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

[Joakim Fors]

On 27 sep 2012, at 15:14, Lester Caine <lester at lsces.co.uk> wrote:

> 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?
> 

A lot of times Bing imagery is distorted in different directions in a small area. Quite easy to see in some places where you have access to very precise ortho imagery or vector data to compare with. For example in Lund where we get ortho imagery from the municpality; Here the Bing layer is distorted in different directions just a few blocks apart… not to mention that the Bing imagery is quite a few years older. Same with some vector data that muncipalities in the region have provided to OSM where it is easy to see that Bing imagery is quite inaccurate.

/Joakim