Impedance loci information is well used by electrical engineers. In power system engineering it is useful too: (i) to see if the addition of static VAR compensation will cause resonance problems; (ii) to enable harmonic filters to be designed and (iii) to assess if a proposed ripple control frequency is practical. In the past, to obtain a good frequency locus, the frequency range was scanned using a constant but small increment in frequency. This is computationally very expensive and interpolation methods have also been used to allow a larger frequency increment in the scan. By allowing the interval between frequency points to vary a better choice of frequencies for impedance evaluation can be found. An adaptive scheme for automatically choosing the frequencies for impedance evaluation is described. A parametric cubic spline is then fitted through the chosen points. This dramatically reduces the number of frequency points that need to be evaluated without losing any essential detail in the loci.