Interpolation is done with a simple bi-linear algorithm. Unfortunately the routine does not average from finer to coarser grids yet, but really interpolates. There are some heuristics to check whether a grid wraps in longitude, otherwise the last two points are sometimes extrapolated slightly.

• Finest: select the grid with the smallest distance between latitude or longitude values and interpolate the other grid to this one.
• Coarsest: select the grid that does not have the smallest gaps and interpolate the other field to this one.
• First: interpolate to the grid of the field that was selected first.
• Second: interpolate to the grid of the field that was selected on this page.

Interpolation turned out to be a very non-trivial problem. Please report problems to me.