The Collaborative International Dictionary of English v.0.48:

Equate \E*quate"\, v. t. [imp. & p. p. Equated; p. pr. & vb.
   n. Equating.] [L. aequatus, p. p. of aequare to make level
   or equal, fr. aequus level, equal. See Equal.]
   To make equal; to reduce to an average; to make such an
   allowance or correction in as will reduce to a common
   standard of comparison; to reduce to mean time or motion; as,
   to equate payments; to equate lines of railroad for grades or
   curves; equated distances.
   [1913 Webster]

         Palgrave gives both scrolle and scrowe and equates both
         to F[rench] rolle.                       --Skeat
                                                  (Etymol. Dict.
                                                  ).
   [1913 Webster]

   Equating for grades (Railroad Engin.), adding to the
      measured distance one mile for each twenty feet of ascent.


   Equating for curves, adding half a mile for each 360
      degrees of curvature.
      [1913 Webster]