[OSM-talk] All you've ever wanted to know about the french cadastre

[Christian Quest]

2012/9/30 Lester Caine <lester at lsces.co.uk>:
> The problem *I* am seeing with the cadastre data I have looked at is that a
> building is not simply one or two profiles, but several seemingly unrelated
> elements all strange shapes and not relating to the imagery. Since there is
> no explanation of the detail my feeling was that these SHOULD all have been
> combined into a single element in the raw processing until such time as real
> detail of a difference was available?

We provided explanations many times...

Separate polygons can come from:
- different ownership of building parts
- different building but same looking roof on the aerials
- different building type: porchs, garages, hangar, without wall
tagged as "wall=no"

It is not data errors, but much more detailed geometry compared to
what you can do by simply surveying or trace on aerials. Some may
think its too much details, some don't.


> I'd EVEN be happy with a 'bot' going
> around the current data and combining adjacent or overlapping buildings into
> one where there IS no other tagging? This would at least give a better
> representation of what is known and I believe would substantially reduce the
> number of building elements?
>

This would not work in most cases like we explained already several
times. Is cities this would make a whole block looking as one
building. My own house would be merged with all the neighborhood
houses.



pnorman wrote:
> The cadastre imports are more complicated. I'm not aware of any
> comprehensive studies on the quality of the imports, but I did some
> analysis[1] previously. Based on this, about 75% of the buildings are
> building=yes wall=yes and 25% are building=yes with no wall tag.

building=* + wall=no should be more in the 25% zone and building=*
without wall=* tag in the 75% (we do not use wall=yes).

Many wall=no polygons are porch, garages in France usually have
walls... and locks ;)

-- 
Christian Quest - OpenStreetMap France - http://openstreetmap.fr/u/cquest