[OSM-talk] Mapping huge lakes as coastline

[Teemu Koskinen]

On Wednesday 13 April 2011 20:24:36 M∡rtin Koppenhoefer wrote:
> I wonder if it would not be better to map really big lakes as
> coastline. This is done somewhere, e.g. here
> http://www.openstreetmap.org/browse/relation/555716
> (Baikal lake)
> 
> but it is not done here:
> http://www.openstreetmap.org/browse/relation/1308279
> (Lake Onega)
> 
> This results in bad rendering for low zoom tiles, with the lake
> showing up on zoom6 but not on zoom5 (in Mapnik).
> 
> A few similar cases (quite big lakes modeled as water and not as
> coastline) can be found all over the world, e.g.
> - http://www.openstreetmap.org/browse/relation/1239458
> (Vänern in Sweden, which by the way has a warning attached that
> suggests it once was a coastline ;-) )
> 
> - http://www.openstreetmap.org/browse/relation/38997
> (Lago de Nicaragua)
> 
> - some lakes in Canada, ...
> 
> Modelling these as multipolygons also might slower the rendering speed
> the more complex the polygons get by adding further detail.
> 
> cheers,
> Martin
> 

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.

Teemu Koskinen