[OSM-talk] Fixing broken multipolygons, some notes

Frederik Ramm frederik at remote.org
Sat Mar 18 22:24:42 UTC 2017


   if I understand you correctly your basic message is "let's try and
improve osm2pgsql's polygon interpretation rules instead of fixing the
data", or at least "while we wait for the data fixing to be completed".

I think this is not a good idea because, as you remark yourself, those
working with the OSM source data directly will not profit from more
magic in osm2pgsql. We would have more situations where someone
processes OSM data with, say, QGIS and thinks "uh, I must have done
something wrong, because on the OSM map there's a forest here and on my
map there isn't".

Fixing the data in OSM is the better approach. In many cases, broken
polygons are a result of a broken import or careless editing and if
MapRoulette prompts someone to open their editor in that location, they
are likely to find quite a few other things worth of attention.

> The “Fixing broken polygons”, especially programmatic/mass fixing, could
> be more dangerous to all users.

I wholly agree that, with perhaps a very few narrowly defined
exceptions, nobody should make an attempt to fix broken polygons
automatically because many things go wrong (and the automatic fix would
not give us the advantage I mentioned above, that someone looking at the
data in the area might find other things amiss).

> Let us look at a map extract from the
> mentioned Scandinavian forests

It has been suggested to completely remove some CORINE-based landcover
import in Scandinavia on the basis that not only there are many broken
polygons, but also that it bears very little resemblance to the reality
on the ground. People have said it is a nicely painted map but not much
more. But I believe the local community said they preferred a nicely
painted and somewhat wrong map over having to trace the forests from
aerial imagery themselves ;)


Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"

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