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-...@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.
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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
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