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UTM grid references

On the locations from maps page, you find that the UTM grid reference for a certain creek junction near Mole Creek is DP476986. However, the instructions in the map legend on that page suggest that the full grid reference should be 55GDP476986. A GPS unit at the creek junction might record the grid reference as 55G 0447621 5398638. Some Tasmanian field workers would prefer 4476 3986. Which of the four ways to write the grid reference is right?

Actually, all four are correct. To understand why, let's get back to basics: getting the grid reference for a place is like finding the x and y coordinates for a point on an x, y graph. In the UTM system, the x distance is called the easting, and the y distance is called the northing*. The x axis is a long way south of Tasmania and is called the false northing origin. The y axis is west of Tasmania and is called the false easting origin. (For more details, see the UTM system page.) Just as you'd report a point in maths with the x value first and y value second, UTM grid references are written as easting first, northing second.

Of the grid reference formats shown above, the most complete is the one from the GPS. It's in three parts: a Grid Zone Designation (55G), an easting to the nearest metre (0447621) and a northing to the nearest metre (5398638). This literally means 447621 m east of the false easting origin in 55G and 5398638 m north of the false northing origin. If you're reporting a location to someone outside Tasmania, it's important to include the Grid Zone Designation. For uses within Tasmania, you can leave off the 55G, since almost the whole State is within this UTM area. (The bit that isn't in 55G is discussed on the King Island problem page.)

You're left with the 6-digit easting 447621 (same as 0447621) and the 7-digit northing 5398638. As explained on the error page, it's highly unlikely that you know the location of the creek junction to the nearest metre, even with a GPS. When you used the map, you only estimated the location to the nearest 100 m. You can therefore round off the easting and northing to the nearest 100 m, to 4476 53986. This literally means 4476 x 100 m east and 53986 x 100 m north of the false origins.

You can simplify further by leaving off the initial 5 in the northing, because all Tasmanian locations have this 5 (see the UTM system page). You now have the 4-digit easting, 4-digit northing format used by some field workers: 4476 3986.

Finally, how do you get from 4476 3986 to the 2-letter, 6-digit DP476986? First, 'deconstruct' each of the 4-digit combinations to get 4  476  3  986. The separated-off 4 and 3 are 100 km (100 000 m) units of distance. Shift those first digits to the front to get 43 476 986, then replace those first digits with their letter equivalents (see below) to get DP 476 986. Still confused? Keep reading:

Each of the 100 km (or 100 000 m) UTM grid lines which crosses Tasmania is designated with both a number and a letter. The line codes, as shown on the map, are: easting B=2, C=3, D=4, E=5, F=6, G=7; northing M=1, N=2, P=3, Q=4, R=5, S=6. The letters are arbitrary, but the numbers have a meaning. For example, the '2' easting line is 2 x 100 km east of the false easting origin.

We can name each 100 km square on the map by giving its easting letter to the west and its northing letter to the south. Our creek junction, marked with a small black cross on the map, is near the top of 100 km square DP.

100 km grid

The grid reference DP476986 can be understood to mean easting 476 and northing 986 in grid square DP. Literally it means 476 x 100 m (47.6 km) east of the 100 km line 'D', and 986 x 100 m (98.6 km) north of the 100 km line 'P' (see map, above).

The 2-letter, 6-digit grid reference DP476986 by itself is ambiguous. It could refer to a location in Tasmania, or it could refer to a Grid Zone Designation somewhere else in the world which contains a 100 km square labelled DP. The best way to write the map-estimated grid reference for the creek junction is 55G 447600 5398600, which gives the location to the nearest metre within Grid Zone Designation 55G as a 6-digit easting and 7-digit northing. Here the 00's take the place of uncertain metres and tens of metres (see the error page).

For general purposes within Tasmania, DP476986 is perfectly acceptable. It specifies a location to the nearest 100 m for a Tasmanian audience. You can also use this format in reporting locations to a non-Tasmanian audience if you make it clear in which Grid Zone Designation the places are located. For example, you could say All grid references reported here refer to UTM Grid Zone Designation 55G. (And don't forget the datum.)

*If only it were that simple! An x, y graph is on a plane, while eastings and northings are meant to work on the curved surface of the Earth. They work, but for some purposes only approximately.