Position summary

With the three concepts summarised in table, we can set up and use a coordinate system.

Coordinate system concepts

Coordinate system concept	Alternative name	Role in positioning
datum	Terrestrial Reference System (TRS)	The set of parameters which defines the coordinate system and states its position with respect to the Earth’s surface.
datum realisation	Terrestrial Reference Frame (TRF)	The infrastructure of ‘known points’ that makes the coordinate system accessible to users
Type of coordinates		The way we describe positions in the coordinate system

We have answered the question ‘What is position?’ in a way that is useful for positioning in geodesy, surveying and navigation. A position is a set of coordinates, hopefully with an accuracy statement, together with a clear understanding of the coordinate system to which it refers in terms of the three items in the table above.

The following two sections are case studies of two coordinate systems in common use in Britain – that used for GNSS positioning, and that used for OS mapping. As we shall see, a close look at either of these examples shows that even within one coordinate system, there are alternative datums and TRFs in use, sometimes under the same name.