In the OS NGD Boundaries Collection, the mean high water (springs) mark is shown to the normal tidal limit (NTL). Areas of salt marsh, mudflats and so on often have extremely complicated tide lines. For ease of digitising and to reduce data volumes, mean high water may be generalised in these areas (not Scotland). The mean high water (springs) mark is represented in the OS NGD Boundaries Collection by a polyline feature. The mean high water (springs) mark is subject to continuous change, but the captured alignment of a tide line is a snapshot on one day. It is not practical to revise tide lines frequently.
