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

[Francois Hugues]

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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