[Tagging] Giant river multipolygons

Tobias Knerr osm at tobias-knerr.de
Mon Jan 28 16:26:42 GMT 2013

Dear tagging list,

as many of you may know, it is documented practice that riverbank areas
can be split into several small parts, as illustrated in this image:

Nevertheless, there appears to be a trend to merge them into a single
area for the entire river via multipolygons. This has been brought to my
attention by a recent changeset
which added previously separate sections of the Danube river into this
multipolygon: http://www.openstreetmap.org/browse/relation/1189126

I dislike this in two of my roles:

As one of the local mappers who have created (and are trying to keep
current) one of the affected sections of the Danube river, I find that
this relation is more cumbersome to handle in editors, history view, and
so on.

As a developer who is using something else than the standard Mapnik
pipeline, I dislike huge multipolygons because they can have devastating
effects on performance.

Of course "write better editing tools" and "tune your applications" may
be the answer to that.

But given the lack of arguments in favour of mapping an entire river as
a single area (the only documented argument seems to be that it feels
vaguely more elegant to avoid the artificial splits), I'd prefer another
answer: Delete these giant relations and return to manageable, small
riverbank sections.

Am I missing something? I'd like to hear your opinions.


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