Scanning tunneling microscopy (STM) is used in situ to study threading dislocation terminations at the surface of GaN grown by molecular beam epitaxy on a Si(111) surface. The associated surface depressions are imaged at the nanometer scale, giving a new insight into the relationship between growth kinetics and dislocation-related surface morphology. Pure edge and dislocations with a screw component are easily differentiated and their density is determined. Pure edge dislocations are located at the boundaries of subgrains slightly misoriented in the surface plane. The direct measurement of the spacing of these dislocations provides the subgrain misorientation determination. Transmission electron microscopy well confirms the findings obtained from the STM study.