### Geodetic Reference Systems

Geodetic reference systems are the fundamental surfaces that best represent the Earth’s size and shape. They are essential to reference and integrate geospatial data for location-based services and scientific analysis. Geodetic reference systems are based on a collection of theories, principles, models, parameters and fundamental constants. They serve to quantitatively describe the position of a point in space. In contrast, geodetic reference frames are materializations (or realizations) of geodetic reference systems, making them “real” at ground monuments. Although the theoretical definition of a reference system is rarely changed, realizations are updated over time to reflect improved measurements and the Earth’s crustal dynamics.

Earth system changes are often derived from position and displacement measurements at several points on the Earth's surface, which can be made from a number of geodetic techniques over many decades. To understand Earth system changes, geospatial data sets must be merged, analyzed, and interpreted in a coherent and self-consistent manner. To connect and combine vector quantities, their components must be referred to a common reference frame in order not to yield mathematically illogical and physically meaningless results. When such outcomes are being pursued, it is essential to work within reference frames that are stable and related to each other with accurate transformations.