Hallade method

The Hallade method, devised by Frenchman Emile Hallade, is a method used in track geometry for 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, i.e. 22 yards. Half chords, i.e. 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 each end of a chord, and the offset at the half chord mark measured. The versine of the chord, which is equal to this measured offset value can be calculated using the approximation of:

${\displaystyle \operatorname {versin} \theta =1-cos\,\theta }$

which is:

${\displaystyle v\approx {\frac {L^{2}}{8r}}}$

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