High Water Mark
In England and Wales, this is the mean level of all the high tides; in Scotland this is the mean level of the spring high tides. In places where there is no Foreshore (e.g. vertical cliffs), the TidalBoundary is classified as the High Water Mark.
Low Water Mark
In England and Wales, this is the mean level of all the low tides; in Scotland, this is the mean level of the spring low tides. When there is Foreshore, this defines the lower limit of the Foreshore.