[postgis-users] ST_Buffer + grid problem

[Pierre Racine]

I guess this is related to a GDAL problem where you cannot create a raster using ST_AsRaster with pixel size smaller than 1. Was this fixed Bborie?

Ed could you reduce the list of geometries passed to ST_Extent to one and write it as ST_GeomfromText so we can debug this more easily?

Pierre

> -----Original Message-----
> From: manohar.kaul at gmail.com [mailto:manohar.kaul at gmail.com] On Behalf
> Of Ed Linde
> Sent: Thursday, March 22, 2012 9:45 AM
> To: Pierre Racine
> Cc: PostGIS Users Discussion
> Subject: Re: [postgis-users] ST_Buffer + grid problem
> 
> Hi Pierre,
> So I am trying to have a 4meter by 4meter cell and I converted 4meters to
> degrees, because I think ST_Extent will compute it in degrees as my geometry is
> in srid 4326. But I get this GDAL error which I have no clue what it means. Any
> ideas how I can fix it? Also I noticed that if I set it to 1.0, 1.0 I get
> 21 rows but the gridid isn't continuous either (which is weird).
> 
> CREATE TABLE vectorgrid AS
> SELECT (gvxy).geom, ((gvxy).x - 1) * rwidth + (gvxy).y gridid FROM (SELECT
> ST_PixelAsPolygons(rast) gvxy, ST_Width(rast) rwidth
>             FROM (SELECT ST_AsRaster(ST_Extent(way_geom)::geometry, 0.000036,
> 0.000036) rast
>                          FROM buffers
>                         ) foo1
>            ) foo2;
> 
> ERROR:  rt_raster_gdal_rasterize: Unable to add band to GDALDataset
> 
> ********** Error **********
> 
> ERROR: rt_raster_gdal_rasterize: Unable to add band to GDALDataset SQL state:
> XX000
> 
> 
> 
> 
> On Thu, Mar 22, 2012 at 2:13 PM, Pierre Racine <Pierre.Racine at sbf.ulaval.ca>
> wrote:
> 
> 
> 	You can control how the grid is aligned by using more ST_AsRaster
> parameters. See:
> 
> 	http://postgis.refractions.net/documentation/manual-
> svn/RT_ST_AsRaster.html
> 
> 	If you want it to align on your point, just align it on your points...
> 
> 	Pierre
> 
> 
> 	> -----Original Message-----
> 	> From: manohar.kaul at gmail.com [mailto:manohar.kaul at gmail.com]
> On Behalf
> 	> Of Ed Linde
> 	> Sent: Thursday, March 22, 2012 9:05 AM
> 	> To: PostGIS Users Discussion
> 	> Cc: Pierre Racine
> 	> Subject: Re: [postgis-users] ST_Buffer + grid problem
> 	>
> 	> Hi Pierre,
> 	> Thanks for the grid idea last night. I have a problem though, I want to
> have a 4m
> 	> by 4m cell sized grid over my buffer geometries which are in SRID =
> 4326. And
> 	> then I have another data set of 2D points which are also in SRID
> 4326...
> 	> wondering how I can figure out from just a given (lat,long) which cell
> ID in the
> 	> buffer grid it would belong to? I am thinking that I might run into
> "alignment
> 	> issues" because shouldn't the extent of the grid be exactly the same
> on the 2D
> 	> point cloud as well, so that the grid cell IDs will match? Unless I am
> missing
> 	> something here?
> 	>
> 	> Cheers,
> 	> Ed
> 	>
> 	>
> 	>
> 	> On Wed, Mar 21, 2012 at 8:46 PM, Pierre Racine
> <Pierre.Racine at sbf.ulaval.ca>
> 	> wrote:
> 	>
> 	>
> 	>       > I am not sure if this is possible, but I have computed (using
> ST_Buffer)
> 	> a sort of
> 	>       > buffer around several LINESTRINGs. Now I would like to lay some
> sort
> 	> of grid of
> 	>       > say 1m^2 cell size on top of this collection of geometries and
> then
> 	> compute the
> 	>       > intersection... so in the end I would like to for example, know
> that grid
> 	> cell ID = 1
> 	>       > intersects with buffers 1 and 10, grid cell 2 intersects with buffers
> 7, 10
> 	> etc..
> 	>       >
> 	>       > Is there a simple way of doing this in postgis? Maybe someone
> could
> 	> point me to
> 	>       > some documentation of how I can generate such a grid in postgis
> and
> 	> maybe
> 	>       > then I can use just ST_Intersect once I have these two
> geometries?
> 	>
> 	>
> 	>       With the raster type you can now easily create a vector grid like
> this:
> 	>
> 	>       CREATE TABLE vectorgrid AS
> 	>       SELECT (gvxy).geom, ((gvxy).x - 1) * rwidth + (gvxy).y gridid
> 	>       FROM (SELECT ST_PixelAsPolygons(rast) gvxy, ST_Width(rast)
> rwidth
> 	>                   FROM (SELECT ST_AsRaster(ST_Extent(geom)::geometry,
> 1.0,
> 	> 1.0) rast
> 	>                                FROM yourbuffertable
> 	>                               ) foo1
> 	>                  ) foo2;
> 	>
> 	>       Make sure a spatial index exist on both tables:
> 	>
> 	>       CREATE INDEX yourbuffertable_geom_idx  ON yourbuffertable
> USING
> 	> gist  (geom);
> 	>       CREATE INDEX vectorgrid _geom_idx  ON vectorgrid USING gist
> 	> (geom);
> 	>
> 	>       You can then perform a normal intersect query:
> 	>
> 	>       CREATE TABLE interresult AS
> 	>       SELECT b.bufferid, g.gridid, ST_Intersection(g.geom, b.geom)
> geom
> 	>       FROM vectorgrid g, yourbuffertable b
> 	>       WHERE ST_Intersects(g.geom, b.geom);
> 	>
> 	>       Pierre
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