[Tagging] Lakes, islands and multipolygons

Sam Vekemans acrosscanadatrails at gmail.com
Fri Oct 16 00:30:48 BST 2009


At least from what i understand...
I wouldnt use the tag natural=coastline  simply because coastline becomes
confusing when your getting to the 'coast' where there are lots of islands..

im still working on it, but i do have a shp file for the islands (just not
imported yet)
http://www.openstreetmap.org/?lat=62.1747&lon=28.6278&zoom=13&layers=B000FTF

What i plan on doing is simply tagging the islands as place=island
.. and leave all the rest as open space. un there are 'natural=marsh'  or
sub_sea=reef's around... then leaving it empty would be fine.

and just having nodes around saying what the is called.

... but. ... on the other hand...  when using mkgmap for garmin.. the
coastline doesnt know what is land or what is water.   ... i think thats a
fault of mkgmap though.
http://www.openstreetmap.org/?lat=48.407422&lon=-123.366209&zoom=18&layers=000BFTTT

As kosmos make it look cool.
(btw, reef is needed for boating & kyaking maps)

Hope that helps,
cheers,
Sam

On Thu, Oct 15, 2009 at 3:51 PM, Jukka Rahkonen <
jukka.rahkonen at latuviitta.fi> wrote:

> 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-
>
>
>
>
>
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>
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