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E. Now What?

1. Who Uses the Horizontal Datum?

Quite simply, anyone who uses spatial data. To allow integration with other data, spatial data are based on formal coordinate systems. Formal coordinate systems are derived form a mathematical Earth model - a datum. 

Even on small area projects, surveyors employing GPS are dependent on the datum:

• Whether calibrating to a formal coordinate system or local one, satellites are in the TRS and measured positions are converted to the end-user system. The TCS is based on the ellipsoid.
• GPS measures geodetic heights of ground points. A geoid height must be accounted for to determine orthometric height. The geoid and ellipsoid are integral parts of the horizontal datum.  

2. Formal coordinate systems

The most obvious datum-dependent product is a formal coordinate system. Despite being unfamiliar with surveying or geodetic concepts, users are comfortable working with coordinate systems. For most, coordinate systems are transparent. All the user has to do is specify the project coordinate system, all the "messy" stuff is handled by software

Designing a coordinate system requires choices be made about distortions. What kind are there? How do they behave? How large is tolerable? Can they be computationally minimized or eliminated? There are many formal coordinate systems based on different needs and acceptable distortion levels.

The Horizontal Coordinate Systems topic describes how a grid coordinate system is designed, tied to the datum, and converting surveying measurements to/from of them.