There are no land features to divide the Celtic Sea from the open Atlantic Ocean to the south and west. For these limits, Holt suggested the marine
contour and the island of
Ushant off the tip of Brittany. The definition approved in 1974 by the UK
Hydrographer of the Navy for use in
British Admiralty Charts was "bounded roughly by lines joining Ushant,
Land's End,
Hartland Point,
Lundy Island,
St. Govan's Head and
Rosslare, thence following the Irish coast south to
Mizen Head and then along the 200-metre
isobath to approximately the latitude of Ushant." The
International Hydrographic Organization defines the limits of the Celtic Sea as follows:
On the North. The Southern limit of the
Irish Sea [a line joining
St David's Head to
Carnsore Point], the South coast of Ireland, thence from Mizen Head a line drawn to a position .
On the West and South. A line from the position South to
49°N, thence to latitude 46°30'N on the Western limit of the
Bay of Biscay [a line joining
Cape Ortegal to
Penmarch Point], thence along that line to Penmarch Point.
On the East. The Western limit of the English Channel [a line joining
Île Vierge to Land's End] and the Western limit of the Bristol Channel [a line joining Hartland Point to St. Govan's Head]. ==See also==