  We present a method of measuring the charge density on dielectric surfaces.
Similar to electrostatic force microscopy we record the electrostatic
interaction between the probe and the sample surface, but at large tip-sample
distances. For calibration we use a pyroelectric sample which allows us to
alter the surface charge density by a known amount via a controlled temperature
change. For proof of principle we determined the surface charge density under
ambient conditions of ferroelectric lithium niobate.
