I would start with something like this... ST_DumpAsPolygons is an expensive function so it might take a while can you explain more what you mean by visualize?
SELECT ST_DumpAsPolygons(ST_Clip(rast,1,geom)) as gval FROM raster_table r inner join (select geom from vector_table where gridcode = 18) poly18 on ST_Intersects(r.rast, poly18.geom) On Wed, Aug 20, 2014 at 4:00 PM, DanielFranco <[email protected]> wrote: > Hi, I'm a newbie in PostGIS Raster use and GIS applications, so this may > be a > dumb question. I'm a little confused by some functionalities and I don't > know if it's possible to do what i'm trying to. Basically, I'm using > postgis > functions either for raster and vector forms of an image and I'm trying to > visualize parts of the raster by using the informations presented only in > the shapefile. For example, I have an TIFF file of a land coverage (one > band) about 49746 x 71819 and it's shapefile with some grid codes. I'm > doing > the following query to extract pixel values from an expecific gridcode (18) > presented in the shapefile imported (vector_table) so I could infer some > grid codes by the pixel value: > > SELECT (gval).val as pixel_value > FROM > ( SELECT ST_DumpAsPolygons(ST_Clip(rast,1,geom)) as gval > from raster_table > join vector_table > on (st_intersects(rast,geom)) > where geom in (select geom from vector_table where gridcode = 18) > ) as intersection; > > The query runs too slow and I don't know how to optimize it. The raster > image is big and I tried different tiles configurations (100x100, 200x200, > 1000x1000) and 100x100 seemed to have better perfomance. I'm using st_clip > for a faster answer. I don't know if these pixels values would retrieve > some > important information and if PostGIS raster is really suitable with area > calculations. Is there a way to optimize the query or postgis raster isn't > suitable for this case? > > > > -- > View this message in context: > http://postgis.17.x6.nabble.com/PostGIS-Raster-Performance-tp5006902.html > Sent from the PostGIS - User mailing list archive at Nabble.com. > _______________________________________________ > postgis-users mailing list > [email protected] > http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users >
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