The GPS for Bushwalking FAQ
1. What is GPS ?
GPS stands for Global Positioning System. It was created by the US
of Defense and uses a constellation of satellites to transmit position
Users can receive these signals with a hand-held GPS unit and accurately
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
or SA. SA meant that the public could only receive GPS signals with an
accuracy of about
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
to turn GPS signals off on a regional basis when its national security is
3. What GPS should I buy ?
There are many makes and models of GPS to choose from. Garmin and Magellan
amongst the better known brands. When choosing a GPS for bushwalking you
battery life, size, weight, waterproofness, ruggedness, and the ability to
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,
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
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
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
is selected, the GPS should display grid references which you can use to
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
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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