Hi all PostGIS Raster fans,
Thanks to Bborie Park form the Center for Vectorborne Diseases at the
University of California, PostGIS Raster now supports map algebra operations on
two rasters. The new function, along with its sister working on one raster,
grants PostGIS with one of the most flexible map algebra operator available
today in any GIS package.
ST_MapAlgebra(raster, raster, expression) -> raster...
-compute a new raster from the expression of two input raster,
-the expression is evaluated by the PostgreSQL parser so that any valid SQL
expression can be used including conditional expressions. an upcoming variant
will allow you to refer to your own custom function instead of using an
expression,
-can compute the new raster over the extent of the first input raster, the
second inout raster, the intersection of the two rasters or their union,
-accept alternative expressions to deal with nodata value occurring in the
first, the second or both raster.
This major milestone which will be part of PostGIS 2.0 is the foundation for
many future cool developments:
-ST_Intersection(raster, raster) and all other overlay operation like
ST_Union(), ST_Difference(), ST_SymDifference()
-ST_Clip(raster, geometry) (which is essentially the same as
ST_Intersection(raster, ST_AsRaster(geometry)))
-ST_BurnToRaster(raster, geometry)
You can obviously build your own raster model from an expression of many raster
layers by imbricating many ST_Mapalgebra calls. e.g to implement r1 = (r2 + r3)
/ r4 you can do
SELECT ST_MapAlgebra(rasttemp.rast, r4.rast, "rast1/rast2") AS r1
FROM (
SELECT ST_MapAlgebra(r2.rast, r3.rast, "rast1 + rast2") AS rast
FROM r2, r3
WHERE ST_Intersects(r2.rast, r3.rast)) rasttemp,
r4
WHERE ST_Intersects(rasttemp.rast, r4.rast))
Feel free to be creative and suggest your own plpgsql or C code to get map
algebra to work!
Thanks,
Pierre
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