The earth's field provides a uniform magnetic field over a wide area in the scale of kilometers. The figures on the right show how a ferrous object, a vehicle, creates a local disturbance in this magnetic field either it is moving or stationary.

The magnetic disturbance of a large ferrous object, such as a vehicle, can be modeled as a composite of many dipole magnets. These dipoles have north-south orientations that cause distortions in the earth's magnetic field, where the distortions are most obvious at the engine and wheel locations but can also depends on what ferrous items are in the interior, on the rooftop or in the trunk locations. Thus the earth's magnetic field distortion has unique correspondence to the shape of the car.