Jackson and Cheng have suggested that the changes in areal dilatation measured in Geodolite surveys by the U. S. Geological Survey may be simply an artifact of the measuring system. Although systematic error could conceivably account for the observed excursions in dilatation, we maintain that the specific criticisms by Jackson and Cheng are incorrect: the excursions in dilation cannot be attributed to the offset correction nor to proportional error associated with temperature. The absence of both errors is demonstrated by using data that are particularly sensitive to the two effects.