So, we've established that having a GPS position with geographic coordinates in degrees isn't sufficient; you also need a map representing the earth at a consistent scale, say in (kilo)metres. How to get from one to the other? This is called 'projection', which can basically be defined as a method of reducing the three-dimensional earth to a two-dimensional representation. There are a large number of ways of doing this, none of which is perfect. To illustrate this, try peeling an orange and then squashing the peel flat in such a way that it accurately represents its form before being peeled - impossible, eh!
I'm no expert on projections, so I'll just mention the main types I use on this site, which are all in Europe.
Mercator
Start with Mercator's, first produced in 1569 and still going strong. Although it's no longer used much in atlases, it is used by the commercial online mapping systems, such as Google. Here's the world according to Google:
Though well-known, this suffers from one obvious deficiency: it's fine on the equator but becomes ever more useless as you approach the poles. All projections are inevitably distortions, but Mercator's makes Greenland, for example, look the same size as Africa, which is extreme. Its advantage for Google et al is that it's geometrically simple and scales simply; the distortions also diminish as you zoom in.
Transverse Mercator
Because of the problems with a simple Mercator, a variation that is widely used by national cartographers, who only have to concern themselves with their own local area, is the Transverse Mercator. This turns Mercator on its side, so the 'equator' becomes a latitude meridian. This means the distortions are minimised around that meridian. The Ordnance Survey in Britain uses a transverse Mercator projection, with the planar coordinates being the National Grid.
A development of transverse Mercator is the Universal Transverse Mercator (UTM) which divides the earth into 60 transverse Mercator zones (so 6° each) and is widely used in Europe. Normally, it is based on the WGS84 ellipsoid, but versions based on ETRS89 and ED50 are also used in Europe.
EPSG Codes
The European Petroleum Survey Group defines a Geodetic Parameter Data Set which lists most projections and coordinate systems. Each projection has a code which is widely used in software to identify and convert from one to another. EPSG:41001 is the code for Mercator's projection; EPSG:4326 for WGS84 geographic coordinates; EPSG:4230 is ED50 (EPSG:230nn are UTM subsets of this); EPSG:4258 is ETRS89 (EPSG:258nn are UTM subsets of this). EPSG:3785 is a relatively new code to describe the form of Mercator used by Google et al which treats the earth as a sphere not an ellipsoid.
External Resources
- Wikipedia is a good place to start to get more information on projections, with pages on the main ones and plenty of references to other sources
- spatialreference.org is a good place to explore projection definitions, and see the EPSG Registry itself
- see Winwaed Software's demos for an excellent summary of equal-area projections which, as you can see, are much better for atlas-type general maps than Mercator's, as they represent the curved nature of the earth much better