Changing grid references from Memory Map to Garmin

[mow]

I have a Garmin eTrex Vista Cx running with Topo Maps. However I use Memory Map to route plan and download data to the GPS.
What I can’t understand is that the grid references differ once the download is complete i.e. 31212 on Memory Map becomes 31214.
I can’t think of any reason the transfer of data would adjust the figures.
Can anybody help ?


Mike

[philpugh]

The only thing I can think of is map datum. GARMIN units always use WGS84 as the datum for all data transfer irrespective of what you have set the GPS unit to for day to day use.

I'm not sure I understand the 'grid ref' you quote in your question but...

I assume Memory Map is using WGS84 as it's datum. Check what datum your GPS is set to. A different datum will cause an apparent shift in your position. Datums can cause a lot of confusion.

[mow]

I am getting a bit confused about this Map Datum. I set my Garmin to British Grid and hence the Map Datum automatically chooses Ord Srvy GB. Only if I select “hddd mm” etc etc do I get the choice of Map Datum and can select WGS 84. Are you saying that the selection of British Grid is the problem ?

[philpugh]

Almost certainly YES!

Using OSGB display with WGS84 datum will plot your reported position off by as much as 50-100 mtrs.

Datum catch most people out the first time they come across them. If in doubt - set everything to the same datum. If you want to check your current postion against an OS map then you will have to set the GPS to OSGB datum and use the Eastings/Northings display to allow you to do this. Your OS map is already at OSGB datum

More confusing is the fact that GARMIN units use WGS84 exclusively when uploading/downloading data (waypoints etc) irrespective as to which datum it is set to use for display - so your PC based mapping s/w needs to be able to do datum changes also. A lot of free/shareware apps don't. OziExplorer is a fine example of a Topo program that handles datum issues very well.

[philpugh]

Sorry - another point. I have an old GARMIN that allows LAT/LON display with OSGB as the datum - this is valid as OS maps do have Lat/Lon calibration on the side - but it is easier to use Eastings/Northings!

You obviously have a newer GARMIN. I've just bought (on Ebay) a GARMIN GPS60 as a replacement for my GPSII+ for my outdoor activities. I will have to try it out and see if it allows Lat/Lon wioth OSGB datum!

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