Atlas location references

To: <>
Subject: Atlas location references
From: "Paul Taylor" <>
Date: Sat, 14 Jul 2001 00:05:40 +1000
> Anthea has been told that she has to give LAT/LONG references rather
> than UMG. As many of the maps I use have UMG only I have the GPS set for
> UMG; even when using 1:25 000 survey maps UMG is much quicker for
> locating yourself on the map than LAT/LONG.
> I'm certain that someone out there has copies of the algorithms so that
> I can feed the UMG figures to the computer and get the L/L answers.

Things are a little trickier than this.  Lat./Long. and UTM (Universal
Transverse Mercator) coordinate systems are simply means of representing
location data; the values for a given location vary depending on which
Datum is used.  For Australian Maps the datum is AGD66 or AGD84 (I've 
only seen AGD66 maps.)  When people refer to GPS coordinates, they 
usually mean WGS84 datum coordinates.  The difference between the AGD
and WGS datums can be up to 200 metres.

Some GPS units (e.g. Garmin) have a swag of datum and grid systems to 
choose from so that you can use them with either WGS84 Lat./Long. 
coordinates or the local mapping system (for Australia, UTM grid and 
AGD66 or AGD84 datum.)  This can be very handy, but can also be a source 
of confusion (and error) e.g. if someone is using a local datum on their 
GPS and just quotes "GPS coordinates" (which are assumed to be WGS84.)

The good news is that these problems will eventually go away - the new
Australian coordinate system, GDA94, is essentually identical to WGS84
so there will be no difference between it and "GPS" coordinates.  How 
long it will take to be adopted on maps etc. is anyone's guess...  

AUSLIG has some good information of Australian datums, coordinate systems
and GPS at:

They also have the formulae (and Excel spreadsheets) for converting 
between the different datum and coordinate systems at:
Don't ask me how to use them - the formulae are pretty complex!

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