Interestingly the small GPS's used in Yacts and Fishing boats tend to be more accurate than Admiralty Charts, at least on the smaller chart scales.

Never fix your "way points" to directly at a Navigation Mark - Without a good look out and change of course you could end up with loud expensive crunching noises !

Fact - I've cox'ed the marker launch in some Radar/GPS surveys on the Medway.

Gareth