solve it as an orthogonal distance problem. computing the distance**2 of the points from the asumed circle and summing up you obtain a ordinary least squares problem in three unknowns. even easier: plug your data in odrpack (from netlib). they even have a driver for orthogonal distance regression for ellipses (driver2). hope this helps peter original posting: I have a number of x,y data points that form an arc of constant radius. Do you know a good technique for finding the center and radius of the arc? I've tried the following, but I'm not sure of it's reliability: take first data point, find line running through point but also tangent to the arc, take second data point and find line tangent again. Normalize both lines and find the intersection. This will give me the center, from here the radius is easy.