Interpolating world data to a local grid in Python
As @gansub implicitly suggested, you may have a better outcome by defining the output grid as well. 
Check this very similar question: 
How to interpolate scattered data to a regular grid in Python?

After comments, I realize I missed the irregular target grid. Maybe this answer helps:
https://stackoverflow.com/questions/3242382/interpolation-over-an-irregular-grid