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 the expression-based map-algebra function.

The second map-algebra function (ST_MapAlgebra(), ST_MapAlgebraFct(), or ST_MapAlgebraFctNgb()) requires the user to provide an appropriate PostgreSQL function to be called for each pixel. The function being called can be written in any of the PostgreSQL PL languages (for example, PL/pgSQL, PL/R, PL/Perl), and be as complex as needed. This type is more challenging to use than the expression map-algebra function type, but it has the flexibility to work on any number of raster bands.

For this recipe, we use only the expression-based map-algebra function, ST_MapAlgebra(), to create a new band with the temperature values in Fahrenheit, and then append this band to the processed raster. If you are not using PostGIS 2.1 or a later version, use the equivalent ST_MapAlgebraExpr() function.