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

FROM buff_temp
GROUP BY id
ORDER BY id;
5.8 Aggregate Functions
Figure 5.7: Different temperature values in a polygon [23].
The ST_Union(rast, band, expression) function returns a single band raster from the union
of a set of raster tiles. If no band is specified for the union, band number 1 is assumed. The
resulting raster’s extent is the extent of the whole set. The resulting raster pixel values are defined by
expression parameter. User can define their own expressions or use predefined functions like FIRST,
LAST, MIN, MAX, SUM, MEAN and COUNT. The default function is LAST when no expression
is specified. The LAST (respectively FIRST) function merges all rasters together by unioning their
extents and burning the last (first) raster without taking the other raster pixel values into account.
SELECT (ST_Union(rast, ’last’)
FROM dummy_rast;
5.9 Create a High Resolution Analysis Grid
Even though, to compute road and river length, mean temperature and etc, a vector format can
be used to store each cell of a grid as a polygon. Then necessaire operations are applied to the set
of polygons. It sounds like raisonnable but with a huge set of polygon (500 000 x 300 000 polygons)
resulting from a large area, these polygons are unmanageable. In contrast, by using raster, users can
choose to encapsulate them into small blocks as far as they want for easy analysis.
For example, all of USA can be stored as a grid of 500 000 x 300 000 pixels or cells (each cell
represents an area of 10 m). In raster format, these polygons are manageable in 15 000 000 tiles
of 100x100 pixels. The rest is to intersect vector layers with the raster tiles and apply appropriate
operations to the intersection result.
5.10 Create a Specialised Web or Desktop GIS Application
Complete SQL GIS
With the raster API, PostGIS is now a very complete SQL GIS as:
• It provides a single set of overlaid functions that work seamlessly on raster and vector files. So
an easy SQL API to manipulate and analyse graphical data.
• All geographic data (vector and raster) is spatially indexed. So there is no need to write complex
C,C++, Python or JAVA code to manipulate complex data.
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