Read full details on how to read grid references. TF73G - identifies tetrad G within 10-km square TF73 TF722336 - identifies a point to the nearest 100 m H64 - identifies a 10-km square in Ireland (occasionally written as IH64) TF73 - identifies a 10-km square in Britain All British grid references should begin with two letters, those in Ireland starting with one letter, and followed by a series of numbers (and/or the tetrad code). A correct grid reference does not include the map sheet number. Reading grid referencesĬorrectly reading grid references is crucial for many BTO surveys. The remaining areas of both the OV myriad and the O pentad lie in. It is also the only land representative of the much larger 500km grid square (or pentad) which is known by its prefix letter O. A transparent overlay for the Landranger maps showing a 10-km square and tetrads was available on request during the Atlas survey period. OV0000 at Beast Cliff in North Yorkshire is the sole land representative of the OV 100km national grid square or myriad. In Ireland the Ordnance Survey of Ireland Discovery Series (1:50,000) maps are most useful. Ordnance Survey Landranger maps (1:50,000) or Ordnance Survey Explorer maps (1:25,000) are perfect for this purpose. The city of Savannah, Province of Georgia, was laid out in 1733, in what was colonial America, around four open squares, each surrounded by four residential ('tything') blocks and four civic ('trust') blocks.The layout of a square and eight surrounding blocks was known as a 'ward. Part of the fun of Atlas projects is poring over maps, deciding on the best route to take you past this pond, or that promising-looking wood. We strongly recommended you got an up-to-date map of the area you proposed to survey. Read more detail on giving complete grid references. A tetrad is then given its full reference as the 10-km square code followed by the tetrad letter (e.g. There are 25 tetrads within each 10-km square and these are labelled A to Z, excluding O (to avoid confusion with zero), from bottom left (SW) to top right (NE). A tetrad (from the Greek tetras meaning four) on the other hand is a collection of four 1-km squares arranged into a 2 km by 2 km square. A 10-km square as its name suggests is a square 10 km by 10 km. There are various scales of resolution, but for the Atlas we worked with 10-km squares and tetrads. Like many bird surveys the Atlas used the Ordnance Survey National Grid as a means of identifying locations of bird records.
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