Geodesy is the study of the Earth's physical nature, including its shape, orientation, size, and gravitational field. It is a relatively new field that relies on technological advancements, especially satellite and space measurement technologies (starting with the launch of Sputnik in 1957). It relies on accurate measurements using fixed constants usually perceived from space, whether that be the north and south pole or cosmological orbits of the moon and planets.
There are many techniques used to get centimeter accurate measurements, including widely know ones such as global positioning system (GPS) and light detection and ranging (LIDAR). Using them, scientist can find global estimates of sea level change or map the motion of tectonic plates. Also, mixing two techniques such as the primarily elevation based LIDAR and rainwater precipitation sensitive GNSS (similar to GPS) can be used to see how aquifers recharge after rainfall.
The only way information such as the graph above can be made is because of the precise and repetitive nature of satellite orbits. Data from the 1980s can be used with more current data to provide information on the extent of surface deformation due to plate movement, this in turn can provide insight into the slight changes in Earth's gravitational fields that can only be seen in large scale.
Another inference made came from the the use of GPS measurements of glacial movement and loss. After the melting of ice caps in the last ice age the Earth's mantle and crust warped due to the changes in mass above them. Measuring this change has led to the determination of how viscous the mantle is.
Geodesy is also important in understanding and predicting natural disasters and subtle changes in lived in areas. Multiple threats can impact places like Thailand where the constantly rising sea levels are magnifying the impact of normal floods.
Some other applications (above), including aquifer response to water pumping and landslides along crests.
Source: Wdowinski, S., & Eriksson, S. (2009). Geodesy in the 21st Century. Eos, Transactions American Geophysical Union, 90(18), 153-155.