Grid to grid interpolation performs bilinear interpolation, generating each point of the output grid from its four neighbouring points in the input grid. The weights applied to the four input grid points are calculated:

- by performing a linear fit along each line of latitude;
- normalising the two partial weights for each point;
- performing a linear fit in the north-south direction.

The interpolation scheme also handles quasi-regular gaussian grid input fields. An output latitude line may be generated from two input latitude lines, one to the north and one to the south, which have different grid intervals.

A gaussian field cannot have lines of latitude at the poles. To generate a `line' of latitude points at the North or South pole , the interpolation scheme performs a linear interpolation of points on the gaussian grid line nearest to the pole and then puts these values into the output grid. For U and V wind component values, this provides grid points at the pole which have a `directional' value.

ISLSCP processing does bi-linear interpolation using four neighbouring points. Neighbours are used if they have the same land/sea characteristic in the old (ECMWF) land-sea mask as the new point in the new land-sea mask. If the four neighbours do not all have the same type, the nearest neighbour of matching type is used. If all four neighbours have different type from the new point, they are all used.