Hallade method

The Hallade method, devised by Frenchman Emile Hallade, is a method of surveying, designing and setting out curves in railway track.

It involves measuring the offset of a string line from the outside of a curve at the central point of a chord. A standard chord length is used: in the UK this is conventionally one chain, ie 22 yards. Half chords, ie half a chain intervals, are marked on the datum rail using chalk. The string, which is one chain long, is then held taut with one end on two marks at the ends of a chord, and the offset at the half chord mark measured. The versine of the chord can then be calculated using the approximation of versin(theta)=1-cos(theta) which is:

v = (L^2)/8r

where v = versine L = chord length r = radius of curve

By comparing the surveyed versine figures to the design versinces, this can then be used to determine what slues should be applied to the track in order to make the curve correctly aligned. This is often done using pegs which are driven into the ground in the cess beside the track to be aligned. The process of putting the pegs in the correct positions is known as 'setting out'.