We present an indoor positioning system that measures location using disturbances of the Earth's magnetic field by structural steel elements in a building. The presence of these large steel members warps the geomagnetic field such that lines of magnetic force are locally not parallel. We measure the divergence of the lines of the magnetic force field using e-compass parts with slight physical offsets; these measurements are used to create local position signatures for later comparison with values in the same sensors at a location to be measured. We demonstrate accuracy within one meter 88 percent of the time in experiments in two buildings and across multiple floors within the buildings.