[Paraview] Ridgeline finding

[Karl Merkley]

I have some DEM data and I am attempting to extract the ridgelines  
from the data.    I'm looking at watershed type information so for my  
definition the ridgeline is location where water flows in different  
locations from the top of an elevated region.  The gradients are a  
clue but I don't know how to create a polyline that traverses the  
ridge.   Does anyone have any suggestions for a good way to do this?

   Thanks,
        Karl