Why are many people so hung up about metre, centimetre, millimetre, accuracy when it just isn't needed for navigation
If you are down to needing such accuracy; you should be outside on the wheel
and not even looking at the plotter. The 10m or so that GPS typically gives these days is more than enough accuracy to get you to where you need to go. The charts
in most of the world are much less accurate than that even when corrected for datum, spheroid, etc..
I have seen chart errors, when referred to GPS, of more than half a mile and I know there are bigger variances out there. In one case the GPS position on a chart moved from having a large (quarter mile or so) error to the east at its eastern margin to a similar error west at the western margin. This chart covered about 15 miles. The same data as the paper chart is in C-MAP
etc.. But the the chart is perfectly usable for pilotage - again get your head
outside and off the screens when in close.
I have a photo
of the plotter screen
tiled with radar
showing a smiliar offset which I'll dig out and post one day.
Ships use extreme accuracy at times, for docking
etc.. but they are not going to be using opencpn
but dedicated systems with a whole array of sensors, GPS of which is only a part.