[Tagging] Lakes, islands and multipolygons

[Jukka Rahkonen]

Hi multipolygon masters,

How would you tag some island-rich Finnish lakes, for the one here:
http://www.openstreetmap.org/?lat=62.1747&lon=28.6278&zoom=13&layers=B000FTF

Currently big lakes in Finland have been tagged as coastlines which is a
good workaround and looks correct in slippy maps.  However, it is not
right, because they are not seas but just lakes.  Additional drawback is
that these fake-seas are hard to use for non-OSM purposes because they
won't get imported by osm2pgsql.  Cloudmade shapefiles are also very
messy, islands can be water and water dry land even if coastline tagging
and ring directions are OK.  But tagging big lakes in some other way is
problematic because the outer rings tend to be rather large and there can
be quite a many holes in the lake polygons.  Such an OSM multipolygon
relation could be a bit painful to edit afterwards.  And actually each
inner polygon should be an own island/islet polygon as well with own tags,
at least a name tag.
The biggest lake polygon in the data from National Land Survey of Finland
has more than 280000 vertices. Osm data is not as accurate and has less
nodes, but even OSM lakes can be quite complicated.

By the way, why do we call all polygons having holes as multipolygons,
even if they have only one outer ring? But of course they can be
considered as simple multipolygons.

-Jukka Rahkkonen-