Magnetometres work better when magnets are not sitting on top of them

A magnetometre is a device which measures the earth's magnetic field in two axis and allows you to work out the direction of magnetic north. It basically allows you to create a 'digital compass'. Our testing involved taking measurements at 15degree increments to allow us to determine the error involved in the measurement (generally a under a couple of degrees) and also appropriate offsets to calculate a bearing measurement. That all worked well - using a compass and the diagram shown to make the measurements.

Using it in the plane is a different story through, about 3 cm above the magenetometre sits the GPS antenna - and in this configuration it gave extremely erratic results. After about 5 minutes it clicked that the antenna has a couple of magnets in the base (designed for car mounting) - when these are removed - unsurprisingly the magnetometre works again.