What is "equating for curves"

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.

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

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.