A new technique for estimating geomagnetic parameters at or close to the rigsite is described and shown to allow reduction in the errors associated with main-field model predictions of geomagnetic field parameters. Valid reduction in the magnitude of certain directional error-term values becomes possible, resulting in significant reduction in wellbore positional uncertainty for magnetic-survey systems. These can then approach levels normally reserved for high-accuracy gyros. Wider application of the use of geomagnetic measurements to enhance both the accuracy performance and quality control of magnetic surveys is now much more feasible because continuous measurements at the drilling site can effectively be replaced by taking a "snapshot" to establish local baseline geomagnetic conditions. Then, with remote, fixed observatories subsequent variations at the drillsite can be determined. A single set of measuring equipment could service the needs of many drilling projects, and the feasibility of development of the capability for offshore application is now considerably improved.