Garmin GPS 12 BNG co-ordinate wrong

loftcat · Occasional Visitor Joined: Mar 14, 2009 Posts: 4

I have a rather old Garmin GPS 12 unit which I've recently revived after two or three years without use. In the past it's been extremely reliable, but since reviving it the British National Grid co-ordinates are out by a couple of hundred yards in latitude. However if I switch the position format to decimal or long/lat it actually gives an accurate reading. I'm measuring against a 1:25000 OS paper map, and a Navman Pocket PC running Memory Map.
I've checked the GPS 12 map datum is set to WGS 84, but I'm at a loss what else I can do to correct it. It's annoying because it seems to be reading the satellites correctly, but I can't figure out why I'm getting a duff reading for BNG. Can anyone help??

dales · Posted: Sat Mar 14, 2009 11:38 pm Post subject:

Hi, I think you need your map datum at Ordnance Survey GB (OSGB) to be compatible with the British National Grid and with your OS maps.

Dales.

loftcat · Occasional Visitor Joined: Mar 14, 2009 Posts: 4

Hi Dales, I'm not quite sure what you mean by this - I didn't think there was anything else I need to change on these units. I know they are quite old but other than the software updates on the Garmin site there's nothing else you can download for it. I've also used it in the past without problems, so I'm not sure what could have caused the settings to change...

dales · Posted: Sun Mar 15, 2009 6:11 pm Post subject:

Hi Loftcat, you don't need a download for your device. You can alter the GPS12's map datum on its Navigation Setup page. (See page 44 of your manual, available here).

See the comments from BarryD (an Advanced user) in his second entry here. These comments are helpful for those wishing their GPS position display in British National Grid format, to be compatible with printed OS maps using the OSGB datum.

Dales.

loftcat · Occasional Visitor Joined: Mar 14, 2009 Posts: 4

Thank you Dales! With your assistance I've corrected my settings and it's working perfectly! Funny how I sussed this out years ago (probably by stumbling into the OS GB setting) but found myself stuck this time by being quite sure WGS 84 was correct - just shows a little knowledge can be dangerous!

philpugh · Posted: Sun Mar 15, 2009 8:30 pm Post subject:

The older GARMIN units (I have a GPSII+) did allow you to set up some illogical combinations of Map datum and display format. Modern ones don't allow you to make this mistake. Note that any data transfer done from the unit (eg track downloads or route/waypoint uploads are always done in WGS84 datum irrespective of the units map datum setting - another thing to confuse the unsuspecting!
_________________
Phil

loftcat · Occasional Visitor Joined: Mar 14, 2009 Posts: 4

dales · Posted: Sun Mar 15, 2009 11:30 pm Post subject:

Yes, I can confirm that MemoryMap deals OK with your downloaded tracks (and with your waypoints/tracks for uploading) when all the datums on PC and device are set up.

What I didn't realise, was the info from philpugh saying that transfers to/from the device are in WGS84.

However, all becomes clear now, when I remember that the OS maps on the PC (unlike the printed maps) are in WGS84 datum. So the downloads to PC are thereby consistent with the PC's maps. And the device, when set up, must take the WGS84 uploads and perform any necessary conversions. Interesting!

Dales.

dales · Posted: Sun Mar 15, 2009 11:52 pm Post subject:

PS see here, where

	PocketGPS
	Web