Datums & your GPS

Subject: Datums & your GPS
From: "Peter Ewin" <>
Date: Thu, 22 Sep 2005 18:43:06 +1000
Just a quick follow up on Martin's comments. Some older GPS don't have GDA 94. However WGS (World Geodetic System) 84 is exactly the same, and I believe is basically the reason why Australia had to change (to come in line with other parts of the world). They vary slightly depending on where you are, but on average one is about 230 to the North East of the other (can't remember which way).

One other commonly made error is that if you are using latitude and longitude, the values will also change depending on the datum (though technically at the poles or on the equator at least the latitude won't change). This means that the new topo maps will appear slightly different around the edges.

In NSW they are slowly working through the maps, and have done about a third (I think). The new 1:25000 have the GDA logo, area called Series 2, and all the ones I have seen have an aerial photograph on the back.

The Atlas of NSW Wildlife (run by DeC formerly NPWS) used to rely on AGD 66, but now allows data to be entered in either format (as long as it is known).


Subject: [BIRDING-AUS] Datums & your GPS
Date: Thu, 22 Sep 2005 09:21:34 +1000

Brian Fleming raises some important issues regarding Global Positioning
Systems, the next generation of topographic maps (which work together) and
using GPSs while birding .

As GPS owners know, their units have a wide range of datums to choose
from.  All current GPS devices should have the Geocentric Datum of
Australia 1994 (GDA) as an option.  This is to be the standard datum for
Australia I believe.

As Brian mentions, old maps have a different datum (often Australian
Geographic Datum 1966) to that now used on new topographic maps. Depending
on the datum you use your GPS can give a slightly inaccurate locality
reading.  Hence it's important to make sure you datum matches the map you
are basing your survey on.

In Victoria we are lucky to now have a new series of 1: 25 000 topographic
maps which use GDA.  This means GPS users can easily choose the correct
datum when using the relevant map.

Atlassers in Victoria are being encouraged to use GDA as the primary datum
wherever possible.  This is because the mapping products that are now
being produced by them use GDA as the standard datum.

If you have ever completed Birds Australia Atlas sheets or Atlas of
Victorian Wildlife recording sheets, then you know that each asks you to
indicate what datum you have used to determine your locality information.

There is plenty of useful information around on this issue and I've
provided a few of the better web sites below.

1. Datums and Coordinates

2. Geocentric Datum of Australia (GDA)(Federal Government site)

3. Geocentric Datum of Australia (Land Victoria site)

3. GDA and topographic maps (Federal Government site)

4. Moving to GDA (Dept. Sustainability & Environment, Victoria site)

The 'GPS - A Guide for users' can also be found via this site.

Martin O'Brien
Threatened Species & Communities Section
Department of Sustainability and Environment
2/8 Nicholson St. East Melbourne  3002


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