Performing simple map-algebra operations

In the previous recipe, we saw that the values in the PRISM rasters did not look correct for temperature values. After looking at the PRISM metadata, we learned that the values were scaled by 100.

In this recipe, we will process the scaled values to get the true values. Doing this will prevent future end-user confusion, which is always a good thing.

Getting ready

PostGIS provides two types of map-algebra functions, both of which return a new raster with one band. The type you use depends on the problem being solved and the number of raster bands involved.

The first map-algebra function (ST_MapAlgebra() or ST_MapAlgebraExpr()) depends on a valid, user-provided PostgreSQL algebraic expression that is called for every pixel. The expression can be as simple as an equation, or as complex as a logic-heavy SQL expression. If the map-algebra operation only requires at most two raster bands, and the expression is not complicated, you should have no problems using...