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

[Christian Quest]

In France, cadastre is the most official source about buildings and
land ownership. When you buy/sell a building all the documents refers
to the cadastre.

It provides sometime too many details compared to what could be seen
by survey. Does this mean it is wrong to have too many details ?

Considering merging these detailed outlines automatically into a
single block is not a option because in many cases in cities this
would lead to a single building outline for a whole street block
loosing way too many details.

Bing aerial pictures are most of the times a good source, but not the
reference. Cadastre data may be more up to date (we have new buildings
not visible on Bing), and sometimes it is the reverse when Bing
aerials are from 2011 or even 2012. That's usual with different source
of information, that's why I indicate bing year in the source tags
when I trace over bing.

I manually traced building in a village in Burgundy because the
cadastre was only raster there at that time, then the vector version
became available. I updated the whole area and found that vector was
much more accurate and higher quality than what I did manually. I
think it is the case most of the time because of the georeferencing
that is done manually and because inaccuracies/lack of details in the
manual tracing. One important thing when tracing cadastre by hand is
that you cannot cross check with Bing at the same time because of
different projections (Lambert vs Mercator). So cross check with Bing
must be done afterwards, exactly like when using vector data.
That's why I consider manual tracing as a waste of time, and not high
quality compared to using extracted building from vector data.

Buildings, even with some artifacts like the one showed by Frederik
example, are helping a lot adding POI at their right position. This is
also something I learnt after adding POI in my own city and then
adding buildings from vector extraction when it became available (this
summer). I add to relocate a lot of POI added by myself and other
contributors, and adding new ones is now much easier.

--
Christian