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:
shade6rs = resample(shade_sm30,6,bilinear)
Without adding information, smoothing has made the image more readable.
These recipes were derived by trial and error.
Harvey Greenberg . . . . .
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