The Collaborative International Dictionary of English v.0.48:

Surbased \Sur"based`\, a. (Arch.)
      (a) Having a surbase, or molding above the base.
      (b) [F. surbaiss['e].] Having the vertical height from
          springing line to crown less than the half span; --
          said of an arch; as, a segmental arch is surbased.
          [1913 Webster]