Gridfit is very nice. I have written an update that adds bicubic interpolation and improves the internal scaling (makes surface smoothness completely independent of the x/y grid size). If I send you the code, would you update Gridfit?
This function provides an excellent solution for fitting surfaces to irregularly-spaced data using a bicubic fit. Generating a 200,000 element surface from 65,000 GPS measurments takes about six seconds on my laptop.