PostGIS Cookbook - Second Edition

How to do it...

Let's generate a slope raster from a subset of our SRTM raster tiles using ST_Slope(). A slope raster computes the rate of elevation change from one pixel to a neighboring pixel:

WITH r AS ( -- union of filtered tiles 
  SELECT ST_Transform(ST_Union(srtm.rast), 3310) AS rast 
  FROM chp05.srtm 
  JOIN chp05.sfpoly sf ON ST_DWithin(ST_Transform(srtm.rast::geometry,  
     3310), ST_Transform(sf.geom, 3310), 1000)),  
  cx AS ( -- custom extent 
    SELECT ST_AsRaster(ST_Transform(sf.geom, 3310), r.rast) AS rast 
    FROM chp05.sfpoly sf CROSS JOIN r 
  ) 
  SELECT ST_Clip(ST_Slope(r.rast, 1, cx.rast), ST_Transform(sf.geom, 3310)) AS rast FROM r 
CROSS JOIN cx 
CROSS JOIN chp05.sfpoly sf; 

All spatial objects in this query are projected to California Albers (SRID ...