GDA vs GDA 94

Submitted: Tuesday, Apr 01, 2008 at 19:17
ThreadID: 56159 Views:3977 Replies:5 FollowUps:7
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g'day folks,

as i have to consider myself to be a novice when it comes to navigation by GPS i do have following question:

Here at Exploroz the coordinates are given in the map datum GDA 94.
On my Garmin GPS device (60CSx) i do find the map datum GDA but not GDA 94.
Am i save to assume that this is the same for navigational purposes?

looking forward to hearing some advice - cheers - lukas
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Reply By: Member - Doug T (FNQ) - Tuesday, Apr 01, 2008 at 19:24

Tuesday, Apr 01, 2008 at 19:24
Except for a small difference in the flattening term, the reference ellipsoid used with WGS84 is essentially the same as the Geodetic Reference System 1980 (GRS80) ellipsoid used with ITRF and hence GDA94.Given that the ITRF and WGS84 reference frames are also very closely aligned, for most practical purposes GDA94
and WGS84 coordinates can be considered the same and no transformation is required.In other words, when comparing GDA, WGS & ITRF datums, keep in mind that GDA94 coordinates are fixed as at the 1st January 1994 (epoch 1994.0) and that the
Australian plate is drifting in a north-easterly direction about seven centimetres per year. If we were to re-compute GDA now, the coordinates would about half a metre different.


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Follow Up By: Member - Andrew (QLD) - Saturday, Apr 05, 2008 at 15:12

Saturday, Apr 05, 2008 at 15:12
Just to help others out, the reference for that information is found in the ICSM Factsheet - 'What is the difference between WGS84 and GDA94?'

Andrew
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Reply By: Member - Mike DID - Tuesday, Apr 01, 2008 at 19:34

Tuesday, Apr 01, 2008 at 19:34
Hi Lukas

WGS84 and GDA94 are virtually identical.

You may come across some older maps that use an AGD66 Datum and these will have a 250 metre error, unless you change the GPS to AGD66.
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Reply By: Gramps (NSW) - Tuesday, Apr 01, 2008 at 19:46

Tuesday, Apr 01, 2008 at 19:46
Doug/Mike,

I think Lucas' question was related to GDA (on his Garmin) Vs GDA 94 on EO maps. Coincidentally, that happened to be the subject line of his post :)))



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Reply By: Member - John R (QLD) - Tuesday, Apr 01, 2008 at 20:52

Tuesday, Apr 01, 2008 at 20:52
Perhaps the following quotes from this website:
http://www.geoproject.com.au/gda.faq.html
will help:
"The Geocentric Datum of Australia (usually referred to as GDA94, or just GDA) is a coordinate system for Australia" (so Garmin is just abbreviating);
"Thus for most mapping, exploration and GIS uses, WGS84 and GDA94 coordinates will be the same. If you use a GPS receiver (alone, or with a differential service) and the GDA datum is not a built-in option, you can select WGS84."
"The difference between GDA94 and WGS84 is approximately 45cms in 2000."
Cheers, John
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Follow Up By: Member - Oppikofer L (Int) - Tuesday, Apr 01, 2008 at 21:12

Tuesday, Apr 01, 2008 at 21:12
g'day John,

thanks for your reply and in particular for that link!

cheers - lukas
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Reply By: Bonz (Vic) - Saturday, Apr 05, 2008 at 14:44

Saturday, Apr 05, 2008 at 14:44
yer well hmmm Doug is spot on but wow what a complicated way to say... GDA and GDA94 are the same thing and that within the accuracy of your GPS (approx 5m) WGS84 is also the same thing
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Follow Up By: Member - Andrew (QLD) - Saturday, Apr 05, 2008 at 15:09

Saturday, Apr 05, 2008 at 15:09
Well the person who wrote it is spot on.....

Andrew
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Follow Up By: Bonz (Vic) - Saturday, Apr 05, 2008 at 15:12

Saturday, Apr 05, 2008 at 15:12
As I said, I am now researching some of what Doug said to fill in my own gaps. He knows stuff!
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Follow Up By: Member - Andrew (QLD) - Saturday, Apr 05, 2008 at 15:14

Saturday, Apr 05, 2008 at 15:14
I didn't say he didn't...just that the information is from a standard factsheet not Doug's own words.

Andrew
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Follow Up By: Bonz (Vic) - Saturday, Apr 05, 2008 at 15:16

Saturday, Apr 05, 2008 at 15:16
Yedr hmm sorry Andrew a bit slow on the uptake today
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Follow Up By: Member - Andrew (QLD) - Saturday, Apr 05, 2008 at 15:31

Saturday, Apr 05, 2008 at 15:31
No need to be sorry...1/2 my fault :-)

Andrew
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