§ Mr. FearnTo ask the Secretary of State for the Environment, Transport and the Regions if he will make a statement on the accuracy of the information available from the Ordnance Survey on the mean high water springs tide-contour around Britain's coastline. [144942]
§ Ms Beverley HughesOrdnance Survey tide lines have been surveyed at the basic scale of survey in the region and may have been surveyed at 1:1250, 1:2500 or 1:10000 scale using photogrammetric or ground survey methods. Interpolation between tide gauges around the coast is used to obtain the tidal heights and/or tide times depending on survey method along intervening stretches of coastline.
In England and Wales, mean high water is the high water mark of an ordinary or average tide. In Scotland, mean high water springs is used and is the high water mark of an average spring tide. Mean high water heights for example can vary along the coast from a minimum of 0.09 meters to a maximum of 5.62 meters above Newlyn Datum. Ordnance Survey uses the Admiralty Tide Tables as the source of tidal height.
For an area with 5 meter contour intervals. The vertical accuracy of the tide line equates to ±1.25m. The horizontal accuracy of the tide line depends upon the slope of the coastline.