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Datums

The latitude/longitude system and the UTM system of grid lines are anchored to the real Earth in a very complicated way. The anchoring is based on an array of fixed points with known locations, plus a mathematical model of the Earth's shape.

The standard anchoring arrangement adopted around Australia since 1994 is called the Geocentric Datum for Australia, or GDA, or GDA94. Up-to-date Tasmanian maps are based on this datum, and drawn with the Map Grid of Australia 1994, or MGA94.

Older Tasmanian maps were based on the Australian Geodetic Datum 1966, or AGD66. Maps using this datum were drawn with the Australian Map Grid 1966, or AMG66. The two map grids are about 200 m apart. That 200 m may not sound like a lot, but it's 10 times bigger than the uncertainty in a GPS reading, and four times as big as the uncertainty in estimating a UTM grid reference to the nearest 100 m from a 1:25 000 scale map. Note that datums also apply to latitudes and longitudes. When Australia moved from AGD66 to GDA, Australian lat/lons also shifted by about 200 m.

It's important to understand that grid references and lat/lons taken from older maps aren't 'wrong' just because they're based on AGD66. Those old grid references are 100% correct within the system based on AGD66. However, they're different from grid references based on GDA, so datums are a possible source of confusion and error, especially when you're dealing with older Tasmanian spatial data.

How to tell which datum is being used? If you're getting a location from a paper map, check the map's legend. It will clearly state whether the map is based on AGD66 or GDA. If you have older spatial data and no idea whether it's based on GDA or AGD66, check the metadata to see if the datum was specified. If it wasn't, it's best to ask the data provider for assistance!

If you have an AGD66 grid reference, you can convert it to GDA by adding 112 m to the easting and 183 m to the northing. (Eastings and northings are explained on the locations from maps and UTM grid references pages.) Thus 514326 5467173 (AGD66) becomes 514438 5467356 (GDA). To convert a GDA grid reference to AGD66, subtract 112 m from the easting and 183 m from the northing.

In the lat/lon system, to convert from AGD66 to GDA (or WGS84; see below), subtract 5.4 seconds from latitude and add 4.8 seconds to longitude. Thus 43°22'06"S 145°47'11"E (AGD66) becomes 43°22'01"S 145°47'16"E (GDA). To convert from GDA or WGS84 to AGD66, add 5.4 seconds to latitude and subtract 4.8 seconds from longitude.

If you're using a GPS, check your 'units' settings to see whether the datum is GDA or WGS84. For everyday purposes these two datums are the same. The WGS84 datum is global and is the datum used by Google Earth and the GPS satellites. The GDA datum is only used in Australia.

As it happens, the Australian Plate is moving northeast towards Papua New Guinea at about 7 cm per year, carrying Australia and its GDA system with it. I'm writing this in 2014, when there's more than a metre horizontal difference between WGS84 and GDA. That's not enough to worry about yet, unless you're a surveyor or need very accurate locations. That one metre is way less than the uncertainty in a reading from a handheld GPS.

It's a good idea to give the datum when reporting locations from Tasmania. A simple way is to add the datum in brackets: 42°50'14"S 145°33'51"E (GDA). When reporting a long list of locations you could say, for example, Latitude/longitude data are based on the WGS84 datum.