  A method is described by which a function defined on a cubic grid (as from a
finite difference solution of a partial differential equation) can be resolved
into spherical harmonic components at some fixed radius. This has applications
to the treatment of boundary conditions imposed at radii larger than the size
of the grid, following Abrahams, Rezzola, Rupright et al.(gr-qc/9709082}. In
the method described here, the interpolation of the grid data to the
integration 2-sphere is combined in the same step as the integrations to
extract the spherical harmonic amplitudes, which become sums over grid points.
Coordinates adapted to the integration sphere are not needed.
