Which interpolation method is the best suitable for hill/valley areas and normal ground with undulations and rivers? When working with Aster 30mX30m DEMs, which method will give the most realistic elevations? I am trying with different methods and getting different contour distribution and shapes but don't know which will give the most accurate and realistic values.
I wonder why do you want to interpolate your DEM? It is already gridded!
In case you want to interpolate your DEM to smooth your surface I can recommend the "Multilevel B-Spline Interpolation" tool in SAGA-GIS. I made good experiences with it. You can adjust the smoothing-parameter and therefore control the magnitude of smoothing.