Text and photographs copyright of Jim Shead.
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HistoryGreat Britain is probably the best mapped part of the world and nearly all our maps are based on those of the Ordnance Survey (OS). In 1791 the Board of Ordnance set up a body to survey the land and produce maps for military use. When the Napoleonic Wars ended in 1815 these one inch to the mile maps soon found a use in the development of Victorian Britain, with its increasing civil engineering, housing, and drainage needs. Over the years the range of maps and the information on them was greatly improved and in 1945 the OS National Grid was introduced giving a map referencing system to the whole country.
Ordnance Survey Maps TodayAlthough OS produce a large range of general and specialist maps, ranging from a large scale 1:1,250 (or 50 inches to a mile) maps to the Routeplanner of Great Britain 1:625,000 (or 1 inch to ten miles) map, it is probably the 1: 50,000 scale (or 11/4 inches to a mile) Landranger Maps that most people would identify as OS maps. These detailed maps are ideal for walking or exploring an area by boat, bicycle or car.
The OS National GridThis system splits the country into 100 kilometre squares each of which is given a two letter Identifier, for example ST is the square containing Bristol, Taunton and Weymouth. The Landranger Maps are divided into kilometre squares, which are numbered from 00 to 99 in each axis within the 100 kilometre grid reference. OS grid references can be given to any degree of accuracy by extending the number of digits used but for most purposes a two letters and six digit reference is used.
Giving a Grid ReferenceFirst quote the letters for the 100 kilometre square in which the point is situated e.g. ST for a point in Bath. Next quote the Eastings, that is the numbers running from west to east across the bottom of the map. Take the two digits printed on the line plus an extra digit to represent the tenths of a square in which the point is situated, e.g. Bath Bottom Lock is to the right of grid line 75 but not quite half way across so we estimate the reference as 754. We then go through a similar process with the Northings, that is the numbers running from south to north up the side of the map. For Bath Bottom Lock this gives us a reference of 643. Putting these three elements together gives us an OS grid reference of ST754643.
OS Map Numbers and Grid ReferencesOn the waterway tables of distances I have (in most cases) quoted the grid reference of each point and the Landranger Map number on which the location can be found.