Method of obtaining an alignment versine signal by an extrapolated chord based measurement comprising the steps: - of obtaining alignment versine signals by a measurement device, - of application of an impulse response filter to the alignment versine signals, said impulse response filter being characterized by a polynomial function whose co-efficients are determined from an inverse transfer function of said measurement device and a transfer function characterizing a second chord based measurement device with a longer chord length, characterized in that the method of obtaining an alignment versine signal by an extrapolated chord based measurement comprises a preliminary step of determination of the co-efficients of the polynomial function of the impulse response filter, said step comprising: - a sub step of obtaining numerical values of the modulae and arguments of a transfer function characterizing the measurement device, - a sub step of obtaining numerical values of the modulae and arguments of a transfer function characterizing the second chord based measurement device with a longer chord length, - a sub step of numerical inversion of the modulae and of taking the opposite of the arguments of said transfer function of the measurement device, - a sub step of multiplication by the modulus of the transfer function of the second chord based measurement device and of summing of the argument of the transfer function of the second chord based measurement device, - a sub step of polynomial estimation of the ratio of the transfer functions characterizing the second chord based measurement device and the measurement device from the ratios of the modulae and the difference in the arguments of the transfer functions. - of obtaining alignment versine signals by a measurement device, - of application of an impulse response filter to the alignment versine signals, said impulse response filter being characterized by a polynomial function whose co-efficients are determined from an inverse transfer function of said measurement device and a transfer function characterizing a second chord based measurement device with a longer chord length, characterized in that the method of obtaining an alignment versine signal by an extrapolated chord based measurement comprises a preliminary step of determination of the co-efficients of the polynomial function of the impulse response filter, said step comprising: - a sub step of obtaining numerical values of the modulae and arguments of a transfer function characterizing the measurement device, - a sub step of obtaining numerical values of the modulae and arguments of a transfer function characterizing the second chord based measurement device with a longer chord length, - a sub step of numerical inversion of the modulae and of taking the opposite of the arguments of said transfer function of the measurement device, - a sub step of multiplication by the modulus of the transfer function of the second chord based measurement device and of summing of the argument of the transfer function of the second chord based measurement device, - a sub step of polynomial estimation of the ratio of the transfer functions characterizing the second chord based measurement device and the measurement device from the ratios of the modulae and the difference in the arguments of the transfer functions.