[Talk-GB] Boundary_line at the coast

[Edward Bainton]

I'm interested in boundaries marked at Mavis Grind
<https://osm.org/go/e7tUHqY_g-> (thanks to SK53 for the waterway=portage
<https://wiki.openstreetmap.org/wiki/Tag:waterway%3Dportage> tag - Mavis
Grind is an old Norse portage, still in use by Shetland Canoe Club).

1. Does anyone know if county boundary lines at the coast are set at mean
low water? There's a gap between coastline (which I understand is MHW) and
the county boundary: https://osm.org/go/e7tUHoc5F--?m=&way=669235281

2. "coastline" is very coarse - is it ok to make it follow the coast more
finely, or is it some important legal line where it stands? (I've already
done this on the east side of the portage, but then thought perhaps that
was a no-no: https://osm.org/go/e7tUNAM7G?layers=D&m=&way=669235281)

3. Also, there are two walls visible on aerial imagery that all but match
the doglegged county boundary as it crosses the isthmus. Is it safe to
assume that these mark the actual boundary, and can I tug the boundary to
match them? Or maybe assume the boundary is definitive, and the imagery is
misaligned, so I should move the walls? Or leave well alone?
West side: https://osm.org/go/e7tUHqdxn?m=&way=669235281
East side: https://osm.org/go/e7tUHrpqY?m=&way=669235281

Thanks as ever, Edward
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