This is the fastest (though not most accurate) method of fitting a circlle to data points on a plane (given by their xy-coordinates). It returns the circle center (a,b) and radius R. It was proposed by I. Kasa in article "A curve fitting procedure and its error analysis", IEEE Trans. Inst. Meas., Vol. 25, pages 8-14, (1976). Another version of this circle fit has been posted by Izhak Bucher (file 5557).