The gravitational constant varies as a result of variations in the distribution of mass. On a global scale, the variation in distance to the moon and sun provide for diel variations in gravity which lead to earth tides. Another diel change in mass is provided by slight variations in atmospheric pressure. These changes cause variation in hydraulic head that can be readily monitored using accurate pressure transducers.
On a local scale, variation in the mass of groundwater in storage leads to a local change in gravity. These changes can be monitored by instruments established to monitor change in gravity over time. The variation of gravity due to differences in rock density can also be measured by moving a gravity measuring instrument over the land surface. This can be achieved on a local scale or a regional scale. The latter is the principle behind the GRACE satellite measurements. Examples of these different data sets are described in this Chapter.