1. A method of calibrating a CD-SEM technique of characterisation by scanning electron microscopy in order to determine a critical dimension, said method comprising: determining a mapping function transforming at least one parameter obtained by modelling a measurement provided by the CD-SEM technique into a function of at least one parameter representative of a measurement provided by a reference characterisation technique different from the CD-SEM technique, wherein the mapping function is characterised by a plurality of coefficients; making measurements on a plurality of physical patterns chosen to cover a field of validity desired for the calibration, wherein the measurements on the plurality of physical patterns are made using both the CD-SEM technique of characterisation by scanning electron microscopy requiring calibration and the reference characterisation technique; determining, from the measurements, a set of coefficients of the mapping function minimising a distance between the functions of the parameters measured by the reference characterisation technique and applying the mapping function to the parameters obtained by modelling the measurements provided by the CD-SEM technique; using the set of coefficients when using the calibrated CD-SEM technique.