We demonstrated some of the advanced uses of PostGIS's map-algebra functions by computing the three-band and two-band EVIs from our MODIS raster. This was achieved using ST_MapAlgebra() and ST_MapAlgebraFct(), respectively. With some planning, PostGIS's map-algebra functions can be applied to other uses, such as edge detection and contrast stretching.

For additional practice, write your own callback function to generate an NDVI raster from the MODIS raster. The equation for NDVI is: NDVI = ((IR - R)/(IR + R)) where IR is the pixel value on the infrared band, and R is the pixel value on the red band. This index generates values between -1.0 and 1.0, in which negative values usually represent non-green elements (water, snow, clouds), and values close to zero represent rocks and deserted land.