Why are Memory Map GPS Waypoints off by 50m?

Q. Hello

Could you please advise on the following:

I have downloaded the GPS Waypoints for the Pennine Way Trail which I am planning to walk in the summer.

I have also got Memory Maps Pennine Way National Trail CD and when I compare the waypoints on their CD map to the ones taken from the download there appears to be a difference in the actual locations.

For example:

PW009 (SK 06166 98592) which is marked as A628 SW of Crowden actually comes up as being in Torside Reservoir!

Which is the accurate reading on the ground - Your GPS waypoints or the Memory Map CD?

I am going to use the GPS as a backup on the walk but before leaving home I would like to make sure that the backup is as accurate as possible.

Thanks

Guy Teasdale

A. It depends on which Map Datum you have set your GPS device to.

All go4awalk.com GPS Waypoints use:

Position Format = British Grid
and
Map Datum = ord srvy GB

and you must set you GPS Device to this Position Format and Map Datum (see Setup and then Units on your GPS unit) for the readings to be accurate.

This datum matches the Ordnance Survey Grid Reference system used on all Ordnance Survey Maps.

Memory Map uses a different datum - WGS84 - and as a result the GPS waypoints are all 'off' by 50m. (Why Memory Map uses this datum for British mapping is unclear - and why you cannot change the datum within the software is even more unclear. It may be because they are an American company).

So to answer your question, you should set your GPS device to: