ST_TPI — Returns a raster with the calculated Topographic Position Index.
raster ST_TPI(raster  rast, integer  nband, raster  customextent, text  pixeltype="32BF" , boolean   interpolate_nodata=FALSE  );
Calculates the Topographic Position Index, which is defined as the focal mean with radius of one minus the center cell.
                 
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                 This function only supports a focalmean radius of one.  | 
            
Availability: 2.1.0
-- needs examples