  A combination of analytical and numerical approaches obtains the complete
dispersion curve for surface excitations in a condensate held in a plane linear
potential. This improvement on previous approximate results yields an accurate
formula for the local Landau critical velocity for vortex nucleation at the
surface of a sufficiently large condensate, which agrees very well with recent
experiments. The dispersion curve for surface modes at a hard wall potential is
also presented, for contrast.
