Salut Frank, long time no see
> ... What if you get an extract of OSM, and you're only
> interested in the forests, because you want to calculate the percentage
> of forest coverage. You don't get information about lakes, heath and
> other land uses when you don't cut out holes from multipolygons.
You're right but that's a moot point.
From my point of view, OSM is not a tool for GIS professionals. It's a
community project ("activité citoyenne"). From a scientific and rigorous
perspective, a forest must have a hole for any 2D entity (lake, river, road,
street and building). But, if you go that way, editing the map will become so
complex (or arduous as you say) that no normal contributor will be interested
any more.
Look at this "monstruous" way:
http://openstreetmap.org/way/175486790
It's a CanVec import with 1420 nodes, part of relation 2344036.
The way was imported after many of the streets have been created. So the
forest is "on top" of streets and roads and there's no hole for them.
Do you really think a human being will get satisfaction and pride if he/she
have to dig holes around every street?
And look at the above example. The golf course is on top of the forest
(without a hole) albeit it has a significant area.
It the goal is to use the OSM database as a rigorous GIS ressource, then the
tools designed for humans (Id, Potlatch, JOSM) will have to be modified to
automatically create a hole around any 2D object. If they are not modified, you
may say goodbye to normal contributors.
You may also broadcast this message to Europe because the first european forest
I looked at do not have holes for lake, rivers and roads.
http://www.openstreetmap.org/#map=15/52.1965/-3.8386
Regards,
dega
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