[OSM-dev] coastline polygons

[Robert (Jamie) Munro]

Robert Hart wrote:
>> One main problem with storing it in OSM is that renderers can't do
>> anything useful with it, since you can't tell the inside from the
>> outside.
> 
> I think that should read "One *minor* problem with storing it in OSM is
> that renderers can't *render the sea blue and the land green*, since you
> can't *currently* tell the inside from the outside."
> 
> Personally I think the advantages of having the coastline in the OSM
> dataset do outweigh the disadvantages.

What advantages? I don't know any. We can put the coastlines on the
slippy map without them being in the OSM database, so why duplicate?

Disadvantages:
* The OSM database is struggling as it is. Coastlines are thousands of
extra nodes that we don't need.
* Coastlines are much too large to be represented by a single way - it
will always be "incomplete" when you download it, and things will break.
* Having the coastline in multiple ways breaks the topology of the
system, and requires horrible hacks in renderers to make use of.
* The public coastline data derived from STRM and other places is
adequate - we can't usefully improve it with our GPSs and wiki-like
editing methods.

> A simple rule like "sea is on left" or some
> such, would be more than sufficient to solve your "main problem"

No it wouldn't. That would only solve the problem if you are rendering
something that actually had a stretch of coastline in it. If you find a
completely empty area, you have no idea if it is in the sea or in the
desert.

Robert (Jamie) Munro

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