Confused About Garmin Etrex reading

[mickstar001]

HEY FELLOW AUSFISHERS WONDERING IF YOU COULD SHED SOME LIGHT ON THIS

I HAVE A GARMIN ETREX - WHILST OUT AT PEEL LAST WEEKEND FOR A BIT OF SHTIS AND GIGGLES MOTORED UP TO A MARKER ON THE NORTHERN SIDE - COMPARED IT TO THE BEACON TO BEACON - THE MARKS IN THE BEACON TO BEACON AND THE MARKS WHICH MY ETREX WAS SHOWING WERE TWO COMPLETELY DIFFERENT MARKS. THE REASON FOR THE QUESTION IS HAVE JUST PURCHASED AN EPRIB AND ARE WANTING TO ATTACK THE 24'S OFF THE GOLD COAST - WANTED TO MAKE SURE I WILL BE IN THE RIGHT PLACE!! HAVE THE SETTING ON MY ETREX TO WGS84 IF THIS MAKES SENSE AND MY OWNERS MANUAL DOES NOT ELABORATE ON THIS TOO MUCH AT ALL.

THANKS IN ADVANCE

MICK

[25_ponies]

Mick, when you say "two completely different marks", do you mean the lat+long displayed on the GPS was different to what you expected, or you had a mark/waypoint that you had already saved into the unit that wasn't where you expected it to be ?

[Blackened]

G'day
He mentioned marker, not marks so my guess is it's a beacon. I dont really know what may be the issue but how accurate is beacon to beacon? WGS84 is what most people use, but for greater accuracy get a proper chart and cross reference with this.
Dave

[SatNav]

1. You would need to first confirm a) The datum of beacon to beacon, b) The accuracy of beacon to beacon if not an actual coordinate c) how up to date is beacon to beacon relative to the current position of the marker d) the format your etrex is displaying

[25_ponies]

Mick, just as a quick check I loaded the 2 GPS waypoints shown in the B2B into Google Earth. The both look fairly accurate (see pic). How far off do you think you were, are we talking meters or kilometers ?

[mickstar001]

Yeah i was talking about the closest gps waypoint to peel - from memory i think it was a couple of hundred metres off.

[25_ponies]

Could it be that you transposed a couple of digits when you entered them into the GPS, or maybe even converted from DMM.MM to DMS format incorrectly ?

Those are typical errors that result in a couple of hundred meter variations.

[SatNav]

1. Format transpositions will be variable and range from a few metres to approx half a nautical mile

2. The mention of "a couple of hundred metres off" is generally an immediate indication of different datums as first thing to check

[25_ponies]

2. The mention of "a couple of hundred metres off" is generally an immediate indication of different datums as first thing to check

True, but Mick said that his Etrex was set to WGS84, and the B2B also uses WGS84.

[mickstar001]

Yeah 25 Ponies you get my drift i think, perhaps numbers were inputted incorrectly but dont think so. ?????????

[SatNav]

1. If it's not a datum issue, not a transposition issue or a format issue and there's this 200 metres difference then you still have a problem.

2. Where the decimal parts are more than 59 then it becomes fairly obvious in mistaking unintended formats but it has been know to have occured, more often than some would acknowledge.

3. The only way any GPS would allow coordinates with decimals greater than 59 would be a D MM.mmm or decimal format. If, and then it's a big if, if those closest coordinates had by chance been converted to DMS then entered as DM.m the difference is a bit over 800 metres, so no match.

4. What coordinate format is this beacon to beacon book?

5. If none of the above then the only thing left is the beacon has moved? Somehow I would check the obvious and check the obvious again before thinking the beacon has moved "a couple of hundred metres"

[mickstar001]

never thought of the beacon moving satnav - dont know im really confused now - may have to stop a fellow boatie soon and compare gps's i think!!!

[SatNav]

1. Now if the beacon hasn't moved and this is why you probably need to check the obvious then re-check again, eliminate movement as a factor (which it probably isn't anyway) and then it all comes down to a user issue on one side or the other.

[jimbamb]

Maybee im mile off track.. not trying to confuse the issue......... you have to be moving to get accurate readings off handheld ..................did you motor up to the mark or turn it on when you got there???could be the problem..

[SatNav]

1. No GPS handheld or otherwise has to be moving to get accurate positional readings, heading or direction you do but this is not part of the problem here.

2. Now you could get incorrect readings if for example you got to the mark, switched the GPS on and immediately noted/logged the coordinates before the GPS had time to initialize. But these coordinates would most likely be at the location the GPS was last switched off.

3. What coordinate format is this beacon to beacon book? Anybody know?

[25_ponies]

. What coordinate format is this beacon to beacon book? Anybody know?

It's WGS84

[SatNav]

1. WGS84 is a Datum, coordinate formats are things like Degrees Minutes Seconds, Degrees Minutes decimals etc

[25_ponies]

yep. good point. It's DD MM.MM - see my post above with the image, the coordinates in that are exactly as per the B2B