Wrong GPS Datum Used

Submitted: Wednesday, Mar 31, 2010 at 12:25
ThreadID: 77340 Views:3508 Replies:2 FollowUps:12
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Hi all, I have recently been reviewing some old plot files. Several were obviously taken using the wrong datum in the GPS for the map loaded into the Laptop (Natmap raster 1;250K series) as the trails show as badly offset from marked objects on the maps such as roads, lakes etc.

I now use the WGS84 as my default datum and have no problems, even tho it's still not the correct datum for the Natmaps (because the correct datum isn't on my GPS unit's datum list). But it's close enough for me.

Is there any way that I can now correct the old plotfiles posthoc to WGS84 so the trails show more accurately on the maps pls?

Cheers.
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Reply By: Robin Miller - Wednesday, Mar 31, 2010 at 13:04

Wednesday, Mar 31, 2010 at 13:04
Hi John

depends a bit on which software you are using and type of plots but conversion is possible , I have loaded plots into a GPS , changed datum and read them back out.

If you are using plt files as per oziexplorer there is a tricky little thing I did to convert some of my oldies.

If you look at plt file with a text editor program then the second line displayed in the file is the datum.

e.g. Australian Geodetic 1966

if you change that line to read WGS 84 , then save the file and reload to Ozi then it will shift the whole plot to the new co-ordinates.

This way you can change anything to anything.






Robin Miller

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Follow Up By: Member - John Baas (WA) - Wednesday, Mar 31, 2010 at 16:54

Wednesday, Mar 31, 2010 at 16:54
Thanks Robin. I'm not quite sure what a text editor program is, could you suggest one pls.

However, I have since looked at the notepad for the file, by clicking on the file in Windows Explorer, and it shows that I actually did use WGS84 on two of the trails that show as offset, so now I seem to have different problem.

That is, using WGS84 more recently gives me no probs while it appears to have done for earlier files about a year ago????


Cheers.
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Follow Up By: The Explorer - Wednesday, Mar 31, 2010 at 18:32

Wednesday, Mar 31, 2010 at 18:32
Hello

See below - you need to change WGS84 to Australian Geodetic 1984, not the other way around.

Also the datum shown in the plot file is no indication of what datum was used by the GPS it is just a default setting in OziExplorer (look in Configuration - Data File Datum) ie no matter what datum you enter data in it will be converted and stored in this datum. OziExplorer must however be told what datum is being used in first instance - looks like in your case datum of incoming NMEA data was Australian Geodetic 1984 (or 66) but OziExplorer was expecting WGS84 i.e. setting was wrong on the Configuration GPS page for GPS NMEA OUTPUT DATUM.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Follow Up By: Robin Miller - Wednesday, Mar 31, 2010 at 20:28

Wednesday, Mar 31, 2010 at 20:28
Hi Guys

John , I should not assume to much - the windows program Notepad is a simple text editor and it looks like you can now see the datum that the file was made in , so just change the datum around with Notepad and you can see the file move around on a map.

Most datums we use are less than 150m out by the way.
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Follow Up By: Member - John Baas (WA) - Wednesday, Mar 31, 2010 at 20:34

Wednesday, Mar 31, 2010 at 20:34
Thks Robin, everyday I learn something new from this site.

Cheers.
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Follow Up By: The Explorer - Wednesday, Mar 31, 2010 at 20:54

Wednesday, Mar 31, 2010 at 20:54
Difference between WGS84/GDA94 and previous datums used in Australia (Australian Geodetic 1984/1966) is closer to 200m than 150m. Actual difference depends on where you are but range is between ~200m and ~180m. Difference between WGS84 and GDA94 is curently less than 1 metre (but increasing as we drift north), difference between Australian Geodetic 1984 and Australian Geodetic 1966 is ~3m.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Follow Up By: V8 Troopie - Thursday, Apr 01, 2010 at 01:45

Thursday, Apr 01, 2010 at 01:45
Greg, would you know in which direction that 200m offset is?

Reason I'm asking is my track on lake Argyle (on a boat) was off by that on my chart plotters Navionics chart whereas it showed the track on the Kimberley coast spot on.
Your Kimberley series of 50K raster charts were spot on, on the lake and along the coast, BTW.

The Navionics chart of lake Argyle is an exact copy of the paper chart, unfortunately there is no datum shown on the paper chart. Its quite old I'm told but no new ones are available AFAIK.
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Follow Up By: The Explorer - Thursday, Apr 01, 2010 at 10:39

Thursday, Apr 01, 2010 at 10:39
Hi

If you plot coordinates recorded in AGD 84/66 incorrectly as WGS84/GDA94 they will be positioned ~200m south west of the correct position (This was the case in the problem John had). And visa versa - If you plot WGS84/GDA94 data incorrectly as AGD 84/66 it will be north east of its correct location by about ~200m.

As mentioned I suspect the cause of Johns problem was that he had the display datum on his GPS set to AGD 84/66 which (on some GPS units ..not all) initiated the NMEA data stream to be sent in that same datum. Problem is that OziExplorer, by default expects data in WGS84, unless you change the setting in configuration. Most GPS units send NMEA data in WGS84 irrespective of the display datum chosen on the GPS (i.e. the datum the user sets the coordinate display to show).

I am not sure as to why your navionics unit was not plotting correctly in one location but OK in another. I am not familiar with the unit/software. Is it like OziExplorer and uses scans of paper charts? If so maybe the calibration is incorrect i.e. they used wrong datum for the Lake Argyle map. If your track was offset to the north west by ~200m (and we assume the NMEA data stream was in WGS84) then the grid on the map (used to calibrate the map) is AGD84/66 but was entered as WGS84.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Follow Up By: V8 Troopie - Friday, Apr 02, 2010 at 02:23

Friday, Apr 02, 2010 at 02:23
Thanks a lot Greg. My track was out by an estimated 150m to NE from my actual position. The correct chart plotter datum is important on the lake to find the submerged old river bed (marked on the map) which is deep while just off the submerged banks the depth was 1m or less in places.
Now I know which datum to enter should I return to the lake, its certainly well worth the long tow from Perth.

You asked about the chartplotter charts. Navionics software uses the identical information as is on official Australian marine charts. This is why I choose that brand, easy to compare to the paper charts which I also carry.
The Lake Argyle map is an odd one, it has marine info on it but is obviously a land map. It was made when the lake first was flooded AFAIK (1970's).
I only found out by accident it was included on the Navionics chart SD while panning up the Ord river from the mouth the lake map suddenly opened on the screen.

Navionics has somehow changed the marine (and lake) charts to a vector system, it will not run on Ozi, only on their plotter. I had to buy a special SD card reader to be able to open these charts on my laptop as well.
I guess they goofed when they scanned in the lake map, assuming it has the same datum as the official marine charts.
Klaus
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Follow Up By: The Explorer - Friday, Apr 02, 2010 at 02:36

Friday, Apr 02, 2010 at 02:36
No worries - looks like you've got it sussed.

Also just realised after reading your post, and re-reading mine that I should have said "If your track was offset to the north EAST by ~200m" not "north west". Sorry about that.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Reply By: Member - The Bushwhackers -NSW - Wednesday, Mar 31, 2010 at 17:16

Wednesday, Mar 31, 2010 at 17:16
Hi John

You might have the 'right' datum in your gps, but the original plot may have been recorded in a different datum

If your gps is set to WGS 84, and the trail and waypoints are out approx 220metres to the south west, then they may have been recorded in '66.

Well, thats what the book says.........

Cheers, Dave

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Follow Up By: Member - John Baas (WA) - Wednesday, Mar 31, 2010 at 17:22

Wednesday, Mar 31, 2010 at 17:22
The plot is out by about that much and in that direction Dave, but as above the notepad for the plot says it (the plot) is in WGS84 as follows:

OziExplorer Track Point File Version 2.1
WGS 84
Altitude is in Feet
Reserved 3
0,2,16711680,Track Log - 21/03/2009 12:40:36 PM ,0,0,2,8421376
448
-29.977342, 114.976413,1, -2.0,39893.1531250, 21-Mar-09, 3:40:30 AM
-29.977722, 114.976918,0, -5.0,39893.1532407, 21-Mar-09, 3:40:39 AM
-29.978122, 114.977403,0, -4.0,39893.1533333, 21-Mar-09, 3:40:47 AM
-29.978538, 114.977838,0, -5.0,39893.1534259, 21-Mar-09, 3:40:55 AM
-29.978922, 114.978257,0, -7.0,39893.1535185, 21-Mar-09, 3:41:03 AM
etc


So it's still got me stumped.

Cheers.
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Follow Up By: The Explorer - Wednesday, Mar 31, 2010 at 18:23

Wednesday, Mar 31, 2010 at 18:23
Hello
I am not 100% certain what has caused your problem, however it looks like the GPS you were using at the time was sending the data in Australian Geodetic 84 or 66 but you had OziExplorer set to accept data in WGS84. Anyway solution is relativley simple. Either use method suggested by RM and change WGS84 to Australian Geodetic 1984 in track file using Notepad....or

Open the offending track file in OziExplorer.
Select SAVE - EXPORT TRACK - TO TEXT FILE
Select WGS84 (or GDA94) as the EXPORT FILE DATUM
RECORD FORMAT doesnt matter but go for degrees etc or UTM
Click SAVE and save the file to a known location on your PC as a *.txt file

Now Click LOAD - IMPORT - TRACK FROM TEXT FILE
Select the txt file you just saved
Select Australian Geodetic 1984 as the IMPORT FILE DATUM.

Track will plot ~200m north east of current location (along powerline in example provided?)

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Follow Up By: Member - John Baas (WA) - Wednesday, Mar 31, 2010 at 18:31

Wednesday, Mar 31, 2010 at 18:31
Greg, that's done it. Great stuff.

Also, thks to Robin.

What a great Forum!

Cheers.
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