Hill Skill Series – Understanding grid references

A grid reference is a series of letters and numbers that defines a unique square on a map, the more digits used the greater the accuracy and the smaller the square. Every country has its own unique grid, the lines are aligned north-south and east-west, forming a series of squares.

Irish grid system
Fig. 1 – The Irish grid system.

In Ireland the grid is divided into squares 100 kilometres x 100 kilometres (1 kilometre is a thousand meters).

There is a datum point set off the south west coast, which defines the 0 point, and each 100 km square is measured from here. The Irish grid is 500 km x 500 km and gives 25 squares in total.

Each square is represented by a one letter code, with the exception of I, which could be mistaken for a 1. See Fig. 1.

These 100 km squares are then subdivided further into smaller squares, each one being 1 kilometre across.

These 1 km squares are depicted on maps as blue numbered lines running north-south and east-west respectively. They are individually labelled using the ascending numbers 00, 01, 02, 03 etc… all the way up to 99.

Fig. 2 - 1 kilometre square on a 1:50 000 scale map, in this case the square 79 09.
Fig. 2 – 1 kilometre square on a 1:50 000 scale map, in this case the square 79 09.

The numbers along the bottom of the map, which increase towards the east are called Eastings , those numbers that are running up the side of the map and increase towards the north are called Northings.

When writing down a grid reference we first quote the Eastings then the Northings.

This can be more easily remembered by the saying along the hall and up the stairs’.

We can define a given 1km square by first giving the 100km square box letter and then the 2 numbers for the Eastings followed by the 2 numbers for the Northings.

In the example in Fig. 2 the highlighted square is in the Slieve Mish mountains of Dingle, in the 100km grid square Q (see Fig. 1), the Eastings are 79 and the northings 09. It is written Q 79 09 and is known as a 4 figure grid reference.

The 1 km box can be further subdivided into one hundred 100 metre x 100 metres squares, ( these squares are not shown on the map).

This now allows us to define an area of land 100 meters square, (see red box in Figure 3), and is called a six figure grid reference.

Fig. 3 - Dividing a 1 km grid square into smaller 100 metre squares.
Fig. 3 – Dividing a 1 km grid square into smaller 100 metre squares.

The extra numbers needed are not shown on the map and must either be estimated or obtained more accurately using a Romer, (found in the top right hand corner of your compass).

 The corner of the Romer is placed on the point to be identified and the numbers are read off where the Romer intercepts with the grid.

In the example in Fig. 4 the red dot has an easting of 5 and a northing of 7.

Grid references can be further used to accurately define a point on a map down to a 10 metre square, these are eight figure grid references.

Fig. 4 - Use the Romer scale on your compass to accurately read the grid reference.
Fig. 4 – Use the Romer scale on your compass to accurately read the grid reference.

A grid reference is important information, it allows you to inform others of where you are, (for example Mountain Rescue), and also allows you to locate features or a position on a map when given to you by someone else.

This article was written by Russ Mills of Mountaintrails, who provide navigation training and Mountain Skills courses in Ireland.

More information on our navigation courses can be found here: Mountaintrails Navigation Courses.

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Russ Mills

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