Earth Reference Surfaces
An ORS defined to approximate the shape of the Earth is termed an Earth Reference Surface (ERS)
- The extent of the Earth’s surface over which an ERS is a sufficiently accurate model of the shape of the Earth is limited by the extent of available measurements
- Such bounded ERS are termed datums
- In practice, datums have historically been limited to the extent of continental landmasses, or less; with the advent of satellite geodesy, accurate planet-spanning measurements have become available to support the definition of unbounded ERS’s
- As a result, legacy (bounded) datums are recognized as local, and newer unbounded datums are recognized as global
- Local and global datums are represented identically, excepting that the former have a bounded, application-dependent, region of applicability given a specific accuracy requirement
Both local and global datums may be used to locate and orient coordinate systems
- In practice, the process of establishing location and orientation parameters for the Earth is traditionally handled separately in the horizontal and vertical dimensions