PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

4.18 Conversion
In PostGIS, when using analysis functions, it is no matter whether the layers are raster or vector
type as PostGIS Raster provides a set of functions to support conversion between raster and vector
data.
4.18.1 Raster to vector
With ST_DumpAsPolygons() function, raster data is converted to vector form, along with a set of
geometric objects.
Example:
SELECT ST_DumpAsPolygons(rast)).geom, ST_DumpAsPolygons(rast)).val
FROM raster_table ;
The result of this function is a set of geomval formed by a geometry and a pixel value, where each
geometric object is the union of all pixels of the same band that have the same pixel value. So the
ST_DumpAsPolygons() function is useful for polygonizing rasters in which a single raster will be split
and transformed into multiple polygons. For example, as the circle in Figure 4.45 is composed by two
half-circles that have different values, when applying the ST_DumpAsPolygons() function, it results in
two geometric half-circles by definition.
4.18.2 Vector to raster
Figure 4.45: Conversion from raster to vector [22].
Inversely, a raster object can be build from a geometric object using ST_AsRaster() function. The
vector to raster conversion can be performed without loss of information as PostGIS Raster introduces
the concept of raster object. This concept postulates that geographic entities can be stored as raster
tiles at variable sizes instead of polygons. In the following demonstration, the request will output a
raster containing image of a black geometric circle taking up 150 x 150 pixels as the one in Figure
4.46.
Select ST_AsRaster(ST_Buffer(ST_Point(1,5),10),150, 150, ’2BUI’);
Figure 4.46: Conversion from vector to raster [22].
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