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

[Christian Quest]

2012/9/26 Lester Caine <lester at lsces.co.uk>:
> Christian Quest wrote:
>>
>> So cross check with Bing
>> must be done afterwards, exactly like when using vector data.
>> That's why I consider manual tracing as a waste of time, and not high
>> quality compared to using extracted building from vector data.
>
>
> Christian
> I've now seen your source data, and that is showing buildings as 'blocks'.
> In the PDF file are those blocks drawn individually when there are are more
> than one building side by side? Or are the vectors simply the outline of the
> whole block?
>

There is no single answer as there is many different cases.

Most of the time, a building is defined by a single polygon.
Sometimes, in may be split into multiple polygons if the land/building
ownership has evolved during the past or if the building is composed
of different parts. Auto-merging is almost impossible and manual
merging is not obvious.
Sometimes, one building polygon is covering different building parts.
Auto-split is also almost impossible and manual split also not
obvious.

> I'm sure that over time improved quality data will evolve, but the current
> vector data that many of us have access to is not ideal.

I doubt we will ever have access to a better source nation wide.
Some cities that stepped into opendata do provide building vector data
but they are a minority and most of the time it is the same data
because cities are updating the cadastre data locally then send it to
the nationwide administration.

Cities and town had/have the responsibility to create the vector
version. Some parts of France are fully vectorized, some are still
very late (25% of our 36000+ communes still).


> Personally the
> problems I am finding is where we have semi detached houses drawn as a
> single block, and splitting that into two blocks is a pain on potlatch ...

Well... working with complex objects may be a pain in Potlatch but
this is not typical of building coming from the cadastre.
Some building with wholes in them are even using multipolygon relations.


> I've not tried on JOSM yet. But a tool on my 'wish list' is one I can use to
> select vector lines from a 'staging layer' and combining them to a closed
> way to which I can then add extra tags. This I think is the best way to use
> this 'poor quality' vector data and convert it to better quality data? I can
> also see a 'split' tool, where the imported vectors are for a 'semi' or
> 'terrace' and you want to split each out to separate buildings.
>

I agree that a tool in JOSM (or whatever is your editor of choice) to
split a polygon into 2 smaller polygons could be really helpful, not
only for buildings.


> http://www.openstreetmap.org/?lat=52.048913&lon=-1.85734&zoom=18 will show
> my own experience with all of this. What is not visible is the Opendata
> streetview layer, and just how bad the vector data is with respect to the
> current status. 'New' buildings are actually not too bad, but none of the
> extensions on the houses on Smallbrook Road are present on the OS layer,
> which seems to be stuck with 40+ year old data. Some how I expect the same
> sort of discrepancies in most data, and so I would not use the OS data in
> the same manor you are using the French data, although with my historic hat
> on it WOULD be nice to retain the history of the additions of these
> extensions over time. Importing buildings from the Opendata streetview layer
> would fill up the UK map, but we do not know what date the buildings relate
> to and then updating and we still need to split and add address data ...
>

The cadastre is not perfect, but it is not 40+ years old data, it is
closer to 1 or 2 years old in most cases.
The updates are available more or less once a year on a town by town basis.
Cadastre is used to collect taxes and our administration is quite
efficient in that area ;)
In the source tag, "dgi" means Direction Générale des Impôts (Impôts = taxes).

What's missing from time to time in the cadastre data are state/public
buildings as they don't pay taxes !


One additional thing we have been able to extract from vector cadastre
data is a rough length of highways/path/tracks in each town. This
allows to have a rough idea of the existing/missing ratio as displayed
by this layer: http://layers.openstreetmap.fr/?zoom=7&lat=47.45473&lon=2.19472&layers=B00FFFFFFFFFFFFFFFFFT

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