During a rocket flight in the dayside auroral oval on December 17, 1974, several examples of velocity dispersions of precipitating electrons were observed. It is shown that the commonly employed interpretation of such dispersions as resulting from a purely temporal modulation of the source is not unique: the same dispersion effects can also be caused by a moving source of finite width and constant intensity. This spatial model is further improved by adding the dispersion originating from a superimposed convection velocity. In general the source altitudes implied by the two models (temporal and spatial) differ. For one of the events analyzed the temporal model yielded a source altitude of ~3000 km. Because of special circumstances this did not differ significantly from the result obtained with the spatial model. The second case analyzed was inconsistent with the temporal model and required further modification of the spatial model to include an energy dependent source width, in line with expectations from an acceleration in a V shaped electric field pattern. Applying the latter model we derive a source altitude of ~3500 to ~5000 km.