ST_DumpAsPolygons — Returns a set of geomval (geom,val) rows, from a given raster band. If no band number is specified, band num defaults to 1.


setof geomval ST_DumpAsPolygons(raster rast, integer band_num=1, boolean exclude_nodata_value=TRUE);


This is a set-returning function (SRF). It returns a set of geomval rows, formed by a geometry (geom) and a pixel band value (val). Each polygon is the union of all pixels for that band that have the same pixel value denoted by val.

ST_DumpAsPolygon is useful for polygonizing rasters. It is the reverse of a GROUP BY in that it creates new rows. For example it can be used to expand a single raster into multiple POLYGONS/MULTIPOLYGONS.

Availability: Requires GDAL 1.7 or higher.


If there is a no data value set for a band, pixels with that value will not be returned.


If you only care about count of pixels with a given value in a raster, it is faster to use ST_ValueCount.


This is different than ST_PixelAsPolygons where one geometry is returned for each pixel regardless of pixel value.


SELECT val, ST_AsText(geom) As geomwkt
SELECT (ST_DumpAsPolygons(rast)).*
FROM dummy_rast 
WHERE rid = 2
) As foo
WHERE val BETWEEN 249 and 251

 val |                                                       geomwkt
 249 | POLYGON((3427927.95 5793243.95,3427927.95 5793243.85,3427928 5793243.85,
 		3427928 5793243.95,3427927.95 5793243.95))
 250 | POLYGON((3427927.75 5793243.9,3427927.75 5793243.85,3427927.8 5793243.85,
 		3427927.8 5793243.9,3427927.75 5793243.9))
 250 | POLYGON((3427927.8 5793243.8,3427927.8 5793243.75,3427927.85 5793243.75,
 		3427927.85 5793243.8, 3427927.8 5793243.8))
 251 | POLYGON((3427927.75 5793243.85,3427927.75 5793243.8,3427927.8 5793243.8,
 		3427927.8 5793243.85,3427927.75 5793243.85))

See Also

geomval, ST_Value, ST_Polygon, ST_ValueCount