A procedure for the assessment of the maximal power frequency magnetic flux density from a power substation is proposed in the paper. The present technique is based on a multiquadric approximation of magnetic flux density obtained using the discrete measured data. The approximation sufficiently handles multidimensional multiextreme functions by interpolating their scattered values accurately. Subsequently, the maximal flux density value is evaluated by minimizing the negative multiquadric approximation via a stochastic optimization method - differential evolution. Therefore, the procedure provides the maximal flux density assessment on the basis of a limited number of measured values, thus reducing the measurement effort