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

Figure 4.44: In-database raster.
directly seen from Quantum GIS (an open source desktop geographic information systems). For
instance, the first pixel value for the first, the second and the third band are respectively FD, 4E, 46
in hexadecimal. These values correspond to 253, 78 and 70 in decimal.
4.16 Indexing
PostGIS Raster creates indexes over raster data itself. In this way, user can do really complex
operations on raster data, like getting a set of geometry and geometry value from a given raster
band (ST_DumpAsPolygons() function). Unlike Oracle GeoRASTER that creates indexes over spatial
extent, it has more sense when PostGIS Raster creates indexes over raster data.
In PostGIS, loading a new raster table and creating indexes over the raster column can be done
by executing these two lines:
python raster2pgsql.py -r path/*.tif -t raster_table -s 4326 -k 50x50 -I
-o raster_sql.sql
psql -d database -f raster_sql.sql
The -s option georeferences the resulting rasters using SRID 4326.
The -k option splits image files into tiles of 50x50 pixels (one per row). All these tiles do not
overlap and their upper left corners follow a regular block grid. The global extent of the layer is
rectangular and not rotated.
The -I option informs the loader to create the spatial index on the raster table. This index
is very important as it restricts PostGIS Raster’s computing efforts only to the tiles involved in
spatial operations. For example, when finding the intersection of the raster_table and a set of
geometry points, this operation will be performed only on the tiles that actually intersect with the
geometry points. So it is much faster to search for those tiles if they are spatially rather than try
them one after the other sequentially from the raster table. If the -I option was not specified when
using raster2pgsql.py, the index can always be created afterward by executing the following SQL
command:
CREATE INDEX rast_gist_idx
ON raster_table
USING GIST (ST_ConvexHull(rast));
The ST_ConvexHull() function returns the convex hull geometry of the raster including pixel
values equal to BandNoDataValue(). Further, a spatial index can also be made on the geom_points
table to make sure that the intersection operation will be done as quickly as possible:
CREATE INDEX point_geom_idx
ON geom_points
USING GIST (the_geom);
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