Information about Grid References.

Brief Introduction
To be useful maps generally have to provide a means of identifying places exactly.  For most small scale maps such as 1:50,000 scale this is done by printing a grid of lines representing 1 km squares over the map.  
All small scale maps produced in the last 50 years in Ireland have used the national grid, even the old 1 inch to a mile and the city street maps.  For many practical purposes the 1:50,000 map is the most useful.  Every map includes instructions for specifying the grid reference for a place.
Ireland is covered by 89 1:50,000 maps.  71 are published in the Republic and 18 in the North.  The good news is that the same grid system is used for all of Ireland.  A different grid system is used in Britain.
In the national grid, Ireland is described as a square 500 km by 500 km.  This is further divided into 25 100km squares, each of which are given a letter.


Picture from Peter Dana.

A grid reference in Ireland looks like this:  O 285 376 for a point on Howth in County Dublin.  The initial letter, "O" in the example above, indicates the square.  The two following numbers are the "easting" and the "northing".  Within the O square there are 100 times 100 kilometre squares.  The "285" indicates the 28th column of kilometre squares, the 5 means halfway across the square.  The "376" indicates the 37th row of kilometre squares in "O".  The 6 indicates 6/10 up that square.  More information about this is included in textbooks and on any 1:50,000 Ordnance Survey map. 
A simple grid reference therefore can resolve down to 10 points in each dimension within a kilometre square that is to 100m which for many purposes is fine.  It means that any place in Ireland can be represented by just 6 digits and a letter.  All that is required to do describe a position of a place is a map (well .. assuming that you know where the place is).

Coordinate systems in various countries, including Ireland. 
Irish Ordnance Survey
Northern Ireland Ordnance Survey


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