contour lines from site values:

[Admin. scarab J.M. ROQUES 5322]

Hi,

I was wondering which function may draw Isovalues contours from a site list wit
h numerical values as attributes (with very sparsed points) in Grass.

As i need to realise this outside of Grass (I use Grass to validate), does some
one can tell me some algorithm to do this too ?
i thought of using something like nearest neighbours interpolation, but can't f
igure out the best way to avoid computing the whole area values; is there a way
 to track the isovalue line?
I think this algorithm is surely famous but I cannot find a description anywher
e...)
Thank you for answering
JM Roques