# Position summary

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

### Coordinate system concepts

<table data-full-width="true"><thead><tr><th>Coordinate system concept</th><th>Alternative name</th><th>Role in positioning</th></tr></thead><tbody><tr><td>datum</td><td>Terrestrial Reference System (TRS)</td><td>The set of parameters which defines the coordinate system and states its position with respect to the Earth’s surface.</td></tr><tr><td>datum realisation</td><td>Terrestrial Reference Frame (TRF)</td><td>The infrastructure of ‘known points’ that makes the coordinate system accessible to users</td></tr><tr><td>Type of coordinates</td><td></td><td>The way we describe positions in the coordinate system</td></tr></tbody></table>

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 pages on [Modern GNSS coordinate systems](https://docs.os.uk/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/modern-gnss-coordinate-systems) and [Ordnance Survey coordinate systems](https://docs.os.uk/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/ordnance-survey-coordinate-systems) are case studies of two coordinate systems in common use in Britain – those used for GNSS positioning and 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.