A GPS receiver (GPSr) can be used to identify your current location to an accuracy of 4m or less, tell where you have been and tell you where you need to go. They can be connected to a computer and data can be transferred both ways. Data collected can be overlaid onto a map to show graphically where you have been. They can calculate areas of fires, show sunrise and sunset, moon cycles and even the best times to go fishing. They can store the location of hazards, water tanks, dams and structures, in fact anything you wish to enter (or "mark").

A GPSr receives signals from satellites and by calculating the difference between the time the signal is sent and the time it was received, it can calculate its distance from the satellite. Using at least 4 satellites, it can then work out its location on the earth's surface! This signal is generally not affected by clouds or weather but may be affected by very dense foliage, cliffs and solid structures.

The GPSr MUST have acquired satellite reception before it is useful. A common mistake is to read the location from the screen before it has locked onto satellites; the location being shown will still be where it was last time it was used! A message should indicate "ready to navigate", "3D NAV" or similar. Be aware some models use the message "tracking satellites" when trying to locate them, whereas other models use this message to indicate it is using them...beware! Become familiar with your model.

A GPSr is not a compass. A GPSr is only valid for navigation and direction when it is moving. After all, it does not know which way you are holding it, but it tends to assume it is facing with the aerial pointing in the direction you are travelling.

A GPSr is not manufactured to be country specific and will work anywhere in the world. As such it comes with an inbuilt list of "formats" under which it can display information. This can be latitude/longitude, British grid, German grid, NZ grid, many other country grids and UTM.

In Australia, we should set our GPSr to either "lat/lon" OR "UTM/UPS", depending on who will be using the data. Pilots often work with "lat/lon"; firefighters tend to use "UTM", which is what we commonly use with our topographic maps to give our usual "6 figure grid reference".

One can change the format at any time...it does not affect the stored data, only the way it is presented. A GPSr can actually be used to convert UTM readings to lat/lon (and vice versa) if needed!

Whenever you intend to use a GPSr, it is important you establish what datum it is set at. If you give someone a grid reference "off a GPS" they should ask you "under what datum is that?" and you need to be able to tell them confidently the setting! As we have recently received all new GDA maps in the Hawkesbury we should use the newer GDA datum setting. If you have a GPSr without this you can use WGS84 instead.

If you go out and use a GPSr to collect data in the field, plot dams, structures and the like, ensure you also record the datum setting in the GPSr at the time. As long as it is known, programs can be used to convert this information to other datums.