northing and the datum together.

A runway near the edge of a zone has a larger scale error than one near the central meridian. A 3,000 m length on the ellipsoid measures 2,998.8 m on the grid at the central meridian. It measures 3,001.8 m where the scale factor is 1.0006, about 284 km from the central meridian at the equator.

An aerodrome can be near a zone boundary. At a grid zone junction, NGA shows two sets of grid lines, one for each zone. The same place has a different easting in each zone. Check that all data of one survey use one zone before you merge them.