Post Code
There are a number of ways of specifying the location of a particular place. The most obvious is the full postal address, although these days, giving just your Post Code and house number, is sufficient for a trader to find your full address from a readily available database.  Here we are at:

OX10 6LA

LAT/LONG
This is perfectly fine for a postman, or if you happen to have a large scale map showing street names. However, to find a location in the world or within a particular country, you need to use a different method of reference. Historically and traditionally, the world is divided up using lines of LATITUDE and LONGITUDE. Latitude is used to specify a position North or South of the Equator and Longitude specifies a position East or West of the Meridian, which happens to pass through Greenwich in London. Since the Earth rotates approximately once in every 24 hours and there are 360 degrees in a circle (derived from the approximate number of days for the Earth to circle the Sun in a year, and where 360 can be divided by so many other whole numbers), the imaginary lines of Longitude have been divided up into 360 parts, or 180 degrees to the East and 180 degrees to the West of the Greenwich meridian at 0 degrees. Stating a distance between lines of 1 degree is not easy, since at the Equator, this is about 70 miles, because the circumference is some 25,000 miles (meaning the Earth rotates at just over 1000 mph), but as you move towards either of the poles, this diminishes towards zero. However, in all positions, it takes 1 hour to rotate 15 degrees, or to put it in an astronomical way, the Sun (and stars) seem to move an angle of 15 degrees across the sky every hour. Using degrees alone is not sufficient to identify a particular spot on Earth, so degrees are sub-divided into 60 parts called minutes of arc and these are further divided (not surprisingly) into 60 seconds of arc. This means that the distance of 70 miles between degrees of longitude at the equator can be reduced to about 0.02 miles between seconds, a much more useful distance in terms of referencing a point. Longitude  is obviously useless on its own, since this specifies any position on a line which encircles the earth. Latitude lines are therefore used to complete a grid reference, where the angular measurements using degrees (still about 70 miles apart throughout), are again sub-divided into minutes and seconds of arc.

Global Position
This method of referencing a particular point on earth is used in Atlases, marine and aeronautical charts. Originally devised by taking accurate measurements of time, angular position of the sun and the angular position of known landmarks, such global positioning can now be derived from 24  satellites positioned some 12,000 miles above the Earth. Known as a Global Positioning System (GPS), these satellites were originally launched by the USA and used for military purposes. For many years however, the signals have also been available to anyone with a GPS receiver, to determine position, height, speed etc. It is envisaged that the Russian and European Space Agencies will also provide their own systems in the future. GPS receivers are available separately (approx. £100) or integrated into navigational systems in ships, aircraft, weapons and more recently, in cars. Such separate units will give basic indications of position in terms of providing a Lat/Long reference, but if plugged into a PC or PDA (Personal Digital Assistant) running some kind of mapping or navigational software, will provide full colour maps showing you your current position. Some even speak to you, to direct you to a particular predefined destination. Using one such device, our global position is found to be:

Latitude: 51º 37' 22'' (51.623º) N

Longitude: 1º 04' 38'' (1.077º) W

Grid Reference
In many respects, this method of positional reference is a little complex when using a more local mapping system. As far as the British Ordnance Survey, Great Britain is divided up into a grid of squares measuring 100 kilometres by 100 kilometres, each identified by two letters. Each square is further divided up into squares measuring 10 km by 10 km. Each of these can then be divided up, and so on.

The origin of the grid occurs at a point some 70 Km west of the Scilly Isles at an Ordnance Survey reference of SV000000. Ewelme falls within the SU Grid. It is then a matter of determining the distance of the location from its South West origin Eastwards (easting) to coordinate with the distance of the location Northwards (northing). The Ordnance Survey Reference (O.S) for Ewelme is:

SU639920

This basically means we are 63.9 Km East and 92.0 Km North from the origin of the SU grid (Wimbourne, Dorset). Another way to describe the reference, is to use the origin of the grid point off the Isles of Scilly, and define purely numerical eastings and northings as two groups of more precise numbers. Therefore, the GB Grid Ref of where we live is:

 463986, 192000

This is the equivalent of describing that we are 463.986 Km East and 192.0 Km North from a specific point off Scilly.

Mountain or Hill
Having a GPS system can also determine one's elevation or height above a mean sea level. The elevation at the entrance to our house is:

251 feet, 76.5 metres

More information can be found about GPS systems by clicking here.

Find out more about Navigation Software from here.

To visit the Ordnance Survey site click here.