Interpolation of z-values for breaklines

[Andreas Paukner-Ruzicka]

Hello,
I am searching for a good and most exact way to calculate z-values for breaklines
(rivers, faults, ridges etc) which only have x- and y-values. The base for this calculation is 
a DEM with accidentally distributed points. The calculation should be in vector format. 
How to do this with GRASS or ARC/INFO ?

Many thanks in advance !

Andreas


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