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

[Lester Caine]

THEVENON Julien wrote:
>
> *>>>> *Many of the buildings moving away from the one identified simply do not
> even fit the footprint on the bing imagery ?
> What make you so sure that the thruth is in imagery and not in cadastre ?
> particulary considering that Bing is often several year late and that offical
> french maps (IGN) are also relying on cadastre for building ?
> There are a lot of examples of places where there are some buiding referenced in
> cadastre that do not appear in Bing but that you can see IRL.
> You will decide to remove them because you they are not on Bing or you will
> trust local contributors that introduce them because they know they are real ?
> how do you make the difference between the guy that has the knowledge and the
> one that has not ?
Not having access to the cadastre layer I can't comment on the differences 
between what has been traced and the source data, but I can SEE a distinct 
positional difference between the bing layer and the OSM buildings. If there was 
a general offset, then I would accept that there was simply a referencing error, 
but the buildings were offset in different directions across the areas I looked 
at. I also have buildings that do not yet appear on Bing ... no problem with 
that, but the ones that appear on both SHOULD be in the same place? You have 
already said that the cadastre data can't be trusted for detail?

> *>>>> *Now that I have scanned some of the French material I must say that it is
> of very low quality and all of the stuff I have reviewed needs at least SOME
> work to bring it up to a better standard.
> *>>>> *At best all one can say currently is 'there are some buildings round
> about here' ... and stripping unsubstantiated detail would at least be a start.
> According to the way you say that I assume that you have directly check in IRL
> or perform a comparison with a better quality and reliable source to decide that
> the wole cadastre is of very low quality...
> Could you share with us you criteria and methodology to be so affirmative and
> allow us to determine which details are unsubstantiated ?
> After all we are just mappers that concentrate on part of world  where we are
> living and that we know, if I remember well this is the base of Openstreetmap
> crowdsourcing ?

http://www.openstreetmap.org/edit?lat=44.273069&lon=0.500865&zoom=18
YES local knowledge is needed to clean the data up, but some of the buildings 
line up nicely with the image, while others are well offset so if the 
information was reviewed before importing HOW was it reviewed? Certainly not 
against what I would refer to as the base location reference? If the cadastre 
data is providing the fine detail then OK, but I see a lot of what looks like 
lean-tos and porches identified as separate buildings and strange shapes over 
what look like rectangular buildings.
If this was an area I was working on, then I would have concentrated on the road 
structure first and then checked on what businesses are present. This allows a 
safe way of identifying commercial buildings and if they are listed, local house 
sales help add more detail.

-- 
Lester Caine - G8HFL
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