to measurements at other survey points to improve the accuracy of surveys and the efficiency of drilling operations application_number US 35937203 A description 0037 fig1 shows the effect of bias errors in x and y magnetometers on magnetic measurement for perfect magnetometers the diagram is a circle centered at the origin with biases in magnetometers the diagram is still a circle but is off centered the displacements between the center of the circle and the origin of the b x and b y coordinates are the bias errors in x and y magnetometers respectively prior art techniques known to one skilled in the art can been used to determine the bias errors and corrections can been made to x and y magnetometers after corrections the diagram is a circle centered at the origin when there are errors in scale factors in x and y magnetometers the diagram is in general an ellipse in fig2 the diagram corresponds to a case where y magnetometer scale factor is smaller than the correct value prior art short collar algorithms can be used to correct the bias errors so that the corrected diagram is centered at the origin however it remains elliptical in shape in this invention the.