Following a request from this thread
http://www.cartographersguild.com/showthread.php?t=2473

where I tried to fix up some issues with some terrain noise using a spatial filter, I have been asked to explain a bit more about the process. Its not an easy process to understand but I have uploaded my old help file and clipped some app specific bits out of it.

It seems that Gimp at least has an FFT plugin which does exactly the same thing though the screen images might be slightly different, the principle is the same.

Maybe RobA can do some screenies of the images posted from this next link using Gimp and we can see the same process being done.

Spatial Imaging Help