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-boun...@postgis.refractions.net [mailto:postgis-users- > boun...@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-boun...@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-boun...@postgis.refractions.net [mailto:postgis-users- > > boun...@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-boun...@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-boun...@postgis.refractions.net [mailto:postgis-users- > > > boun...@postgis.refractions.net] On Behalf Of Francois Hugues > > > Sent: Saturday, May 26, 2012 5:29 PM > > > To: postgis-users@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. > > > > > > > > > > > > > > > > _______________________________________________ > > postgis-users mailing list > > postgis-users@postgis.refractions.net > > http://postgis.refractions.net/mailman/listinfo/postgis-users > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
