What is a grid reference and how do I use it?

A grid reference is a unique expression comprised of two letters and two groups of numbers, each group consisting of three digits. It is used to identify a specific location, or to give a specific position, when used in conjunction with a suitable Ordnance Survey map. The type of grid reference used to specify a general location is called ‘a six figure grid reference’.

The worked example that follows uses the North sheet side of the OL28 Ordnance Survey map of the Dartmoor National Park. This map has a scale of 1:25000 making it a very popular map for walkers in the South West.

Example grid reference: SX. 664 869. The spacing between elements has been introduced for the sake of clarity only.

General information:

Notice that the map has a network of blue grid lines forming squares superimposed on it.

Observe that the start and finish of each line is numbered on the border margins and at intervals across the map. These numbers that uniquely identify each line are in the range of 0 to 99. Observe also that between adjacent lines the border that surrounds the map is subdivided into ten equal parts. On maps of this scale each square measures 40mm X 40mm which equates to 1 kilometre X1 kilometre (1000m X1000m) on the ground. It follows that each subdivision of the border between adjacent lines represents 100 metres on the ground. It further follows that providing the grid reference is expressed to the nearest subdivision the accuracy of that reference will be within 50 metres of the location.

Procedure steps:

(1) Look for the first two digits of the first group of numbers, in this case 66.

(2) Locate this number on either the top or bottom border margin of the map against a vertical grid line.

(3) This line, in common with all other vertical lines is called the easting, it forms the left hand side of the square in which the location referred to by the grid reference lays. The easting is always the first group of numbers, (forget the third digit that will be explained later).

(4) Repeat this process using the first two digits of the second group of numbers, in this case 86. Locate this number on either the left or right border margin against a horizontal grid line.

(5) This number is called the northing. It forms the bottom side of the square in which the location referred to by the grid reference lays. The northing is always the second group of numbers, (again for the moment forget the third digit).

(6) The square that contains the location that the grid reference refers to is now identified, so the process of finalising the reference can now begin. The process of finalising the reference involves using the third digit of each group to produce the accurate position of the location.

(7) Note the third digit of the first group of numbers (the eastings) in this case 4.. Measure that distance on the top or bottom border scale (the number of subdivisions) from the left hand vertical of the square in which the location lays and then transfer it to that square – then lightly pencil in a vertical line at this distance. The initial measurement of the border scale and the transfer to the square in which the locations lays may be marked on a piece of paper or card if a rule is not available

(8) Repeat this process using the third digit of the northing (the second group of numbers), in this case 9. Measure the distance on the left or right border scale from the bottom horizontal of the square in which the location lays and then transfer it to that square, once more pencilling it in lightly. The intersection of the two lines is the stated grid reference – in this case Round Pound.

(9) If the numbers of either the eastings or northings of the reference cannot be located on the map then the location is not on that map. Devon is basically covered by four maps, the prefix SX covering the south west, SY covering the south east, SS covering the north west and ST the north east, numbers repeat at 100 kilometre intervals. For full information on this subject go to: http://www.ordnancesurvey.co.uk/oswebsite/mapshop/index.html. This shows the full location details of maps covering the British Isles together with full information on the various types of map available.

(10) It is vital that a grid reference is both written and used in the correct order ie. eastings first and northings second. Failure to follow this in the event of an emergency could result in emergency services being sent to a point miles away.

If difficulty is found in remembering this a useful mnemonic to use is as follows. “Along the corridor and up the stairs” – along horizontally and then (vertically) up. Continued use of grid references will make them ‘second nature’ even to the extent of estimating the third digits of the reference. For perfect accuracy use a corner scale, the construction details of which will not be discussed here but will be available from experienced walkers.