In the users guide, the explanatory note for nearest_neighbor for categorical datasets is
And I want to know if it has more algorithm details for this interpolation option.For categorical datasets (i.e., datasets that have
type=categorical in their index files), this option actually causes the geogrid program
to consider all source pixels that lie within each WRF grid cell, and to find the fraction of
the WRF grid cell that is comprised of each category in the source data.