Grid Reference Guide
Northern Ireland and the Republic of Ireland use the same grid reference system. It is independent from the system used by Britain, but is based on the same principles. The island of Ireland is covered by a grid of 100 x 100 km squares, each denoted by a single-letter code for example, J. In Northern Ireland, the letters are C, D, G, H and J.
These squares are further divided into hectads of 10 x 10 km, and the hectads are further divided into 1 km squares. Irish grid references consist of one letter (representing the 100 x 100 km square) and two to ten numbers (representing squares from 10 x 10 km down to as small as 1 x 1m in size). A grid reference refers to the bottom left hand corner of the square which contains the record, not the actual site of the record.
In some computer systems, there is a requirement to enter a two letter code at the start of the grid reference, as the system in Britain uses two letters. In this situation, the letter I (for Ireland) is inserted before the one letter used in the Irish grid system for example, IJ 35 75.
The one letter code on the Ordnance Survey (OS) map you use forms the first part of your grid reference. This letter is found in the corners of the map and, if your map covers more than one square, they will also be printed where one square joins the next on the map. If in doubt, check the legend which will show you a small scale diagram of the squares covered.
To work out a grid reference for a site, you need to look at the grid lines on the OS map. The Eastings are the numbers running (increasing) from left to right along the top and bottom edges of the map and the Northings are the numbers running from bottom to top (increasing) at each side of the map.
To determine a four-figure grid reference (for a 1km x 1km square), first find the square in which your record was made. Keeping your finger on that square, read the Eastings by running your eye from left to right along the bottom, or top, of the map until you come to a line which, if followed up or down towards the site of your record, represents the left-hand side of the square in which your record was made. The two numbers of that line form the first numerical part of your four-figure grid reference.
Next, run your eye up the left-hand, or right-hand, side of the map (Northings) until you reach the line which forms the bottom of the square in which your record was made. The two numbers of that line form the last two numbers of the four-figure grid reference.
The Eastings must come before the Northings in a grid reference. This rule is easily remembered by thinking of it as going along a corridor and then up the stairs.
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In the example to the left, the whole square is the hectad J 37 (10km x 10km). A four-figure grid reference for x would be J 35 75 (1km x 1km). The map margins on the 1:25,000 Ordnance Survey maps are further calibrated into 100-metre divisions, which enable you to imagine the square with the x in it split into 100 smaller squares. Below is an enlargement of the square J 35 75. To give a more detailed six-figure grid reference, you need to insert a number in the middle of the four-figure grid reference and another one at the end, i.e. J 35? 75?. If you imagine the lines are numbered in the same way as for the hectad, you can count the lines along the bottom of the square until you reach the line forming the left-hand side of the smaller square containing the x and you come to the number 7. Count the lines up the left-hand side of the square until you come to the line that forms the bottom of the square where the x is and you come to the number 8. A six-figure grid reference for x would therefore be J 357 758.
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Tetrads are a widespread scale for mapping species. This is an area of 2 x 2 km – a block of four 1km squares. However, it is not possible to use tetrad data to generate 1km scale distribution maps, but it is possible to produce tetrad maps from Vertebrate Officer 1km square or better information. For this reason, it is usual for all records to be submitted to recording schemes on a 1km square or higher scale.
