If you’re still struggling with chart datums for GPS, YM’s postbag indicates that you’re not alone! Search an Admiralty chart and you’ll find a statement like this one. Once you know your chart’s datum – in this case ‘Ordnance Survey of Great Britain (OSGB) 1936 – you have two choices if you intend to plot onto it. Ideally, re-set your GPS receiver (search the setup menus) to this datum. If you can’t do that, use the ‘Satellite Derived Positions’ information to adjust your GPS plot, but only when super-accuracy really matters. Most of the time out at sea such issues are of academic interest only, but when you’re looking for a buoy in dense fog, they can make all the difference.