ST_Polygon
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Name

ST_Polygon — Returns a multipolygon 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.

Synopsis

geometry ST_Polygon(raster rast, integer band_num=1);

Description

Changed 3.3.0, validation and fixing is disabled to improve performance. May result invalid geometries.

Availability: 0.1.6 Requires GDAL 1.7 or higher.

Enhanced: 2.1.0 Improved Speed (fully C-Based) and the returning multipolygon is ensured to be valid.

Changed: 2.1.0 In prior versions would sometimes return a polygon, changed to always return multipolygon.

Examples

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;
MULTIPOLYGON(((3427927.75 5793244,3427928 5793244,3427928 5793243.75,3427927.75 5793243.75,3427927.75 5793244)))

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;
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;
MULTIPOLYGON(((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)))