The emission of electrons from an insulating crystal deposited on a conductor occurs at a macroscopic electric field of a few volts per micrometer, three orders of magnitude below the field emission from a clean metal. This is due to the local field enhancement induced by the presence of the insulating crystal. The emission profiles depend on the shape of the conductive substrate; analyzing these profiles enables the local difference in electric potential and the opening angle to be traced. Given the thickness of the crystal, the local difference in potential indicates the local field enhancement of a few volts per nanometer applied to the conductor.