[postgis-users] get unique values from raster intersecting a polygon

[Francois Hugues]

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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