# 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](/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/what-is-position/types-of-coordinates.md), we review the different types of coordinates we commonly need to work with. In [We need a datum](/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/what-is-position/we-need-a-datum.md), 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.os.uk/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/what-is-position.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.