  Six point generalization of the van der Pauw method is presented. The method
is applicable for two dimensional homogeneous systems with an isolated hole. A
single measurement performed on the contacts located arbitrarily on the sample
edge allows to determine the specific resistivity and a dimensionless parameter
related to the hole, known as the Riemann modulus. The parameter is invariant
under conformal mappings of the sample shape. The hole can be regarded as a
high resistivity defect. Therefore the method can be applied for experimental
determination of the sample inhomogeneity.
