IMHO CN and GPS should only be used for voyaging and bringing you up to the coast. Once there, to me GPS is never more than a check against MarkI Eyeball navigation more commonly known as pilotage.
If CN and GPS are used in this way, a 1 mile variance is actually more than good. Sure a GPS is good when visabilty is poor but it should never be depended on in proximities closer than 1 mile. Many charts are not that accurate.
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