The GPS for Bushwalking FAQ

1. What is GPS ?

GPS stands for Global Positioning System. It was created by the US Department of Defense and uses a constellation of satellites to transmit position fixing signals. Users can receive these signals with a hand-held GPS unit and accurately determine their position on the surface of the earth.

For general GPS discussion, visit the sci.geo.satellite-nav newsgroup.

2. What is Selective Availability (or SA) ?

When GPS was first introduced the civilian signals were intentionally degraded to reduce their usefulness to users other than the US military. This was called Selective Availability or SA. SA meant that the public could only receive GPS signals with an accuracy of about 100 metres.

On 1st May 2000, SA was turned off. GPS users could then receive signals with an accuracy of about 10 metres. The US is committed to leaving SA turned off, but now has the ability to turn GPS signals off on a regional basis when its national security is threatened.

3. What GPS should I buy ?

There are many makes and models of GPS to choose from. Garmin and Magellan are amongst the better known brands. When choosing a GPS for bushwalking you should consider battery life, size, weight, waterproofness, ruggedness, and the ability to download your recorded positions to a computer.

Aditional features which may be of use to some users are an external aerial (for use under heavy vegetation cover such as rainforest), and the ability to upload data from a computer and display maps on the GPS screen.

You can buy GPS units from bushwalking and camping shops, electronics shops, boating shops, or online.

4. What are the limitations of GPS ?

GPS units need to be able to see at least 3 or 4 GPS satellites to get a fix on your position. This means that they need a clear view of the sky. In narrow canyons or under heavy rainforest vegetation it may be difficult to get an accurate position.

5. Is GPS a replacement for the compass ?

Although it is possible to navigate with GPS alone, it is not recommended. Compasses cost a fraction of the cost of a GPS and are inherently more reliable. Compasses do not require batteries and do not need to have a view of the sky to work. Many bushwalkers prefer to navigate with map and compass, keeping a GPS handy in case further confirmation of their position is required.

6. What coordinate system should I use ?

If you're working with topographic maps in Australia, you should set your GPS to use the Universal Transverse Mercator (UTM) projection. In other parts of the world, other projections may be required.

7. What map datum should I use ?

It is important to set your GPS unit up to use the same map datum as the maps that you use for navigation. Most GPS units have a menu of supported map datums. Once the correct datum is selected, the GPS should display grid references which you can use to read your position directly off a topographic map.

Older Australian maps use either the AGD66 or AGD84 datum. These two datums differ by only about 5 metres. NSW, Victoria, Tasmania, and the NT use AGD66. SA, Qld, and WA use AGD84.

New Australian maps use GDA94 (Geocentric Datum of Australia). This new datum was introduced to bring our maps in line with the World Grid System (WGS84) so that they are more easily used with GPS units. (If your GPS does not support GDA94, use WGS84 since this is virtually identical).

AGD66/AGD84 positions and GDA94 positions differ by about 200 metres. A program converting between these datums is available on my software page. See also my page on grid references and datums.

New Zealand uses its own map datum, NZMG. This is not compatible with the Australian map datums. However, New Zealand has recently introduced a geocentric datum similar to Australia's new datum called NZGD2000.

8. How can I connect my GPS to my PC ?

Many GPS units now have the ability to download and upload data to a PC. Usually you have to buy an extra cable to allow this connection, and it plugs into either the serial port (RS232) or USB. In order to connect to the GPS, you also need software. Free software such as EasyGPS will do the job and allows you to save waypoints, tracks, and routes. The EasyGPS program stores info in a GPX file. If you want to convert this to a KML file for viewing in Google Earth, use my GPX2KML program.

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