In the framework of the European project HITRIplus and the SIGRUM collaboration, the National Centre for Oncological Hadrontherapy (CNAO) has launched initiatives to design and build a superconducting gantry for hadrontherapy. Such a gantry is based on short and strongly bent superconducting dipoles that require an appropriate description of the quality of the produced magnetic field. In accelerator magnets, the field quality is conveniently described by a set of coefficients known as field harmonics or multipole coefficients. Generally, two different approaches can be used for determining the field harmonics. The first is the Fourier expansion of the field component along a circle.