ST_Polygon — Returns a polygon geometry formed by the union of pixels that have a pixel value that is not no data value. If no band number is specified, band num defaults to 1.
geometry ST_Polygon(
raster rast, integer band_num=1)
;
-- by default no data band value is 0 or not set, so polygon will return a square polygon SELECT ST_AsText(ST_Polygon(rast)) As geomwkt FROM dummy_rast WHERE rid = 2; geomwkt -------------------------------------------- POLYGON((3427927.8 5793243.75,3427927.75 5793243.75,3427927.75 5793243.8,3427927.75 5793243.85,3427927.75 5793243.9, 3427927.75 5793244,3427927.8 5793244,3427927.85 5793244,3427927.9 5793244,3427928 5793244,3427928 5793243.95, 3427928 5793243.85,3427928 5793243.8,3427928 5793243.75,3427927.85 5793243.75,3427927.8 5793243.75)) -- now we change the no data value of first band UPDATE dummy_rast SET rast = ST_SetBandNoDataValue(rast,1,254) WHERE rid = 2; SELECt rid, ST_BandNoDataValue(rast) from dummy_rast where rid = 2; -- ST_Polygon excludes the pixel value 254 and returns a multipolygon SELECT ST_AsText(ST_Polygon(rast)) As geomwkt FROM dummy_rast WHERE rid = 2; geomwkt --------------------------------------------------------- MULTIPOLYGON(((3427927.9 5793243.95,3427927.85 5793243.95,3427927.85 5793244,3427927.9 5793244,3427927.9 5793243.95)), ((3427928 5793243.85,3427928 5793243.8,3427927.95 5793243.8,3427927.95 5793243.85,3427927.9 5793243.85,3427927.9 5793243.9,3427927.9 5793243.95,3427927.95 5793243.95,3427928 5793243.95,3427928 5793243.85)), ((3427927.8 5793243.75,3427927.75 5793243.75,3427927.75 5793243.8,3427927.75 5793243.85,3427927.75 5793243.9,3427927.75 5793244,3427927.8 5793244, 3427927.8 5793243.9,3427927.8 5793243.85,3427927.85 5793243.85,3427927.85 5793243.8,3427927.85 5793243.75,3427927.8 5793243.75))) -- Or if you want the no data value different for just one time SELECT ST_AsText( ST_Polygon( ST_SetBandNoDataValue(rast,1,252) ) ) As geomwkt FROM dummy_rast WHERE rid =2; geomwkt --------------------------------- POLYGON((3427928 5793243.85,3427928 5793243.8,3427928 5793243.75,3427927.85 5793243.75,3427927.8 5793243.75,3427927.8 5793243.8,3427927.75 5793243.8,3427927.75 5793243.85,3427927.75 5793243.9,3427927.75 5793244,3427927.8 5793244,3427927.85 5793244,3427927.9 5793244,3427928 5793244,3427928 5793243.95,3427928 5793243.85), (3427927.9 5793243.9,3427927.9 5793243.85,3427927.95 5793243.85,3427927.95 5793243.9,3427927.9 5793243.9))