The making of ground-based vector magnetic field observations and the application of results

In the UK we have 41 repeat stations with each being occupied once every four years. The measurement technique uses a fluxgate theodolite, two proton precession magnetometers and a geodetic-quality GPS for determining true north with a north-seeking gyro attachment as a the back-up. The data are then processed to reduce the effect of time-varying fields and then a model is fitted to them. The model forms the basis of the service to provide grid magnetic angle to the UK mapping agency for publication on their map products. This ensures navigation using their maps and a compass is possible. The usefulness of such ground-based vector magnetic observations around the world for assessing local and global model uncertainties is demonstrated. A global database of observations is maintained and can be accessed at http://www.geomag.bgs.ac.uk/data_service/data/surveydata.html.