The equal error method of mathematical processing is to make the fitting errors of each segment equal when fitting the contour curve with a straight line. According to relevant data, the error of fitting non-circular curve with straight line and arc line is called fitting error. When the contour curve is fitted with a straight line, the fitting errors of each segment are equal. This mathematical processing method is called equal error method. The three-point circle method is based on the equal error method that the straight line segment approximates the non-circular curve, and each node is obtained by making an arc of three adjacent nodes.