Geodesists measure and monitor the Earth to determine the exact coordinates of any point.
Using a wide variety of tools, both on the land and in space, geodesists are experts at measuring things. Here are a few examples of what geodesists can measure.
With the precision of atomic clocks and lasers, geodesists can measure the pull of gravity so accurately, they could detect changes of one billionth of your body weight.
With tools that monitor the noise from outside our own galaxy, geodesists are able to measure the distances between two points on Earth to less than a millimeter.
By bouncing signals from satellites located hundreds of kilometers above the ocean, geodesists are able to track the rise of the mean ocean surface to about 1.7 millimeters per year.
And, probably most well-known, by using signals generated by GPS satellites that are located approximately 20,000 kilometers above the Earth, geodesists are able to accurately determine the positions of points to a few centimeters in just a matter of minutes.
Within the United States, this accurate determination of positions forms the scientific basis for all geodetic control, known collectively as the National Spatial Reference System (NSRS). Every non-military federal geospatial product of the United States is tied to the NSRS so that they may all overlap and align accurately.
Geodesy is the science of accurately measuring and understanding three fundamental properites of the Earth: its geometric shape, its orientation in space, and its gravity field as well as the changes of these properties with time.