In this section a method is introduced that determines the quality of the match between a potential field and a data distribution {}. The overlap is computed between the data distribution and a region of same volume enclosed by an isopotential curve (figure B.1). The method relies on the data points being uniformly distributed over a closed region with volume A (as it is the case for the ringlinesquare and vortex distributions).

Let B_{c} be the volume of the closed region defined by {  p()c}, which is the set of points surrounded by an isopotential curve with value c. The volume B_{c} was calculated using MonteCarlo integration.
The computation of the quality measure has two steps. First, choose c, such that B_{c} = A. Second, count the number of data points fulfilling p()c. The quality measure is the percentage of this number on the total number of data points.