Grid Reference - It is usually an eight character item of the form SK123456 where SK is the position in the picture on the right and the 123456 defines a location within that square to within 100 metres.

Any in depth description of co-ordinate systems is given in the OS document.

Ramblers Groups around the country publish a walks programme where members and the public can attend and walk in our great country. A leader guides the group of walkers around the route.

Note: if the public wish to continue attending these walks after the first few (3?) they are then expected to join the Ramblers

Longitude and Latitude

Ordnance Survey (OS) is a national mapping agency in the United Kingdom which covers the island of Great Britain. It is one of the world's largest producers of maps. Since 1 April 2015 it has operated as Ordnance Survey Ltd, a government-owned company, 100% in public ownership. The Ordnance Survey Board remains accountable to the Secretary of State for Business, Energy and Industrial Strategy.

Postcodes

Is a device that obtains it position in the world from satellites, and will then give the user directions to a specific location.

Satellite navigation

NASA Shuttle Radar Topography Mission (SRTM)  is a NASA mission to map the elevation of the landmasses of the world. See wiki

What3words is a geocode system for the communication of locations with a resolution of three metres. What3words encodes geographic coordinates into three dictionary words; the encoding is permanently fixed. For example, the Omphalos of Delphi, believed by the ancient Greeks to be the centre of the world, is located with ///spooky.solemn.huggers.

What3words differs from most other location encoding systems in that it displays three words rather than strings of numbers or letters. What3words has a website, apps for iOS and Android, and an API that enables bidirectional conversion between what3words addresses and latitude/longitude coordinates. As the system relies on a fixed algorithm rather than a large database of every location on earth, it works on devices with limited storage and no Internet connection.

According to the company, its revenue comes from charging businesses for high-volume use of the API that converts between 3 words and coordinates; services for other users are free of charge.