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

SELECT ST_AsJPEG(rast) As rastjpg
FROM dummy_rast
WHERE rid=1;
5.6.3 Raster to PNG File
The ST_AsPNG(raster, band/numberbands, compression) function returns the selected raster bands
as a single PNG image (byte array). Where:
– band: a number that specifies a band for single band export.
– numberbands: an array of bands used to export. Bands are mapped to RGB or RGBA space.
For example, ARRAY[3,2,1] means map band 3 to Red, band 2 to green and band 1 to blue.
If both band and numberbands are not specified, then all bands are used if the raster has 1, 3, or
4 bands and. If it has 2 or more than 4 bands, then only band 1 is used.
– compression is a number from 1 to 9.
SELECT ST_AsPNG(rast) As rastpng
FROM dummy_rast WHERE rid=2;
5.7 Intersect Rasters with Vectors
One approach to make this operation is to convert all raster tiles to a set of geometries and do a
traditional intersection query between geometries only. The problem is that vectorizing all the tiles
will be too long and it is impossible to load a huge number of geometry in GIS software. Since it is
mostly impossible to convert those tiles to vectors, the PostGIS Raster approach will vectorize only
what is needed to do the requested operation.
In the intersection, if a geometry completely overlaps a nodata value area of one tile, st_intersects(rast,
geom) will return false for this tile/geometry couple and st_intersection(rast, geom)
will return an EMPTY GEOMETRY without vectoring this tile. If the geometry partially overlaps a nodata
value area and intersects other parts of the tile, st_intersects(rast, geom) will return true for this
tile/geometry couple and st_intersection(rast, geom) will return only values of parts of the tile
that intersects the geometry.
5.7.1 Intersect Rasters with Points
Practically, user can use intersection operation to extract ground elevation values for lidar (Light
Detection And Ranging) 1 points.
SELECT pointID, ST_Value(rast, geom) elevation
FROM lidar, srtm WHERE ST_Intersects(geom, rast)
Figure 5.4: Intersection of raster and geometric points [23].
In the above request, the intersection(raster,vector) operation is not necessarily needed. This is
due to the fact that each geometric point is represented by a pixel or a cell of a grid. So to extract
1 http://en.wikipedia.org/wiki/LIDAR.
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