[BUG] Coastal DEM percision gap leaving artifacts

[SilconFinta]

Coastal terrain is usually deformed, and due to the gap between DEM data sets, it is left unkept and may present huge holes in some places. The extension of the waterfront is disabled since it will destroy some terrain, but this has caused some other weird things.

[github-actions]

Found 1 possible duplicate issue:

1. [FEATURE] Better coast generation #1118

If this resolves your issue, please close it.

🤖 Automated triage — may be inaccurate; a maintainer will follow up.

[SilconFinta]

Seems accidentally fixed it in Japan regions, by disabling fallback AWS tiles and smearing the NaN value. GSI maps have a clear coastal boundary; therefore, all regions with water are left at 0, or sea level. This method may be used in other regions with appreciable DEM cover and clear coastlines.

clearly better shapes than filling NaN values,

,
much better than the default fallback and whatever.