Smoothing a 30-meter DEM

I recently needed to make a shade-relief image of a 30-meter DEM.

The results were not impressive.

The biggest problem is that Z values are in integer meters, causing stairstepping in the flat areas. This can be solved by smoothing over a relatively large distance. Working at the arc/INFO "Grid:" prompt, I smoothed over a 90-meter sqare and over a 180-meter diameter circle.


smooth_sq = focalmean(delldem) /* This is a DEM of the Dell Quadrangle

smooth_c3 = focalmean(delldem,CIRCLE,3) /* That's a radius of 3 cells

But we want to apply this smoothing only to the flat areas, which we determine with:


focalran_sq = focalrange(delldem)

focalran_c3 = focalrange(delldem,CIRCLE,3)

I combined them with a con function. This is ARC/INFO's compact form of an if-then-else statement:

 
smooth30 = con(focalran_c3 le 3,smooth_c3,focalran_sq le 2,smooth_sq,delldem)

shade_sm30 = hillshade(smooth30,270,40,shade)

This eliminates many artifacts in the plain without changing the mountains.

However, the large cells are distracting, so I smoothed the shaded imgae:

 
setcell 6

shade6rs = resample(shade_sm30,6,bilinear)

Without adding information, smoothing has made the image more readable. These recipes were derived by trial and error.