An Irish Grid reference is the description of a location's position within the Irish Grid coordinate system. An understanding of how Irish Grid references work allows you to work out two-dimensional positional information for any location marked on a map which is based on Irish Grid. Working backwards, this skill also allows you to identify a location from a supplied Irish Grid reference.
In today's world of GPS, it is all too easy to forget about fundamental navigation skills like using Irish Grid references. Even if you are a die-hard GPS fan, we encourage you to take out your paper map every once in a while for some practice. This can be undertaken at your desk or at the kitchen table. If you are unsure of the process, follow our step-by-step guide below.
You can see the positions of Irish Grid's twenty-five lettered 100km x 100km squares (black lines) and how they relate to local lines of longitude (blue lines) in the interactive map below. Feel free to use this to identify which square contains the location you are interested in.
In today's world of GPS, it is all too easy to forget about fundamental navigation skills like using Irish Grid references. Even if you are a die-hard GPS fan, we encourage you to take out your paper map every once in a while for some practice. This can be undertaken at your desk or at the kitchen table. If you are unsure of the process, follow our step-by-step guide below.
You can see the positions of Irish Grid's twenty-five lettered 100km x 100km squares (black lines) and how they relate to local lines of longitude (blue lines) in the interactive map below. Feel free to use this to identify which square contains the location you are interested in.
Step-by-step guide
Follow these six simple steps to find the Irish Grid reference for any location in Ireland using a medium scale map. We demonstrate the process using Rocky Mountain summit (the one overlooking Annalong Valley in the Mourne Mountains) as an example.
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Step 1
Identify which lettered 100km x 100km Irish Grid square covers the location you are interested in. You can do this using the interactive map above. The printed map you are using should tell you the relevant lettered square(s) for that sheet. In this example, Rocky Mountain is located in grid square J (see Fig.1). It is worth mentioning that the centre of grid square J has an Irish Grid reference of 3500,3500 to an accuracy of 100m, where the first 3500 means 350km+0m east of Irish Grid's origin and the second 3500 means 350km+0m north of Irish Grid's origin. It is exactly the same as reading from those algebraic graphs in maths class. It is also worth mentioning that each 100km x 100km Irish Grid square can function as a standalone mini-grid coordinate system with its own origin at its southwest corner. If using this method, you must quote the letter at the beginning of each Irish Grid reference and omit the first digit of the easting and the first digit of the northing. As such, the centre point of grid square J to an accuracy of 100m would be written as J500500 where the first 500 means 50km+0m east of J's origin and the second 500 means 50km+0m north of J's origin. |
Fig.1
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Step 2
Find the location you are interested in on a relevant printed map which covers that location. In this example, Rocky Mountain is covered by "The Mournes" from Ordnance Survey of Northern Ireland's 1:25,000 Activity Map series (see Fig.2). If you need help deciding which map to use, we have plotted the coverage area of every medium-scale map for Ireland on a handy interactive map. Click here to access it... |
Fig.2
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Step 3
Using the paper map, identify which lettered 1km x 1km Irish Grid square covers the location you are interested in. In this example, Rocky Mountain is located in J's 1km x 1km grid square which is bounded by the easting lines labelled 35 and 36, and by the northing lines labelled 25 and 26 (see Fig.3). These numbers tell you that Rocky Mountain's summit is between 35km and 36 km east of J's origin, and between 25km and 26 km north of J's origin. To enhance the level of precision, this 1km x 1km grid square should now be treated as a micro-grid coordinate system with its own origin at its southwest corner. In this case, the origin is 3350,3250 or J350250 to an accuracy of 100m. |
Fig.3
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Step 4
Determine your chosen location's easting to your required level of precision by measuring eastward from the 1km x 1km grid square's west edge. On a 1:25,000 scale map, every 4mm represents 100m. On a 1:50,000 scale map, every 2mm represents 100m. Most maps have a scalebar printed somewhere around the margin to assist with this. Some compasses also have popular scales etched onto their baseplates for measuring distances between 1km-spaced grid lines on maps. Fig.4 shows a series of vertical green lines within the 1km x 1km grid square at 100m intervals (in this example, 4mm intervals on a 1:25,000 map). Rocky Mountain's summit can be seen in the first interval, meaning it is less than 100m east of the origin. By eye, it appears to be about halfway to the first 100m, making 50m an acceptable estimate. With its easting component underlined and in bold, the origin of this 1km x 1km grid square was found to be 3350,3250 or J350250 (see Step 3). As we move 50m east from the origin, the easting component for Rocky Mountain summit becomes 335050 (or 35050 if the J is in front). Notice that the 0 remains as 0 because the first 100m interval was not fully crossed, and that the 50m was added on at the end. |
Fig.4
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Step 5
Determine your chosen location's northing to your required level of precision by measuring northward from the 1km x 1km grid square's south edge. Fig.5 shows a series of horizontal green lines within the 1km x 1km grid square at 100m intervals (in this example, 4mm intervals on a 1:25,000 map). Rocky Mountain's summit can be seen in the third interval, meaning it is less than 300m north of the origin. By eye, it appears to be about halfway between 200m and 300m, making 250m an acceptable estimate. With its northing component underlined and in bold, the origin of this 1km x 1km grid square was found to be 3350,3250 or J350250 (see step 3). As we move 350m north from the origin, the northing component for Rocky Mountain summit becomes 325250 (or 25250 if the J is in front). Notice that the 0 becomes 2 because the first and second 100m intervals were both fully crossed, and that the 50m was added on at the end. |
Fig.5
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Step 6
That's it! All you have to do is put the components together and you have the Irish Grid reference for your chosen location to an accuracy of about 25m. In this Rocky Mountain example, the Irish Grid reference can be written in several different ways, including the following:
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Fig.6
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Slideshow
