Forth Valley, Fife & Tayside Area Ramblers

About Grid References

 Page in development stage. updated/sc 23-Nov-17

Are you a Grid Reference Guru?

Or does it seem like a black art - and why don't we just use post codes?
I'm assuming that if you are reading this, it is because you are not yet an expert.

Of course if you are using this site you don't really need to know how to use a grid reference. You can just click on a link in the description in the Walk Programme. The 'OS Map' link will open an OS map in with an arrow pointing close to the grid ref. The 'Google Directions' link will open Google Maps and show the route from wherever you are. There is one little quirk about the way Ramblers handle grid references that you might want to know. Grid references are uploaded to Ramblers as six figure references and are displayed in online listings the same way. But map markers in online listings are placed in the centre of the given grid square - so effectively using an eight figure reference. 111 222 becomes 1115 2225. Nothing really wrong with this when the given GR is correct. [A future discussion in more detail]

Walk leaders are only human and they do occasionally make mistakes with references. Unlike the printed programme [AJP], entries on the website can be corrected. Any changes to walk details will show in all online listings. Only walks that have been correctly notified will appear in the 'Altered & Cancelled' panel.

Post Codes?

postCodes300A post code punched into the sat-nav in your car can be a really easy way to get somewhere. Most of the time it works well and you arrive where you want to be. So why don't we like them? It's that phrase 'most of the time'. Post codes are allocated to groups of buildings - residential or commercial. Our walks often start at forest car parks or lay-bys or such, with no buildings close by. When our walks are uploaded to Ramblers they are given a post code based on the grid reference. This is the nearest post code in a straight line regardless of what lies between the grid reference and the post code. 

The grid reference used in the example below was not chosen at random. It was chosen because it is a prime example of why you should not trust post codes blindly. The illustration shows the location of the grid reference - East Lomond Car Park.  The red line is the only vehicle access. The blue lines all point to postcodes in Falkland, but the green line points south west to the 'nearest' post code at Hangingmyre Farm. The only access to the farm is from the Leslie - Craigmead road. In a straight line there are 670 metres between grid reference and post code - just under half a mile. By car it is 7 miles by the shortest route. There are other similar cases, but this is the worst I know about. Can you beat it?


Demystifying Grid References 

NO251058 is a grid reference as it is shown in this website and in the Area Joint Programme.
Splitting it into its parts will ease the process of learning how to understand it. NO 251 058.
So all grid references that we will be interested in are made up of two letters followed by two sets of three numbers.  Read the numbers in the form  two-five-one, not two hundred and fifty one.

The grid system of the Ordnance Survey maps,  divides the map into squares. At the largest scale these squares are identified by two letters – see image 1. So the NO of our grid reference has already limited the location to a part of east Scotland. These squares are 100km on each side.

Dividing  these sides by ten  gives us a grid 10 x 10 as shown in image 2. Each of these smaller squares is 10km on each side. If we give each line of the grid a number we can identify any square by two numbers.

In image 2 the black numbers associated with the vertical lines are called Eastings and are always the first number in the reference.  The red numbers  associated with the horizontal lines are Northings. Both sets start at zero and finish with 9. The last line on the right will be zero for the next Lettered Square. The topmost line is zero for the Lettered Square above.
So counting along to 2 and up none we find the pink square. We have used the grid reference NO 2 0 [a two figure reference]. This reference is in fact for the point where the lines 2 and zero intersect.  Anything in the pink square will have this two figure reference. The reference for the green square is NO 4 7 and for the yellow square NO 5 9.

In image 3 we are concentrating on the pink square of image 2 and dividing it up in the same way again.  This allows us to define a much smaller area. As was stated above, everything in the pink square has the reference NO 2 0. So we number each line in this square in the same way as before, except that each number is prefixed with 2 for the Eastings and 0 for the Northings. We have now arrived at the same level as is shown on Ordnance Survey maps. One kilometre grid squares. Counting along 5 and up 5 will give us the position of NO 25 05 - the bottom left corner of the purple square. Anything in this square has the four figure reference NO 25 25.

Zooming in again in image 4 until we are only looking at the purple square of image 3, There are no lines – to include them at this level  would  obscure the details of the map. Instead we measure or estimate where the lines would be. Image 4 shows the estimated lines 1 and 8. Where they intersect is the bottom left corner of NO 251 058 This is a square with sides of 100 metres, and is accurate enough for most purposes. Anything in this square has the six figure reference NO 251 058. Grid references can be quoted to ten figures resulting in a one metre square.

Move your mouse over image 4 and you will see the position on an OS Landranger map 1:50,000 That is a scale of two centimetres to one kilometre. OS Explorer maps are 1:25,000 scale and therefor show more detail, but still have the same 1km grid squares.


OS Factsheet - National Grid

OS Factsheet - Map Reading


GR images

image 1
image 2
image 3
image 4   OS map ©Crown copyright
Wednesday, February 21, 2018