Nearest Neighbor

Nearest-neighbor interpolation is a simple method of multivariate interpolation in one or more dimensions. The Nearest-neighbor algorithm selects the value of the nearest point and does not consider the values of neighboring points at all, yielding a piecewise-constant interpolant.

A detailed description of the algorithm is beyond the scope of this documentation
See: http://en.wikipedia.org/wiki/Nearest-neighbor_interpolation for a summary description and references.