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

[Lester Caine]

THEVENON Julien wrote:
> *>>>> De :* Lester Caine <lester at lsces.co.uk>
>
> *>>>> *Now that I understand what is going on, I can see where some off the
> 'extra' lines come from, and the diagonal is probably due to a boundary detail
> from changing sheets.
> This is more often due to split of landuse ownership. There is no differences
> between this lines and the one separating adajacent buildings

> *>>>> *However while the source has two different shades of block for buildings.
> *>>>>*I don't think they can be used at this stage to provide useful extra lines.
> *>>>>*The process that is extracting the vectors should further process the data
> so that each block IS a single continuous outline?
> *>>>>*Later comparison will then be easier as long as say 90% of the area
> matches the previous instance?
>
> No this is not sufficiant I think because some times you have adjacent buildings
> that are not a single building. You also have cases when line separate building
> parts which have different number of levels or which are "light buildings" (
> without wall by example )

Looking at the source material, there is nothing which can be used to separate 
the blocks displayed into separate buildings, and since we have no means of 
identifying different levels of building, adding 'detail' for that seems 
pointless? All that can be 'accurately' extracted from the source material is 
that there is a 'block' of buildings? So if you are not actually surveying the 
buildings and identifying individual buildings, then normal practice is to draw 
a single box. Frederik's example is the sort of thing that SHOULD have been 
tidied up before importing!

> *>>>> *Of cause simply importing thousands of these objects without a visual
> check of every one of them is something completely different to hand tracing
> every one of them.
> *>>>> *I'd prefer that there was some cross check that objects have been verified.
> *>>>> *And in my book, having to manually select objects to import would provide
> that check?
> *>>>> *So I'd block any area select function, so that hundreds of objects can't
> simply be picked and pushed?
>
> Please also notice that this is sometime not easy to distinguish on aerial
> imagery if the split line really exist or not.

Hand tracing hundreds of individual elements and not committing them often does 
not make sense. What I am talking about here is selecting hundreds of vectors 
from a file without checking them, and having to select each individually would 
help the checking process. Then perhaps the sort of questionable mapping 
demonstrated would not happen?

Personally I would prefer to see 
http://www.remote.org/frederik/tmp/funnybuilding.png as a single closed outline 
box. If the vectors are not providing closed objects then there is something 
wrong with the data anyway and in my book it should not be allowed to be 
imported? With a decent editor, one should be able to select the outline of a 
block and simply import that ...

-- 
Lester Caine - G8HFL
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