The practical and scientific phases of geodesy are so intimately entwined that I shall not in this short paper try to treat them separately. What is of scientific value today may be of immense practical value tomorrow and, conversely, work done in the field of geodesy as an engineering operation may later have great scientific value.Until recently, geodesy was looked upon merely as a scientific subject involving the determination of the shape and size of the Earth, but it has expanded greatly from this early objective. We now not only use geodetic observations to obtain the dimensions of the Earth, but we are trying to determine the undulations of the surface of the geoid. we have learned that mean sea‐level is not an equipotential surfac