Thank you, Can I query what the '([rast.x] - 1) * 10 + [rast.y]') section of the equation is? I can't seem to understand what it does. James -- View this message in context: http://postgis.17.n6.nabble.com/ST-Slope-tp4551303p4641609.html Sent from the PostGIS - User mailing list archive at Nabble.com.