3D Interpolation

Hi all,

I have an x, y, and z column. X and Y have thousands of points, while Z has a first and last. I want to fill in the rest of Z somehow (interpolation was my best guess). I'm filling in gaps in bathymetry data with "best guess" points. So far I have linearly interpolated z values using x and y separately, then averaging those two columns. There has to be a more accurate method.

Any advice or code would be helpful.

Thanks!