A dipole like strain state is induced by threading edge dislocations emerging at the surface of gallium nitride (GaN) bulk substrates. This local strain is calculated by means of a three-dimensional elastic deformation potential model, taking into account the free surface of the sample. The calculations are in excellent quantitative agreement with the strain state derived from line shifts of the near band edge excitonic spectrum, measured by micro-photoluminescence (μPL). Scanning surface potential microscope (SSPM) measurements show that the dipole structure is not reflected in the local electrical potential distortions around the dislocations and the potential profile decreases laterally faster than the strain distortion, which is detectable even in several micrometer distance from the dislocation core.