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

[Christian Quest]

2012/9/26 THEVENON Julien <julien_thevenon at yahoo.fr>:
>
>>>>> De : Lester Caine <lester at lsces.co.uk>
>
>>>>> Sees the light :)
> Great !
>
>>>>> SO while we have this type of raster data from as a background in
>>>>> potlatch and josm and some elements of it in vector files from OS and other
>>>>> sources. You are having to stitch together 'pictures' and then your
>>>>> 'automatic processing' is recreating vector data from the 'pictures'?
>
> I don`t know all details but yes this is something like that
>

The OSM data is extracted from PDF vector data.
If the first version of the script, the PDF we can download on the
cadastre web site were converted to SVG which are analyzed to extract
data into .osm format. The actual script works more or less the same
way, so we are working on vector data all the time and not processing
raster pictures.
Before we had access to a vector based PDF, we tried to do image
recognition to generate vector data from them, but this was not very
successful and was used only for administrative boundaries.


>>>>> I understand the problems now ... and it only really works because the
>>>>> pictures can be simply vectorised.
>
> yes has pdf is vectorised but if i rember well what you see as a simple
> yellow rectangle is an overlay of different rectangle inside pdf code
> explaining partly why we have some geomtry problems with contigous
> buildings.
>
>>>>> SO I would be asking if the 'pictures' can be merged to create a single
>>>>> raster overlay for France to use as a background 'source' which could
>>>>> potentially be used to trace from, but can be used in conjunction with BING
>>>>> imagery to corss check?
> The French cadastre licence doesn't allow us to redistribute cadastre data
> as they are so I think it is not legally possible to do that.
> The other issue is related to projection : Mercator for Bing and Lambert 9
> zones for French cadastre. In addition to that not all the french cities
> have vectorised cadastre data. There are still a lot of cities which have
> raster data that are just scan of cadaster paper plan without georeferencing
> ( Feurs in 42-Loire by example )
>

To cross check with Bing, it is quite easy... generate the .osm file
using cadastre.openstreetmap.fr site. Load the file in JOSM, add bing
imagery and you can do all the cross check that is needed.
Depending on the town, cadastre can be better than bing or the reverse.
We also do cross check with geodesic points as well as GPS traces in
the case the difference is between bing and cadastre is too high.
Bing is not always available at high resolution everywhere in France,
Bing is not always perfectly aligned (especially in mountain area).


>>>>> I would classify that as my base import since it's not externally
>>>>> available? We have several versions of the OS data along with the historic
>>>>> maps for the UK, and I feel sure that should be achievable in France as
>>>>> well?
> I don't know if some people are interested in historic maps or such kind of
> map aspects in french community
>
>

I also have a full copy of the June 2009 extracted cadastre data on my
disks (around 5GB bzipped osm data).


>>>>> The vectorised files are the 'staging layer' and I'm sure that since
>>>>> you are providing each building as a shape then in the future it should be
>>>>> possible to maintain a 'building_id' that can be used with the sort of merge
>>>>> tools I am looking for to handle this type of data?
> We have a tool called "bati fusion" that try to perfom a diff between OSM
> files to make merging easier by using building shape comparison I think but
> I don`t know if it is massively used. I personnally know the guy who
> developped it if you are interested
>

Some other tools are under development to clean some artifacts (small
overlaps between buildings).

Osmose (the quality assurance tool developed by the french community)
also displays overlapping buildings (small and large ones) as well as
highways crossing buildings. This helps to detect quick and dirty
cadastre imports by "serial importers" as we sometime call them ;)

One more thing about the lack of translation of the cadastre wiki
page: it is non intentional, but it acts as an indirect way of
limiting who is doing such imports.

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