I have made several edits around the world related to the OSM coastline. My goal is to enable small-scale derivation of land and ocean polygons without resorting to global preprocessed continent geometries assembled from programs like OSMCoastline.

As a primer, the coastline should be mapped as ways with natural=coastline, with land on the left side and ocean on the right side. This is specified on the OSM wiki: Tag:natural=coastline. “Land” in this instance is defined as the non-ocean parts of the world, not solid ground in general; for example, the Great Lakes are represented not by coastlines but as water body features inside “land”.

There are a few implications to this design:

The ocean should be one polygon in the OGC Simple Features sense: it has one outer ring in the clockwise (CW) direction, and a counter-clockwise (CCW) inner ring for every continent and island. The Caspian Sea is the one exception to this single polygon as stated on the Wiki.

The complement of the global ocean polygon is thousands of land polygons representing continents and islands. Each polygon has a single CCW outer ring and zero inner rings. Again, the one exception is that the Eurasian continent polygon has a CW inner ring representing the Caspian Sea.

In theory, this specification should be enough to infer land and ocean polygons from orientation even within a small geographic extract. I discovered and corrected several dozen places where a violation of this specification arises. The image below is an example of data that needs to be corrected:

These problems are sometimes simply mistagged islands within inland water bodies (Changeset 78960812), but can arise even when mappers are mindful of oceans and way orientation. From edit histories this seem to happen because:

A mapper correctly adds island A based on satellite imagery. Years later, a mapper has access to higher resolution imagery and adds island B (way 677293895). However, B is wholly contained within A: the ocean defined by B’s boundary contradicts the land defined by A’s boundary.

Changeset 78947512 : A case where a hydrological feature has been changed from part of the Single Ocean Polygon to a more detailed object, such as a river delta, estuary or bay multipolygon. The outer ring of the Single Ocean Polygon recedes, but now its islands are no longer contained.

I have corrected most occurrences of this problem as of December 2019, which should enable more flexible land and ocean mapping from raw OSM extracts! Inevitably these issues will crop up again as the global coastline is better digitized, so I hope this post can help mappers fix them.