Fast 2-D interpolation in Python with SciPy regular grid to scattered / irregular evaluation
This: http://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.RectBivariateSpline.ev.html
I.e. myspline.ev(xI, yI)
From scipy v0.14.0, RectBivariateSpline.__call__() takes an optional grid= keyword argument which defaults to True:
grid : bool
Whether to evaluate the results on a grid spanned by the input arrays, or at points specified by the input arrays.
So you could use:
splineoutput = myspline(xI, yI, grid=False)