[OSM-dev] Coastlines and water - contact and technical

[Lennard]

Ben Supnik wrote:

Replying to list...

> Please forgive the newbie questions -- I am trying to determine all of 
> the requirements to find the "wet" areas of a finite region of the 
> world.  Basically my goal is to use OSM as a water source for flight 
> simulator scenery...both oceans and lakes.
> 
> Unfortunately the "spec" is sort of spread out among schemas for mapping 
> renderers, wiki pages, some out of date, discussions, etc.
> 
> Do you know whether the major map rendering engines support 
> multi-polygon where the ways forming the outer and inner rings are not 
> closed loops?

I can only speak for mapnik and the associated osm2pgsql import tool, as 
my only experience with osmarender is limited to running a t at h client in 
the past. osm2pgsql will form polygons in the postgis db when the rings 
are closed, but a ring doesn't have to consist of a single way.

For example, an outer way can be made up of a few ways, which together 
form a closed ring, all tagged as role=outer. Once a proper postgis 
polygon is created in the db, mapnik can render it.

Oceans are handled differently. The base colour for the map is the ocean 
colour, and all land areas (continents and islands in the ocean) are 
sourced from one of 2 shapefiles, depending on the zoom level. If you 
want to have the oceans as polygons, you're gonna have to do some 
inversion process on them.

So the layering is:

1) oceans as default base colour
2) land masses from the shape files
3) other landuse, waterway, highway, etc features from the postgis db

-- 
Lennard