What is position?

We have introduced an irregular, dynamic Earth and the concepts of ellipsoid and Geoid that are used to describe its basic shape. Now we want to describe with certainty where we are on that Earth, or where any feature is, in a simple numerical way. So, the challenge is to define a coordinate system with which we can uniquely and accurately state the position of any topographic feature as an unambiguous set of numbers. In the fields of geodesy, mapping and navigation, a ‘position’ means a set of coordinates in a clearly defined coordinate system, along with a statement of the likely error in those coordinates. How do we obtain this?

The answer to this question is the subject of the whole of this section. In 'Types of coordinates', we review the different types of coordinates we commonly need to work with. In 'We need a datum' and 'Realising the datum definition with a Terrestrial Reference Frame' we will look at the two essential concepts in creating a terrestrial coordinate system that gives us a detailed insight into what a set of coordinates (a geodetic position) really tells us.