[Tagging] Giant river multipolygons

Werner Hoch werner.ho at gmx.de
Tue Jan 29 20:25:05 GMT 2013

Hi Paul,

Am Montag, den 28.01.2013, 17:47 -0600 schrieb Paul Johnson:
> On Monday, January 28, 2013, Werner Hoch wrote:
>         There are a few of that monster relations out there:
>         http://www.h-renrew.de/h/osm/osmchecks/02_Relationstypen/planet/bd8a1061c196c9de.html
>         Some are tagged with type=collection. That's not better or
>         worse, just different.
> What's wrong with
> http://wiki.openstreetmap.org/wiki/Relation:waterway?

This relation type only defines relations for center lines of the river.


It's really easy to maintain this kind of relations for center lines of
waterways. The longer the river, the river became wider. The node
density of the center line gets smaller and smaller. Even large rivers
only have a few thousand nodes.

In the opposite the riverbank ways and areas don't scale with the width
of the river. You have to place a node every 10 to 100 meters for the
riverbank ... on both sides .... and islands.

The riverbank relations are not maintainable, as they are much larger
I guess by factor off 10 to 100. I've never seen a complete riverbank
relation of a large river, yet. But few thousand river/waterway
relations of center lines.

The other reason not to collect the riverbanks is, as soon as you have
the centerline, a GIS program can collect all riverbank areas along that
centerline. It is a waste of time to collect them manually.

If you look into the wikipedia articles:
http://en.wikipedia.org/wiki/Danube [1]
http://en.wikipedia.org/wiki/Amazonas_River [2]
and use the globe symbol beside the coordinats (top right), You'll see
the waterway relations on the map (see WIWOSM [3] for details).

[1] http://www.openstreetmap.org/browse/relation/89652   6567 nodes
[2] http://www.openstreetmap.org/browse/relation/2295651  821 nodes

In comparison the Danube riverbank, only 50% mapped to Black Sea
http://www.openstreetmap.org/browse/relation/1189126  28933 nodes

[3] http://wiki.openstreetmap.org/wiki/WIWOSM


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