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The UTM system

The grid of lines on Tasmanian maps isn't an arbitrary one. It's part of a worldwide grid system based on what's called a transverse Mercator projection of the round Earth onto a flat piece of paper.

Most people are familiar with the more common Mercator projection, devised centuries ago as an aid to navigators. In the Mercator projection (diagram below, at left), the flat piece of paper touches the globe along the Equator. As a result, Equatorial countries are represented more or less accurately, but polar areas are wildly distorted. Antarctica, for example, appears much bigger in relation to Australia than it really is (see the world map, below).

In a transverse Mercator projection (diagram below, at right), the piece of paper is wrapped around the Earth at right angles to the Equator.


The most accurate representation in a transverse Mercator projection will be of areas close to a pole-to-pole line (where the paper touches the globe). But which pole-to-pole line? A transverse Mercator map touching a north-south line through Hobart will show a seriously distorted India, and one touching a north-south line through Mumbai will show a seriously distorted Australia.

The problem has been solved to everyone's benefit by choosing 60 different pole-to-pole lines for anchoring the transverse Mercator projections. In other words, there are 60 different projections in the Universal Transverse Mercator (UTM) system. These projections are spaced 6 degrees of longitude apart, and are given a number from 1 to 60. The 60 projection zones are shown below. (For more on 'longitude', see the latitude/longitude page.)

UTM zones

In the middle of each of the 60 zones is a pole-to-pole line (called a central meridian) to which the projection is anchored. Each of the 60 zones is further divided into 8-degree blocks of latitude, as shown in the map above. (For more on 'latitude', see the latitude/longitude page.) These blocks are designated with the letters shown in the left-hand border of the map. The combination of zone number and designator letter gives what's called a Grid Zone Designation.

As you can see from the map, Tasmania is in Grid Zone Designation 55G. Zone 55 runs from 144°E to 150°E longitude, and its central meridian is 147°E. The northern boundary of 55G is 40°S latitude and the southern boundary is 48°S.

Grid references in the UTM system

In the UTM system, east-west distances are called eastings and are measured in metres from the central meridian in each UTM square. Eastings measured east from that line are positive, and eastings west from that line are negative.

To get around the problem of negative eastings, a false origin is used which is 500 000 m west of the central meridian. In effect, this adds 500 000 m to all eastings. For example, suppose a place is 400 km, or 400 000 m, west of the central meridian. The nominal easting of the place is -400 000 m. We add 500 000 to this figure to get a positive "corrected" easting of 100 000 m.

The situation for north-south distances, or northings is similar. In the UTM system these are measured in metres from the Equator and are positive north and negative south. To avoid negative northings, 10 000 000 m are added to northings in the Southern Hemisphere. In other words, in the Southern Hemisphere we have a false origin for northings which is 10 000 000 m south of the Equator.

Now you have the explanation for that mysterious '5' in front of Tasmanian northings in 6-digit easting, 7-digit northing format. For example, Lobster Falls near Chudleigh is roughly 4598 km, or 4598000 m, south of the Equator. Its northing is therefore -4598000 m. We add 10 000 000 m to this figure to get a positive northing of 5402000 m.

For more on grid references and their various formats, see the UTM grid references page.