[OSM-talk] Mapping huge lakes as coastline

[Jukka Rahkonen]

Teemu Koskinen <teemu.koskinen <at> mbnet.fi> writes:

> 
> I converted a few of the biggest lakes in Finland a few years ago to 
> coastlines, and they worked fine, until last year some other user converted 
> them to multipolygons with natural=water -tags. He also splitted the biggest 
> lake (Päijänne) in pieces, which created arbitrary lines across the lakes at 
> random where the lake was divided to different polygons.
> 
> The biggest lakes in Finland have tens of thousands (or even hundreds of 
> thousands) nodes and a LOT of islands, so it's not practical to represent them 
> as (multi)polygons IMO.

It is not practical, either, to represent them as coastlines. For example
osm2pgsql is not importing coastlines into PostGIS at all but users must use the
processed land polygons as shapefiles for rendering these coastline lakes. One
may say it works fine with Mapnik rendering because of this shapefile
workaround. Some could call it as a dirty hack. For example, it gets complicated
when somebody wants to add tags for the lakes and islands.

By the way, i checked that the biggest lake polygon in the data of the National
land survey of Finland is the lake Saimaa, and it has exactly 287273 vertices
and more than 5000 islands. It is a bit heavy to handle in PostGIS and Oracle
Spatial and with GIS programs but not at all impossible.

There is a wiki page about the future of areas in OSM. Handling big lakes is one
more thing to be discussed there, see
http://wiki.openstreetmap.org/wiki/The_Future_of_Areas

In the data