PostGIS
Name
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.
Synopsis
setof geomval +`*`+ST_DumpAsPolygons*(`raster `rast, integer
band_num=1, boolean exclude_nodata_value=TRUE`)`;
Description
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.
Changed 3.3.0, validation and fixing is disabled to improve performance. May result invalid geometries.
Availability: Requires GDAL 1.7 or higher.
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Examples
-- this syntax requires PostgreSQL 9.3+
SELECT val, ST_AsText(geom) As geomwkt
FROM (
SELECT dp.*
FROM dummy_rast, LATERAL ST_DumpAsPolygons(rast) AS dp
WHERE rid = 2
) As foo
WHERE val BETWEEN 249 and 251
ORDER BY val;
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))