If you have a national grid reference you can put this directly into the Ordnance Survey website and see the location on a map. Start at www.ordnancesurvey.co.uk/oswebsite/opendata/viewer/ where you can enter a grid reference, and then zoom in or out to see it in context to other items around.
The Ordnance survey older Get a map service shows a small zoomable map, and you can enter the grid reference or place name.
You can also put the grid reference directly into the search box on Geograph, with no spaces, this shows the location with circle on a small Ordnance survey map. Under this map are two links, one says change to interactive map, and below this is open Interactive OS map Overlay and this opens a far larger map that is zoomable, and can be dragged. Geograph shows you photos taken at or near the location.
Wheres the path, covered in more detail below also allows you to enter grid references and see the location, but in a variety of map views or two views linked at once.
UK Grid Reference Finder also shows a maps and aerial views from many starting points including a grid reference.
You can also put the national grid reference into the search box of www.streetmap.co.uk and select the Smart Search button, although it doesn't say you can also use the grid reference. The map arrives at Zoom level 2 and if you take Zoom level 4 you get the 1:50,000 Ordnance Survey map.
Many Wikipedia articles have a link on the right just below the title shown as Coordinates with latitude and longitude. If you click on this you gat a GeoHack page that gives for British items the OS Grid reference and below this a wide range of map links to this location.
So what is a Grid Reference
Grid references look like SU103700 and are made up of:-
Many of us got to remember the order of these when learning map reading, as along the passage and up the stairs.
Using the concept that all are broke down by 10, we can see a single number is a representation of 10km , and two to 1km and the three a tenth of a kilometre, or its also possible to have different numbers of digits, showing the grid reference to different levels of accuracy for example SU1070, representing a 1km square, but normally you will see the the two letters and 6 figure format being used.
Printed Ordnance Survey maps show two numbers on its grid lines, the third in each case is estimated, usually within the border of the map there is a diagram showing what large squares are included and their letters. The grid reference system is the same, across map scales and going back several generations. The grid lines are 1km apart.
The origin of the grid system is military and dates from around the first world war, it was overprinted in the second world war onto military versions of maps. The grid system we have today is a slightly modified version of this, it is based upon squares starting from a 49 degrees 45 minutes and 58 seconds North; 7 degrees 33 minutes 23 seconds West; a point South West of the Isles of Scilly, chosen so all map references in the UK are positive.
The National Grid is explained at http://www.ordnancesurvey.co.uk/oswebsite/gi/nationalgrid/nghelp1.html
Maps many of us remember from years ago were mostly 1 inch to 1 mile, these were replaced later with slightly more detailed scale of 1:50,000, which are still available today. Maps you are likely to come across from Ordnance Survey are:-
Getting the Grid Reference
An website called Where's the path, allows you to see OS and Google mapping side by side, the default arrangement is an Ordnance Survey map on the left and aerial Google photo on the right, that are linked so that when you move one the other also moves, and putting a mouse pointer at any point on the map, shows the grid reference. Although is says enter a place, in the top left box, you will find you can also enter a grid reference as well. This also offers many other choices, for single and two view combinations, and deserves some time spent exploring all the options available to you.
The Ordnance Survey Get a map service shows a small zoomable map, and you can enter the grid reference or place name. Clicking on the map re-centres it at this point and shows below the grid reference it is now centred on.
If you have other information such as a latitude and longitude or a post code you can get the Grid reference plus see many map and aerial views by using UK Grid Reference Finder.
Converting to other formats
For many systems, including other maps and Satnavs you need a latitude and longitude, these can be looked up at:-
www.aber.ac.uk/cgi-bin/user/auj/locate.pl allows you to put in a grid reference and returns the latitude and longitude, also allows you to connect to the location on Google Maps and Streetmap.
www.nearby.org.uk/coord.cgi allows you to put in a national grid reference and tells you where it is near and the option to find the location on one of a variety of sites (not Google Maps). Setting the output option to ‘coordinate conversion only’ allows you to get the latitude and longitude and a variety of other coordinates. A further option allows you to look up the nearest postcode, and from this link to very many other pieces of information. You can also enter a wide range of other coordinates to start, and convert this in a similar way.
UK Grid Reference Finder shows other formats.
Stand Alone Programs
You can download a stand alone program (freeware) that does a whole range of conversions including converting from the national grid. Here are three to take a look at:-
iPad and iPhone apps, "there's an app for that", look at OS Grid Converter, this app is free and just does the conversion, an iPhone app that also runs on the iPad called Theseus Grid Converter allows you to put in coordinates a number of ways and by default starts by showing you your current location on a map, with grid, lat/long and postcode shown, as well as telling you the time of sunrise and sunset at this location today.
If you put in OS grid converter into the iPhone or iPad App store search you will also see others.
Other phones and pads/tablets. Some of these will also have Apps, and its worth exploring what is available if you don't have an iPhone or iPad.
Find a place from a name
Markets and Fairs in England and Wales Gazetteer 900-1516 (may be useful for locating places now merged into others).
The Meanings of Place Names
Links are available from www.ordnancesurvey.co.uk/oswebsite/freefun/didyouknow/