[postgis-users] ST_Buffer + grid problem
Ed Linde
edolinde at gmail.com
Thu Mar 22 14:29:51 PDT 2012
Thanks I will try and see how I go with all your suggestions tomorrow...
hopefully you will be around to help. :) I have to play around
more with ST_MapalgebraExpr to understand how what you say works... like
what is a "single band" and "double band" raster?
Does that just mean that in a single band the raster cells will have a 1D
value as opposed to a 2D (x,y) in the double band?
Once the intersection between the road buffer geometry and this new raster
is done, then the point cloud stuff I will have to write some code in C/C++
to handle that, so that shouldn't be a problem. One last thing, is it ok to
treat srid 4326's lat/long points as normal x,y co-ordinates?
I ask because when I try to compute if a point falls within the raster
extent or not ... then I can just treat it as trying to figure out if a
point lie within a rectangle or not, where I know the lower left and top
right corner's (lat,long) co-ordinates. Is that still ok? Or am I
overlooking something?
On Thu, Mar 22, 2012 at 10:17 PM, Pierre Racine <Pierre.Racine at sbf.ulaval.ca
> wrote:
> The raster was just a quick way to make a vector index grid. If you can't
> get ST_AsRaster to work by upgrading to GDAL 1.9 use ST_MakeEmptyRaster and
> then ST_MapalgebraExpr with the '([rast.x] - 1) * ' ||
> ST_Width(rast)::text || ' + [rast.y]' expression.
>
> Then convert each pixel to a table of polygons with
> (ST_PixelAsPolygons(rast)).geom & (ST_PixelAsPolygons(rast)).val
>
> Then proceed with a normal geometry/geometry intersection.
>
> We don't have a very fast way to do that in raster mode right now. The
> fastest approach with what we have would probably be to use
> ST_MapAlgebraFct and write a custom function that generate the values by
> doing a point/geometry instersection with the road buffer table.
>
> Pierre
>
> > -----Original Message-----
> > From: postgis-users-bounces at postgis.refractions.net [mailto:
> postgis-users-
> > bounces at postgis.refractions.net] On Behalf Of Ed Linde
> > Sent: Thursday, March 22, 2012 5:03 PM
> > To: PostGIS Users Discussion
> > Subject: Re: [postgis-users] ST_Buffer + grid problem
> >
> > Oh and if you think there is a smarter way to do this, like maybe not
> making a
> > raster like I am trying to etc then please let me know. The idea is that
> the road
> > geometry is way way smaller than the point cloud which is in Terabytes,
> so I am
> > making a grid index in some way around the road buffers, to speed up the
> > computation.
> >
> >
> > On Thu, Mar 22, 2012 at 10:01 PM, Ed Linde <edolinde at gmail.com> wrote:
> >
> >
> > Hi Pierre,
> > The idea is that I want to build this uniform grid on the road
> geometry
> > and like you mention it has dimensions 1020x24798. I want to assign each
> cell a
> > unique cell ID based on some formula like you had earlier ... x*width +
> y to get a
> > 1D cell ID from (x,y). Hopefully I can do this on just (lat.long) of
> srid 4326
> > directly. Then I need to
> > intersect it to the buffers around the roads which I think should
> be easy
> > once I have a raster, so this is just a normal intersect. Now I should
> know each
> > cell and what road buffer it belongs to! Then on my second data set
> which is a
> > massive point cloud with srid = 4326 as well, I just want to pass
> through it ONCE
> > and compute in which cell it falls and hence associate a point with a
> road buffer.
> > Hope that made sense :)
> >
> > Cheers,
> > Ed
> >
> > On Thu, Mar 22, 2012 at 9:53 PM, Pierre Racine
> > <Pierre.Racine at sbf.ulaval.ca> wrote:
> >
> >
> > 1020x24798 so more than 25 000 000 polygons... or pixels...
> >
> > So if I understand well you want to assign some values to
> each
> > of those cells based on a vector coverage (of how many polygons)?
> >
> >
> > > -----Original Message-----
> > > From: postgis-users-bounces at postgis.refractions.net
> > [mailto:postgis-users-
> > > bounces at postgis.refractions.net] On Behalf Of Ed Linde
> >
> > > Sent: Thursday, March 22, 2012 4:43 PM
> > > To: PostGIS Users Discussion
> > > Subject: Re: [postgis-users] ST_Buffer + grid problem
> > >
> >
> > > So far no proper raster, because the following failed,
> but
> > maybe the polygon
> > > gives us an idea? This polygon is the extent of all the
> road
> > geometries in
> > > Denmark. Also I remember that when you did the raster at
> > your end you said it
> > > worked. Is 0.000036 degrees = 4m correct if I want to
> get a
> > 4m by 4m cell sized
> > > uniform raster grid?
> > >
> > > select ST_AsRaster(
> > ST_GeomFromText('POLYGON((8.07734039737749
> > > 54.4984986588244,8.07734039737749
> > 57.7505109647578,15.1919565742587
> > > 57.7505109647578,15.1919565742587
> > 54.4984986588244,8.07734039737749
> > > 54.4984986588244))'), 0.000036, 0.000036);
> > >
> > >
> > > On Thu, Mar 22, 2012 at 9:39 PM, Pierre Racine
> > <Pierre.Racine at sbf.ulaval.ca>
> > > wrote:
> > >
> > >
> > > > Thanks. The raster I need to visualise is a 4m
> by 4m grid
> > on the entire
> > > map of
> > > > Denmark! :) So do you classify that as a large
> raster? If so
> > is there a
> > > way to see a
> > > > portion of it or something?
> > > > I just want to manually check for a few areas on
> the map
> > that the
> > > intersection
> > > > indeed works as expected and things haven't gone
> awry
> > thanks to
> > > projection
> > > > differences or that I had degrees instead of
> meters or
> > some such
> > > thing. Well, I
> > > > still have to first test if this GDAL upgrade
> will fix things
> > and if make
> > > empty raster
> > > > .. makes a difference.
> > >
> > >
> > > That must be big but Danemark is a small country
> ;-) Only
> > width and
> > > height tell you if a raster is big.
> > >
> > > In the database, your grid is a set of rectangular
> geometry
> > (how many?)
> > > or just a big raster now (width & height)?
> > >
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