Fixing the 200 metre error for entered waypoints on your GPS

[ozjeeper]

I recently had an interesting day of geocaching with five DNFs because the GPS kept showing locations that appeared to be around 200 metres away from where the cache should have been.

I had been playing with the GPS settings and had inadvertantly left it set to AGD66 instead of WGS84. All waypoints subsequently entered whilst the GPS parameter was incorrectly set were entered at a location 200 metres from where they should have been. All waypoints that had been previously entered before the parameter had been changed still retined their correct locations.

As the AUSLIG Map-reading Guide states on pages 9 & 10, "GPS is based on a geocentric datum known as the World Geocentric System 1984 (WGS84). A major implication of this change is that GDA co-ordinates, both lattitudes and longitudes and eastings and northings differ from their AGD predecessors by approximately 200 metres in a north-easterly direction".

So, if you're Geocaching (in Australia) and start getting GPS readings that don't make any sense and they appear to be about 200 metres "out", check your GPS parameters and make sure that WGS84 is selected instead of either AGD66 or AGD84.

[bootleg-homebrew]

as a Newbie, and im feeling that i will end up with egg on my face.

I have scoured the site and several others to realise that there is sometimes an easy fix, I have tried these and come to the conclusion that maybe this is not a setup error.

I can't find any logs that confirm that this has happened befour, 200m disctrepancy's with the correct setup, double checking coordinates after a WGS84 change and even checking several times.

Is there any way of getting a location checked on a Benchmark or Trig point?

[caughtatwork]

If you have your GPSr set to something OTHER than WGS84 and you realise an error has occured and you set your GPSr to WGS84 ... you MUST re-enter the co-ordinates into your GPSr.

If JUST change the datum, your GPSr will still point you to the WRONG location as all it does is recalculate the same position in the new datum.

You MUST re-enter the co-ordinates if you change the datum.

[bootleg-homebrew]

Eventually I hunted out another GPS in the area, Convinced myself that mine is within a reasonable tolerance (10m),

Thanks to all those who posted a reply, now to get to an event and pick up some tips of the trade

[Kerry]

If you have manually entered WGS84 coords with the GPS set to some other datum then the only way to fix these is to kill them and enter them correctly. Effectively WGS84 coords entered while GPS datum is AGD can result in TWICE the difference. Note the 200 metres between datums is not an ERROR, the ERROR is in the user

Any waypoints saved (waypoint save option) while the GPS was set to another datum (or basically any local Oz datum) will be ok as these will be converetd when the datum is changed. Note that most units do this these days BUT there are some older models that do not.

Regards, Kerry.