Coastlines in 3D landscapes

I’ve recently encountered some interesting edge cases with regards to coastlines that I would love to discuss with the community.

The natural=coastline tag is used to mark the mean high water springs line along the coastline at the edge of the sea.

According to Tag:natural=coastline - OpenStreetMap Wiki.

This definition leads to some problems when encountering overhanging rock along the coast. An overhang crossing the ocean creates an area that is simultaneously land and sea when seen from above, with land occupying a higher level than the sea.

It seems that this is usually dealt with in OSM by ignoring the problem. The coastline is mapped at the edge of the overhang, where the ocean becomes visible in aerial photography. This approach, however leads to problems down the road. For instance in this case: Way: 1167228743 | OpenStreetMap. Here we have a bay that stretches in under a rock arch and appears on the other side. Following the mapping approach described above leads me to have to map an isolated ocean. This isn’t ideal as it hides the fact that this piece of sea can be traveled to by boat.

An other alternative would be to follow the tag definition more closely and map the coastline as it appears at sea level. That would result in me mapping this as a bay. However, I find this problematic as it hides the fact that the area of the arch is also land.

In Norway I have a similar conundrum, where the ocean goes though a culvert: Way: 1156824482 | OpenStreetMap. In this case I’ve taken the opposite approach. I hope this illustrates why I fint this style unsatisfying.

Is there a standard approach to mapping such land/sea conflicts in OSM that I don’t know about?

What would you do in these cases?

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