[Taxacom] Geodetic datums do matter [SEC=Unclassified]

Dave Watts Dave.Watts at aad.gov.au
Sat Mar 21 03:41:03 CDT 2009

In reply to Karl

It does matter - a geodetic datum refers to a model of world that attempts to fit the shape of the earth and therefore
is an approximation to it. Each datum uses various parameters to define its shape and hence each coordinate of the same place will be different for each datum.

Comparing WGS84  to the Australian standard AGD96  will make a point appear to move 200 metres. Most users of GPS units are unaware of this and complain that the GPS unit / map is in error when they compare the positions  with different datums.

In reference to an earlier post, GPS horizontal accuracies for hand helds is likely to be only about 20 metres. If you wish accurate stuff, you need a base station and do differential measurements. Heights from hand-helds are also very poor.

The bottom line is KNOW YOUR DATUM when recording stuff. It will save you problems in the future as you can transform from any one datum to another with certainty.

Cheers Dave

Australian Antartic Data Centre

On Fri, March 20, 2009 10:10 am, Finn N. Rasmussen wrote:
> Do the programs and gadget that transfer GPS-info to, e.g., cameras,
> also transfer information of the geodetic datum used when recording this
> particular set of coordinates?

The datum when recorded doesn't matter, since when the points are
downloaded they come in whatever datum is current (at least on mine they
do, I assume this is standard).  That allows you to get your points in
multiple datums (data?).  That is, it *shouldn't* matter; I would be
interested to know if there's an error introduced in converting from one
datum to another, or if it matters what datum it's set on when the point
was taken.  Although if there is an error, I'd be surprised if it's
greater than a small fraction of the point accuracy on a typical
hand-held unit without differential correction.

Karl Magnacca
Postdoctoral Researcher
Department of Zoology
Trinity College, Dublin 2


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