[mkgmap-dev] Sea generation

Fri Nov 5 08:21:50 GMT 2010

On 2010-11-04 23:04, WanMil wrote:
> Some loud thinking:
> Would it be helpful if we copy the Mapnik behaviour of well defined
> coastlines? One could define one separate file that contains all
> coastline data (from europe, from the world?). This file could be
> maintained and improved in a better way than it is done in the OSM data.
> natural=coastline would be ignored/removed from the OSM input.
> As a second step the separate coastline file would build a basis and the
> data from the OSM could be compared to this. Differences in the OSM
> files are only accepted if they don't invert the sea polygons and if
> they don't build new gaps.
>
I think it would. With so many inexperienced users one should expect 
that the coastline is broken at points.

According to the coastline wiki page:
"At high zoom levels the coast polygons used are generated from the 
natural=coastline tag -- the data is made available to the Mapnik 
renderer as a large shapefile (processed_p) which is generated every few 
weeks from planet dumps"

I gather that this file should contain a good coastline:
http://tile.openstreetmap.org/processed_p.tar.bz2 (357 MB)

> Question:
> Has anyone extracted coastline data for the whole world/europe/asia/...?
> How many data is this?
>
I've not done it yet, but will do...

> Yeah. The mp code throws all polygons away that don't touch the bounding
> box. Additionally it closes polygons automatically if their endpoints
> lie outside the bounding box.

Would that mean that a polygon is closed inside the map instead than at 
the tileborder if there aren't nodes within the overlap area?

That would be unfortunate because processing time greatly increases if 
the overlap area is increased in an attempt to include more nodes.