Uploading to eTrex

[Ante]

Hi all,

Tested GPS Utility v. 4.10.7 for uploading waypoints to my eTrex. In GPSU I had the grid set to Swedish and the datum to RT 90. Chose a coordinate of the map: E 1490000, N 6235000. Set my eTrex (v. 2.14) to the above: Swedish grid and datum RT 90. The upload went real smooth.

Then I checked the waypoint coordinates on the eTrex. They were shown as E 1489813 and N 6235636.

Can anyone shed some light on what the problem is?

BTW, does Garmin use a 5 parameter or 7 parameter grid conversion when converting from WGS 84?

Cheers

[MikeB]

Ante,

I have come across this problem a few times, although I havent used GPSU. My problems normally come using OziExplorer, Fugawi, Memory-Map and Anquet Maps.

I have found that the only consistent way to manage Waypoints properly is to import and export them in WGS84. That is almost certainly where the error is creeping in.

What happens when you set your GPS receiver to use the WGS84 datum? Do you get the waypoint reported in the correct position? It might be an idea to set your waypoints up in GPSU and then export them in WGS84 datum format to the Garmin.
_________________
Mike Barrett

[Ante]

Hi Mike,

I just tested the following:
Put in the coordinate in GPSU in Swedish grid and RT 90, converted to UTM and WGS 84 and uploaded to the eTrex. Checked the point in the GPS-R set to UTM/WGS 84. The coordinates were identical. Changed the GPS-R setup to Swedish grid/RT 90. The waypoint now read as before:
1489813 instead of 1490000 and 6235636 instead of 6235000.

Can the fault lie in Garmin's conversion?

[MikeB]

I believe what you are seeing is correct.

When you change the datum you also change the position. The main question is what is the correct lat/long and datum. If you can verify these then you should use those as the basis for all your waypoints.

What I am saying is that a point at say 0,0 in one datum may be represented by 12,75 in another and you need all three elements to fix the position. This is not a problem if you only ever work in one system, but problems will occur if you try to apply the position in one datum to the same co-ordinates in the other, it just doesnt work that way.

Having said that I am not an expert at these conversions, I have just done a few and found that converting everything to WGS84 then uploading then converting to the required datum always worked for me.
_________________
Mike Barrett

[Ante]

I think I've found a solution. The Swedish mapping agency, see www.lantmateriet.se, have on their site the parameters required to set up a user grid on Garmin units that makes for a better conversion between WGS 84 and the local grid.

In GPSU, with grid set to Sweden and datum set to WGS 84, I put in the same coordinates, E 1490000 N 6235000, taken from a map compiled with Swedish grid and RT 90 datum. I set my GPS to the customized "user grid" and datum to WGS 84. After uploading the above waypoint showed the exact same value :D

Cheers

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