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

[Pierre Racine]

What is not clear in your description is: How do you want the value associated to each polygon to be computed from the many pixel the polygon is intersecting with? Do you want:

-the raster value corresponding to one point inside the polygon
-the mean of all the pixel centroid falling inside the polygon
-the area weighted mean of all the raster values inside the polygon
-the value most represented inside the polygon
-etc...

There are many ways to compute this value.

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: Monday, May 28, 2012 12:40 PM
> To: PostGIS Users Discussion
> Subject: [postgis-users] RE : RE : get unique values from raster intersecting
> apolygon
> 
> Thanks but i think the way I made my picture was not really clear and I
> understand now how you read it !
> 
> I already know the existence of st_value for points. Here, the red point on my
> picture are the values of the raster and not a table of points : if it is build from a
> DEM, you have the elevation measure based on a regular pattern and finally you
> get a grid and in the centre of each cell the value measured (my red points)
> 
> For my problem I have two table : one made of polygons, the other a raster
> made of tiles. I want to get value where polygons intersect raster but a celle
> must not appear twice. My raster will be totally covered by polygons and at the
> end I want a gid|val table with as many lines as my raster has values.
> 
> I hope it is clearer now but, for sure, my english does not help !
> 
> Thanks,
> 
> Hugues.
> 
> 
> -------- Message d'origine--------
> De: postgis-users-bounces at postgis.refractions.net de la part de Pierre Racine
> Date: lun. 28/05/2012 16:56
> À: PostGIS Users Discussion
> Objet : Re: [postgis-users] RE :  get unique values from raster intersecting
> apolygon
> 
> I this case the right query looks like this:
> 
> SELECT gid, ST_Value(rast, geom) val
> FROM yourraster table, yourpointtable
> WHERE ST_Intersects(rast, geom)
> 
> Make sure to tile and index your raster properly (smaller is better). You can do
> this at import time.
> 
> The result for cell outside any polygon will be "null".
> 
> 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: Monday, May 28, 2012 9:06 AM
> > To: PostGIS Users Discussion
> > Subject: [postgis-users] RE : get unique values from raster intersecting
> apolygon
> >
> > 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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> > postgis-users at postgis.refractions.net
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> 
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