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The King Island problem

Not all of Tasmania is in UTM zone 55, which by definition runs from 144°E to 150°E longitude. (See the UTM system page for an explanation of UTM zones.) As it happens, the 144°E line of longitude runs right through the middle of King Island. This means that the eastern half of King Island is in zone 55 and the western half is in zone 54.

Nevertheless, the map grid used in Tasmania (for Grid Zone Designation 55G) runs right across King Island without interruption. Tasmanian mapmakers extended the 55G grid westward into 54G's territory for simplicity's sake, following the internationally accepted practice on such things. If they hadn't, the grid for King Island west of 144°E would be the map grid for 54G, which is very different from the 55G grid.

You could say, then, that every point on King Island west of 144°E really has two grid references. Tathams Lagoon, for example, is at approximately 234520E 5591390N (BR345914) on the extended 55G map grid, but at 748317 5591965 (YA483920) on the 54G map grid.

Which grid reference should you use? The one on the extended 55G grid, because this is the grid reference you (and anyone else) can read easily from a UTM-gridded map of King Island.

The problem is that the 55G grid reference is the wrong one as far as GPS units are concerned!

Suppose you're sampling freshwater invertebrates in Tathams Lagoon. You mark the location on your GPS unit as a waypoint in UTM format. The waypoint reads 54G 748317 5591965 because the GPS unit knows that you're really in 54G, not 55G. This waypoint won't be much help in marking your location on a map, because Tasmanian maps carry the 55G grid, not the 54G one*. There's also a danger that you'll forget that 748317 5591965 is in 54G. If you mix it with 55G positions, for example in a GIS database, the GIS will plot Tatham's Lagoon somewhere out in the Tasman Sea!

Alternatively, you might record the Tathams Lagoon waypoint as a lat/lon, say 39°47'17"S 143°53'54"E (WGS84). There's no problem unless you want to convert that lat/lon to a grid reference. On the conversions page, I recommend several ways to do that conversion, but all of them, except one, convert the lat/lon to a grid reference in the 54G grid zone. The one exception is LISTmap. If you use the LISTmap 'Locate Me Tool' on a location in western King Island, the UTM grid reference is provided in 55G.

What to do about the King Island problem? Try to avoid it by sticking with lat/lon. If you have to record UTM grid references from King Island, make sure you include the Grid Zone Designation, 54G or 55G as appropriate.


*If you're patient you can slowly work out 54G grid references from maps of King Island with the help of a ruler and a calculator. 54G grid intervals are marked in blue along the map border. These marks are 10 km apart on the 1:100 000 King Island sheet and 5 km apart on the 1:25 000 sheets. Note that the 54G gridlines are not parallel to the 55G gridlines.