[Talk-ca] Canvec 10 and landcover issues

[Frank Steggink]

On 19-10-2012 21:46, Harald Kliems wrote:
> Hi Pierre,
> thanks for the response.
>
> On Fri, Oct 19, 2012 at 3:25 PM, Pierre Béland <infosbelas-gps at yahoo.fr> wrote:
>> I dont know how you conclude that there is no wetlands around this area in
>> Laval.  It is not sufficient to see houses around to conclude that there is
>> no wetland. These are often wooded areas with water all over.  Google
>> physical also shows a stream starting from this area.
>>
>> The link below shows a comparison of this area with Google imagery.  Are you
>> sure that there is no wetland in this area.
>> http://tools.geofabrik.de/mc/?mt0=mapnik&mt1=googlehybrid&lon=-73.91012&lat=45.69989&zoom=17
> This is a misunderstanding. I did not mean that there is _no_ wetland
> in the area. But I'm pretty certain that the boundaries of the wetland
> are wrong:
>
> http://tools.geofabrik.de/mc/?mt0=mapnik&mt1=googlehybrid&lon=-73.90457&lat=45.69533&zoom=17
>
> Aside from the wetland issue (see below), we can probably agree that
> the area is not natural = wood, even if some people might have planted
> trees in their yards.
>
>> The link below shows an aera in Saint-Jean-sur-Richelieu were houses have
>> been built for over 30 years. Look how many houses were flooded last year.
>> Zoom in to see areas that were flooded.
>> http://pierzen.dev.openstreetmap.org/hot/openlayers/inondation-richelieu-2011.htm?zoom=16&lat=45.28568&lon=-73.24907&layers=B000TFFFF
>>
>> My experience, as a volunteer for SOS-Richelieu, last year, showed me how
>> that too often the municipalities have accepted that contractors build
>> houses over wetlands. And this was often the case with Laval.
> Okay, this is a different issue, coming down to the definition of what
> "wetland" is. I'm by no means an expert, but in my understanding you
> can't have a residential area in wetlands. In order to build houses
> you must first use drainage channels etc. to turn wetland into
> developed land. The fact that there can be flooding in a given area
> doesn't make it into wetland to me. The wiki isn't very explicit about
> this (http://wiki.openstreetmap.org/wiki/Tag:natural%3Dwetland) but
> the specific subtypes seem to hint at a definition stricter than
> yours. Maybe someone can tell us what definition is used for Canvec.
>
> Cheers,
>   Harald.
>
> _______________________________________________
> Talk-ca mailing list
> Talk-ca at openstreetmap.org
> http://lists.openstreetmap.org/listinfo/talk-ca
>
Hi Harald,

As Paul just explained, the Canvec data comes from different ages, so 
what this basically tells is that 20 or 30 years ago (or maybe just 10 
years ago) this area was a wooded marsh. Unfortunately, this landcover 
data is the best available. (The lower resolution Landsat data can be 
pretty old too, and its resolution makes it unusable.) It still needs to 
be reconciled with the roads, preferably with the help of Bing imagery. 
I'm not sure if a decent resolution is available in this area. Good 
coverage is pretty spotty in Canada.

Regarding the flooding: areas which used to be wetlands in the past are 
still prone to flooding, unless significant work has been undertaken 
from ever happening again (like drainage, diverting streams, putting 
extra soil on top). Especially when buildings are built within the 
channels which have been eroded by rivers, then you can basically wait 
for a disaster to happen.

Frank