OziExplorer and Raster Chart Datums

Submitted: Tuesday, Jun 20, 2006 at 11:42
ThreadID: 35102 Views:1665 Replies:5 FollowUps:2
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Hi ... have just come across a problem with datums new to me.
Some of my current Raster 250K maps USING OziExplorer show datum as GDA 94 [1994] and my GPS [Lowrance Map 100] utilises standard WGS 84.

The Raster map data advises that the HORIZONTAL Datum IS EQUIVALENT TO WGS 84. Taking a known point on both maps, there are minor differences in 'points of a minute' positioning.

QUESTION....... does this mean that WGS 84 and GDA 94 are equally accurate in my reckonings ......... bearing in mind the comment above " IS EQUIVALENT TO!"

Am currently using this info to locate/identify potential gold bearing locations, for detecting,thusly accuracy is fairly important to me.

Or does this mean that a more up-to-date GPS is called for?

And no ...... if I tell you any specifics on the said locations, I will have to kill you !!!

There must be a few members in this field, with the neccessary skills/knowledge.
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Reply By: Member - Ian S (NT) - Tuesday, Jun 20, 2006 at 11:54

Tuesday, Jun 20, 2006 at 11:54
Hi Pedro,

came acrossthe same problem last week in the desert after updating to the premium Natmap set. Spent ages before I finally twigged that Oziexplorer could be set to the map datum of GDA 94 [1994] and it showed me where I was on the moving map. As we we trying to accurately track access to the Madigan without offending indigenous land areas it was important to be correct.

We use a hand held Garmin on WGS84 for specific waypoints and we did have a slight difference but took the Garmin result as gospel. We use a Holux PCMCIA GPS to drive the moving map on Oziexplorer.

Hope this helps.
Regards
Ian@Mt dare
AnswerID: 179436

Follow Up By: Member - Pedro the One (QLD) - Tuesday, Jun 20, 2006 at 13:42

Tuesday, Jun 20, 2006 at 13:42
Thanks, Ian ............. of much help.
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FollowupID: 435655

Reply By: agsmky - Tuesday, Jun 20, 2006 at 12:33

Tuesday, Jun 20, 2006 at 12:33
Unless you require less than 1 metre accuracy (which the Map 100 wouldn't achieve in any case) then WGS84 can be considered equivalent (the same as) GDA94. The initial requirement when developing GDA94 was have a datum that was capatible with WGS84 at 1m or less. Basicallly they use the same ellipsoid and only differ in their sub-meter accuracy.

The problem more seems to be both with the inaccuracies of your maps, which in most cases cannot be overcome and the data. In any case, the GPS should be ok, the datums are ok, and the factor effecting your outcomes sound like the ability to gain accurate locations (in whatever datum/projection).

Hope that helps

Andrew
AnswerID: 179448

Follow Up By: Member - Pedro the One (QLD) - Tuesday, Jun 20, 2006 at 13:41

Tuesday, Jun 20, 2006 at 13:41
Thanks Andrew ............... your finding confirmed my view anyway, considering the Raster maps comment of 'is equivalent to.'

Me think it 'mazing that we have equipment that can get to 1 meter anyway!

Makes you appreciate what the original explorers/settlers did in their day, ey !!
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FollowupID: 435654

Reply By: Longreach - Tuesday, Jun 20, 2006 at 12:57

Tuesday, Jun 20, 2006 at 12:57
I'm not sure if this is relevant or not, but it might be of interest anyway.
GPS Problems
AnswerID: 179455

Reply By: Member - Jay Gee (WA) - Tuesday, Jun 20, 2006 at 12:59

Tuesday, Jun 20, 2006 at 12:59
I went through the exercise about 2 years ago of trying to work out "what is the practical difference - if any - between WGS84 and GDA94?". After reading mountains of websites the only conclusions I could come to was:
(1) Yes there is a difference
(2) The difference is in the order of about 8 cm
(3) The average GPS is about 16 cm long - the difference is less than the size of your GPS.
(4) The difference is far less than the accuracy of an average GPS.
(5) The difference is even less than the accuracy of a differential GPS (accuracy 1m)
Therefore - for practical purposes - there isn't any real difference

Jeff
AnswerID: 179457

Reply By: bgreeni - Wednesday, Jun 21, 2006 at 22:56

Wednesday, Jun 21, 2006 at 22:56
Even if there was a huge difference, Ozi would take care of it. The datum that the data comes into Ozi is set in the configuration, and the datum of the maps is in the .MAP file.

For xample when living in the UK I had maps with the OS grid, and GPS outputing WGS84. All worked ine.
AnswerID: 179792

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