Openstreetmap-carto: natural=shoal no longer rendered

[Roburetto]

Expected behavior (as it were until recently)

natural=shoal should depict areas as yellow
natural=shoal with surface=sand should depict areas as yellow with a sand pattern over

Actual behavior

natural=shoal shows nothing
natural=shoal with surface=sand shows a sand pattern over blue water

Links and screenshots illustrating the problem

natural=shoal
surface=sand
https://www.openstreetmap.org/way/699086566

natural=shoal
https://www.openstreetmap.org/way/699086565

Rendering suggestion

A possible rendering alternative could perhaps be to use transparency and place shoal areas under water as in the attached pic (where they are intuitively shown as transition areas in a beach - areas below coastline which became exposed at low tide).

[imagico]

That is intentional behavior - goes back to #3738.

There is discussion about some tuning of the appearance of shoal and beach outside the coastline - see #3840, but we definitely want to keep the land-water distinction as the primary visual distinction and not pretend that natural=shoal implies being above the high water line as it has unfortunately been the case previously.

The mapping recommendation - which is meant to be supported by the styling - is to accurately map the coastline at the top level of the regular tidal cycle.

[imagico]

And we don't want to use transparency because of the confusing color mixing this creates in combination with other landcover colors. See #3854.

[jeisenbe]

If the technical changes in #3854 are implemented, then it would be possible to add a subtle rendering for shoals which lack a surface= tag, perhaps a slightly darker and less saturated blue fill, similar to that achieved for tidalflat over water.

Another option would be to use a generic-looking pattern for shoals that lack a surface tag, perhaps reef.svg since there isn't a very clear distinction between when a feature should be mapped as a reef or a shoal - in most shoals are sand and gravel features, and most reefs are rock or coral features, but the wiki definitions do not require this and seem to overlap in some cases.

I would recommend that mappers consider adding the surface= tag when possible, since a shoal can be sand, gravel or solid rock, and these could be rendered with different patterns. But if the material of the shoal isn't known, then it's fine to map it without the surface tag, and we should try to accommodate this tagging with the rendering.

[georgek]

Since it is due to the same issue I should add that natural=bare_rock isn't rendering any more either.

[jeisenbe]

For natural=bare_rock see #3851