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

[Lester Caine]

Frederik Ramm wrote:
>> I think that "drawing all of the nodes and points manually" is an
>> important difference, from a quality point of view.  Each node or way
>> that you draw by hand, is carefully considered and placed, one at a
>> time.  It isn't perfect; nothing is.  I suggest that this leads to a
>> kind of automatic quality control, as the nodes and ways are placed.
>
> To give an example, look at this imported building
>
> http://www.remote.org/frederik/tmp/funnybuilding.png
>
> Note how the main building consists of 8 separate parts plus a strange diagonal
> line, and note how the smallest parts are just about 2 metres wide.
>
> Compare to the aerial image:
>
> http://binged.it/UuYSio
>
> A very careful tracer of the aerial image might indeed have created more than
> just one shape for this, but there is hardly anything there on the imagery that
> suggests *such* a complex edifice.
>
> This is not an example that you only find after a long search; it is a typical
> cadastre import building.

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. 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?

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?

-- 
Lester Caine - G8HFL
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