POVRay and Height fields and resolution...

[cfds]

Perhaps these steps are from the original png-file before using Wilbur on it. If the original file was only 8 bit deep even a lot of blurring and saving as 16bit png can leave shelfs in the heightfield.

What happens when you create a heightfield in Wilbur from scratch (just some cosine hills) and render it? If the steps disappear they where from your original file, if they remain they are a Povray artifact.

[Crayons]

Egad! Zooks!
You have it!
I'm no expert with Wilbur - I only downloaded it the other day to fiddle with my original file. Anyway, the short version. I managed to create the default sinc function and if I render it to approximately the same sort of "rolling hills" effect when scaled - it is as smooth as the proverbial baby's bum.

As a test I loaded and saved that sinc file from Wilbur into PSP X, and resaved from there and it produces the sort of fringing effects again. It would appear to be an issue with Paintshop Pro? It looks like I'll have to work out how to make a nice pseudo random hill terrain in Wilbur - or something!
Excellent answer, many thanks indeed!