GPS Datum Probs

Submitted: Wednesday, May 20, 2009 at 23:26
ThreadID: 69023 Views:2251 Replies:3 FollowUps:3
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Dear all, I'm running the 1:250k Natmap sheets via OziExplorer software using a Garmin GPS72.

These sheets are on GDA94 datum. However the 72 only supports GDA datums GDA66 and GDA84. I have opted for GDA84 but.....

I think this is the cause of my track lines being parallel but well separated from the tracks shown on the screen.

What might be the best alternative datum to use ps? WGS84?

Cheers. John.
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Reply By: bgreeni - Wednesday, May 20, 2009 at 23:47

Wednesday, May 20, 2009 at 23:47
In OziExp configuration - GPS you can set the datum of the incoming data - there is a long list.

I am not familiar with the Garmin, but the "native" datum for GPS is WGS 84.

Don't worry about the datum/projection of your maps. Ozi takes care of that internally and can use maps of various datums/projections, so long as the .MAP file has it correct, and using the Natmap sheets that will not be a problem.

Bruce
AnswerID: 365916

Follow Up By: Sixty at Last - Thursday, May 21, 2009 at 00:18

Thursday, May 21, 2009 at 00:18
Great Bruce. Many thx. John.
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Reply By: Zebra400 - Thursday, May 21, 2009 at 05:04

Thursday, May 21, 2009 at 05:04
John

2 issues here. Make sure your GPS datum is set as WGS84. This is the best datum to use as it is the datum format that your GPS receives from the satelites.

You also need to make sure OziExplorer has the same settings in the config file. From the Ozi menu bar click on 'File' 'Configuration' then select the GPS tab. Make sure the two bottom left boxes have the Datum set as WGS84. If they are different to what you set your GPS datum as, then Ozi will not receive your data in the correct datum. The GPS upload/download is the datum format if you plan to upload or download waypoints, route or tracks, and the GPS NMEA Output Datum is the datum you use with moving map. I have both of them set as WGS84.

Unless you plan to go walking with a paper map, then there is no need to change the datum in your GPS. However, if you do a lot a walking, you will want to change the datum in your GPS to match the datum of the map you are using. Just remember to change it bak when you return to your car.

Now the second issue, is setting the datum for the loaded map in Ozi. You dont have to change it from the datum in your map configuration file. It is up to you. Ozi will still track correctly. To see what the current loaded map is using, look on the line of data below your menu bar. You will see it most likely displays the lat/long and UTM position where you cursor is currently shown on your map. This datum for a particular position will change if you change the datum it displays for your map. (It does not change the datum in your calibration file). You will notice if you move your mouse, the data changes. To the right of this is the Datum of the currently loaded map. If you left or right click on the datum, you will see a drop down box. you can either select your own datum or select the top option which is the datum of the current map. It is up to you whether change the datum, but it will not affect moving map. You will still track in the correct place. It is just your positioning data that will change.

I hope this helps.

Laurie

AnswerID: 365924

Follow Up By: Sixty at Last - Thursday, May 21, 2009 at 11:25

Thursday, May 21, 2009 at 11:25
Much appreciated Laurie, many thks. John.
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Reply By: Member - Mike DID - Thursday, May 21, 2009 at 06:24

Thursday, May 21, 2009 at 06:24
GDA94 and WGS84 are virtually identical.
AnswerID: 365926

Follow Up By: Sixty at Last - Thursday, May 21, 2009 at 11:26

Thursday, May 21, 2009 at 11:26
Cheers Mike. John.
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