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GPS use

In everyday speech, GPS is short for Global Positioning System receiver, as in I used a GPS to find out where I was.

Handheld GPS units are widely available and good ones are relatively cheap. They can store numerous positions in memory and they can help you navigate from position to position. Some have map displays and other useful capabilities. Frills aside, the basic job of all GPS units is the same. A GPS unit is a radio receiver tuned to a particular set of orbiting satellites. It calculates its position on the Earth's surface from information contained in the satellite signals.

How the GPS unit tells you its position depends on its settings. The most important settings are format and horizontal datum. If you don't set these items correctly, your GPS unit's readings won't make sense.

The format setting allows you to select the numerical system in which position is reported. For general purposes in Tasmania, choose either latitude/longitude or UTM. Although these are interconvertible, lat/lon is a better choice for general use and for publishable records. UTM is better if you want to see your position on a paper map, since Tasmanian maps are UTM-gridded. Either UTM or lat/lon (as decimal degrees) is suitable for GIS*, depending on how the GIS is configured.

When set to UTM format, your GPS unit will display a position looking something like this: 55G 0531668 5246449. This is a position given to the nearest one metre, and means easting 531668, northing 5246449 in Grid Zone Designation 55G. (For an explanation of these numbers, see the UTM system and UTM grid references pages.)

The key thing to understand about 55G 0531668 5246449 is that it's nonsense. No ordinary, handheld GPS unit can correctly determine its position to the nearest one metre. The reasons are discussed on the error page. Also on that page are instructions on how to deal with your GPS reading to make it believable. If you're interested in positional accuracy better than ±25 m, you should carefully note the 'accuracy' your GPS reports when it has finished locking on to satellites.

If you're using a GPS unit on King Island, be aware that UTM readings in the western half of the island won't agree with UTM locations on maps. This issue is discussed on the King Island problem page.

With lat/lon as the format setting, a handheld GPS usually offers a choice of decimal degrees (e.g. 42.16362°S), decimal minutes (42°09.817'S) and decimal seconds (42°09'49.0"S). These different lat/lon formats are discussed on the latitude/longitude page and are easily interconverted. Again, please be aware of error in a lat/lon GPS reading. That last digit in 42.16362°S means the position is known ±0.5 metre, which is unbelievable for an ordinary, handheld GPS.

The meaning of the horizontal datum setting is explained on the datums page. In lat/lon format the appropriate datum will be WGS84, while in UTM format the modern datum for GPS use in Tasmania is GDA (or 'GDA94'). For most purposes, WGS84 and GDA aren't different.

* GIS = Geographic Information System; a computer program, not the same as GPS!