coefficient of bilinear interpolation

CF

Hi,

I'm trying to use bilinear interpolation to transfer values from 1 grid to another. Since the orginal pts are irregularly spaced, sometimes the result is not good, giving values out of the range of the original values.

When this happens, it seems that either the matrix cannot be reversed, or the coefficients are not within the 0-1 range, although they still add up to 1 ie a0=6., a1=-4, a2=.5, a3=-2.5. Normally it's something like a0=.2, a1=.4, a2=.1, a3=.3.

Is the latter always true? I'm trying to detect this type of error.

The remedy I used is to rotate all the pts to another orientation to prevent this type of error. Anyone has a better solution?

Thanks