Grid squareA square of the National Grid as used on Ordnance Survey maps. On these maps 100 kilometre squares are designated by the use of two letters, 10 kilometre squares by a combination of two letters and two numbers, for example SK14, and one kilometre squares by a combination of two letters and four numbers, for example SK 1845. By quoting a six-figure reference, for example SK183457, the position of a point anywhere in Britain can be given to the nearest 100 metres. The grid square is being increasingly used in bird distribution surveys, rather than an administrative unit like a county or a vice-county, whose boundaries are subject to change. In national surveys the 10 kilometre (10 x 10 km) squares are used, as in The Atlas of Breeding Birds in Britain and Ireland (compiled by the British Trust For Ornithology and the Irish Wildbird Conservancy), while local surveys often use the 2 kilometre (2 x 2 km) squares ('tetrads').
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver