The long-term drift of magnetometers at geomagnetic observatories is calibrated by a non-magnetic theodolite. We propose a numerical method to evaluate such absolute measurements in a new, more general manner. It is more flexible and helps to identify and correct or discard erroneous measurements. We derive this method and give examples showing how it improves the quality and reliability of the calibrations parameters (the so-called baseline values) of an observatory magnetometer.