[postgis-users] RE : get unique values from raster intersecting apolygon

[Pierre Racine]

In PostGIS you can easily create a grid with ST_MakeEmptyRaster() and ST_PixelAsPolygons()

Pierre

> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-
> bounces at postgis.refractions.net] On Behalf Of pcreso at pcreso.com
> Sent: Monday, May 28, 2012 3:45 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] RE : get unique values from raster intersecting
> apolygon
> 
> You could try something like gdal2xyz.py on you raster to create points for each
> raster cell which can be imported directly into postgis as point geometries, like:
> 
> psql "create table xyz (id serial,x decimal, y decimal, z decimal)"
> gdal2xyz.py <raster file> | psql "copy xyz (x,y,z) from stdin..."
> psql "select addgeometrycolumn()"
> psql "update xyz set geom=setsrid(makepoint(x,y),<srid>)"
> 
> The result is then a simple spatial join using the polygons & the new points,
> based on ST_Contains().
> 
> 
> HTH,
> 
>    Brent Wood
> 
> 
> --- On Tue, 5/29/12, Francois Hugues <hugues.francois at irstea.fr> wrote:
> 
> 
> 
> 	From: Francois Hugues <hugues.francois at irstea.fr>
> 	Subject: [postgis-users] RE : get unique values from raster intersecting
> apolygon
> 	To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
> 	Date: Tuesday, May 29, 2012, 1:05 AM
> 
> 
> 	I want one value per red point (which are raster data).
> 
> 	The gid of one polygon will appear as many times as necessary
> regarding how it intersects the raster.
> 
> 	Hugues.
> 
> 
> 	-------- Message d'origine--------
> 	De: postgis-users-bounces at postgis.refractions.net de la part de Pierre
> Racine
> 	Date: lun. 28/05/2012 14:24
> 	À: PostGIS Users Discussion
> 	Objet : Re: [postgis-users] get unique values from raster intersecting
> apolygon
> 
> 	You want one value per polygon or one value per red point? It is not
> clear to me from your drawing.
> 
> 	Pierre
> 
> 	> -----Original Message-----
> 	> From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-
> users-
> 	> bounces at postgis.refractions.net] On Behalf Of Francois Hugues
> 	> Sent: Saturday, May 26, 2012 5:29 PM
> 	> To: postgis-users at postgis.refractions.net
> 	> Subject: [postgis-users] get unique values from raster intersecting a
> polygon
> 	>
> 	> Hello list,
> 	>
> 	> I have to get values from raster where they intersect polygons from a
> layer with
> 	> numerous polygons which have shared boundaries, but I don't want to
> have
> 	> duplicate values from raster. The final result must be something like :
> gid | val.
> 	>
> 	> To explain my case, I made a picture.
> 	>
> 	> I think ST_intersection could help but I don't know what will be the
> exact result.
> 	> In my example, we have a grid (a DEM for example), each red point is
> the value
> 	> in the centre of each cell and two polygons A and B. I think cells totally
> within
> 	> polygons are not a problem, but in green I have highlighted some
> ambiguous
> 	> cases :
> 	> - 1 and 2 are cells shared by both polygons but not in the same
> proportions and I
> 	> would like to get value 1 for polygon A but not for B and value 2 for
> polygon B
> 	> but not for A
> 	> - What will be the result for cell number 3 and for the other green cell
> without
> 	> number (and for all the ones which are not totally within a polygon) ? I
> think the
> 	> value is the same for the whole cell (st_dumpaspolygon) and will be
> returned
> 	> whatever the part of the cell covered by the polygon, but if someone
> could
> 	> confirm.
> 	>
> 	> If st_intersection use st_DumpAsPolygon, could it be better to work
> directly with
> 	> this polygonized table using its own gist index and dealing with shared
> areas of
> 	> cells between different polygons ?
> 	>
> 	> Hugues.
> 	>
> 	>
> 	>
> 	>
> 
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