Rendering of coral reef at lower zoom levels

[c933103]

Currently, coral reefs like https://www.openstreetmap.org/way/425270917 will only be rendered up to level 10, but from the scale of the coral reef in the picture, wouldn't it be more appropriate to render it up to ~level 6?

[jeisenbe]

Currently we only render natural=reef areas with an overlay pattern of blue dots. Patterns such as this do not work well at low zoom levels, where features are often quite small on the screen. While this bank is quite large, many mapped reefs are only a couple hundred meters across, so would only be 3 or 4 pixels wide at z10: the pattern is not understandable at that size.

We would also like to add specific patterns to distinguish coral, rock, and gravel reefs in the future; these patterns will work even less well at z6 to z9.

But we are currently discussing how features in the intertidal zone, such as beaches and shoals, might be rendered differently. A slightly darker and less saturated blue fill color might work for areas of shallow water such as reefs and shoals, without requiring the use of a hard-to-see pattern at low zoom levels. See #3864, #3854 and #3840 for more.

[imagico]

The wiki currently defines natural=reef extremely broadly but actual use is much more limited to fairly shallow areas. This has reasons mainly in what is practically observable - on the ground and on imagery. This limits practically verifiable mapping of reefs mostly to areas less than about 20 meters deep and that is also what can be practically found in the database tagged as natural=reef. The Macclesfield Bank is on average about 40 meters deep and what you linked to does not seem to be drawn based on verifiable observations.

There are also extensive shallow reefs, in particular carbonate platforms like the Bahama Banks but so far most reef mapping concentrates on smaller coral reefs around tropical coasts.

Current tidalflat rendering starts at z9 (well - except for the general wetland dashing - which has been extended to z5 in #3458, but that does not really make a lot of sense) and i'd say that is a pretty reasonable starting zoom level for reefs as well when rendered in the current form with a pattern.

In the ac-style i recently tried switching to a solid color for reefs for the lower zoom levels:

https://github.com/imagico/osm-carto-alternative-colors/blob/master/landcover.mss#L920

but i have not much evaluated that in rendering yet.

[c933103]

Wouldn't it be more common to use lighter blue to represent shallow water, instead of darker blue?

[jeisenbe]

Both styles are used by different types of maps. From what I’ve seen, it’s common to use gradations of light to dark when terrain relief is also represented by a spectrum of different colors. When there is only one color for shoals or intertidal areas, darker blue or a more brownish color are also common. But lighter, less saturated blue would be worth testing for reefs, shoals and beaches too.