GPS is a form of navigation that uses a network of satellites (often called the GPS constellation) in conjunction with ground-based technology to determine a precise location anywhere on Earth.

What are the GPS Coordinates?

GPS Coordinates for Latitude and Longitude are recorded in degrees and decimal minutes - this is reasonably common on many referenced sites.

Another common Latitude and Longitude format is degrees-minutes-seconds.
 

Topographic maps are detailed maps that detail graphic representations of features that appear on the Earth's surface. These can include any number of man-made features from roads, buildings, urban development, railway, borders and boundaries to name a few. The map would also include lakes, rivers, streams, swamps, coastal flats, mountains, valleys, slopes, depressions, wooded and cleared areas. To interpret the features you would need to refer to the maps legend.

Topographic maps usually also show a geographic graticule (latitude and longitude, in degrees, minutes and seconds) and a coordinate grid (eastings and northings, in metres), to allow you to determine the relative and absolute positions of mapped features.

When referencing to a topographic map you are probably better of using UTM (Universal Transverse Mercator) coordinates - this is just your basic grid references found on most maps. These measurements, usually in meters and kilometres, are from a reference position - near Paris (I think). Try Google on UTM Coordinates for lots of info.

Mapping and coordinate systems are based on a datum, which is a mathematical surface that best fits the shape of the Earth. Australia's previous datum, the Australian Geodetic Datum (AGD) was defined in 1966 and best fitted the shape of the Earth in the Australian region only.

An updated version of this datum, known as AGD84, was adopted by some Australian States in 1984. AGD84 coordinates are based on the same datum as AGD66 and for map reading and navigation purposes can be regarded as being the same.

From the year 2000, all Australian mapping authorities are now using a new datum, the Geocentric Datum of Australia (GDA), which was defined in 1994.

The primary reason for this change is the widespread use of satellite-based navigation systems, such as the Global Positioning System (GPS), which is based on a geocentric datum known as the World Geocentric System 1984 (WGS84). For most practical purposes, WGS84 and GDA coordinates are the same.

Which datum set do I use?

In all cases the most important thing to check is that you have the correct map datum set in your GPS - otherwise you location may be out by up to 250 m. Most modern Australian maps will be on the GDA94 datum (much the same as the WGS84 datum) although many Australian topographic maps are still on an old AGD66 datum. The datum should be listed in the key of the map and you set the appropriate datum in your GPS set-up.

What is the difference between datum set ADGA94 and WGS84?

In actual distance there is approximately 10 cm between the two datum set and in practice there is really no difference for the average user.

Map coordinates

Most map coordinates are shown in one of two ways:

• Geographical coordinates
  given as latitude and longitude values in degrees, minutes and seconds, or
   
• Grid coordinates
  given as easting and northing values, in metres.

Source: This article was derived from a number of sources including Geoscience Australia
Many thanks to John Louis (csu.edu.au) and Brian Sommerville (GPSOZ.com.au)
for answering our questions