Whenever you give a grid reference you should always include this prefix - sending in a sighting of a flock of Tree Sparrows at '7233' means we cannot determine whether they were in Norfolk, on the Isle of Mull or on Snowdonia. Which 100-km square?īy referring to the map's key you can find a diagram showing which 100-kms are shown where on the map sheet - on some maps the alphabetic prefix is also printed in the corners of the map and wherever the 100-km changes (e.g. Note that, because the grid is square and is imposed on our round earth, the grid is not necessarily parallel to lines of latitude or longitude. Figure 1 shows the National grid with 100-km squares labelled with the alphabetic prefix. In Britain these are given a two-letter alphabetic reference whereas in Ireland they are given a single letter reference (rarely given as IB, IC etc). Britain and Ireland are split into 100km by 100km squares (termed 100-km squares). Grid referencesĪ complete grid reference consists of three parts - an alphabetic prefix, followed by numeric eastings and then numeric northings. This grid provided the first truly national map referencing system by which any place in the country could be given a unique reference. In 1947 the first map was published bearing the National Grid on its cover. Prior to the National Grid many counties had different projections and matching maps at county boundaries was notoriously difficult. So it was that the Ordnance Survey began, though we would not recognise it until following the second World War when the recommendations of the Davidson Committee brought about the metric National Grid. This lead to the production of the first Ordnance Survey map in 1801 of Kent. In 1799 Ordnance Survey rose to the challenge of invasion from France and the entire OS staff were sent to the English south coast to survey the counties thought at most risk. In 1791 the Ordnance Survey was founded as Britain's national mapping organisation, mostly to produce maps for military purposes. Here Simon Gillings clarifies how to give complete grid references. Virtually all BTO surveys now rely heavily on the use of Ordnance Survey grid references yet to the uninitiated they can be confusing.
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