The external bounding line of the Boundary-Line dataset is the extent of the realm (EOR). This means the boundary extent is digitised on the alignment of the mean low water (springs) (MLWS) for the seaward extent. Boundary-Line does not contain a separate mean low water (springs) (MLWS), only CODE 0071 representing the mean high water (MHW).
To display the EOR, both MHW and another type of boundary (for example, European electoral region or Westminster constituency) need to be displayed together.
The Territorial Waters Jurisdiction Act 1878 and the Territorial Waters Order in Council 1964 confirm that the EOR of Great Britain as used by Ordnance Survey is properly shown to the limit of mean low water for the time being, except where extended by Parliament.
This means that the EOR will be:
The mean low water mark (mean low water springs in Scotland) except when landward of a seaward extension or as shown in Figure 5a and indicated as Point B.
The mean high water mark (mean high water springs in Scotland), where coincident with mean low water or mean low water springs, but not when landward of a seaward extension or Point B.
Point B
Seaward extensions
In the figure below, the EOR follows mean low water (springs), crossing the estuaries at LWL.
The figure below shows how these features are represented in Boundary-Line.
The foreshore is taken to be the area of land between mean high water (springs) mark and the EOR which will include:
Tidal water within a seaward extension.
Tidal water above Point B, to the normal tidal limit (NTL), together with the area between mean high water (springs) mark and the edge of the channel at mean low water (springs).
Mean high water (springs) mark is represented in Boundary-Line by a polyline feature with CODE 0071. Mean low water (springs) mark is not shown, although it is generally coincident with the EOR.
The area of non-inland is referenced within each administrative unit using the NA attribute.
The following rules govern the representation in Boundary-Line of offshore islands and rocks in the sea:
Offshore rocks and islands will be shown if 0.4 hectares or more in area at high tide.
Offshore islands with buildings will always be shown, irrespective of size.
Offshore islands or rocks will not be shown if they are:
Beyond the defined EOR, irrespective of size.
Covered at high tide.
If an offshore island or rock is shown, it will be contained within all the relevant administrative unit collections.
The mean high water (springs) mark is shown to NTL.
Areas of salt marsh, mudflats and so on often have very complicated tide lines. For ease of digitising and to reduce data volumes, mean high water may be generalised in these areas (not Scotland).
Mean high water (springs) mark is represented in Boundary-Line by a polyline feature with CODE 0071.
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 very frequently.
Mean low water (springs) mark is shown as the EOR, except when contained within a seaward extension, when it is then omitted.
Banks of sand, mud, shingle or rock separated from the main line of mean low water (springs), and covered at high tide, are not shown. When there are large areas of these banks separated from the main foreshore by narrow channels, the channels are ignored and the whole bounded by mean low water (springs).
The extension of the line of mean low water (springs) into a channel above Point B becomes edge of channel and is not shown.
Mean low water (springs) mark, and hence EOR 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 very frequently.
The normal tidal limit (NTL) is the point at which the level of a river or stream ceases to be affected by the tidal flow. This point is often an artificial barrier such as a lock or weir.
A pier under which water flows is not normally considered to be within the realm. There are some cases, however, where a structure has specifically been included within the realm by act or order, in which case mean high water (springs) mark and EOR are shown around the limits of the structure.
This is the intangible line across a channel where the level of the river meets the level of the sea at low water.
Within large channels or estuaries, Point B is defined on the source documents for Boundary-Line and is therefore shown as the EOR.
In small channels, Point B is not shown on the source documents, so is assumed to be along the general line of low water and is shown as the EOR.
These are artificial extensions to the realm that have been made by an Act of Parliament. They extend the local government and parliamentary areas seaward of mean low water (springs) mark.
Breakwaters are structures of wood, stone, metal or other material built to break the force of waves. They can be separate from or joined to the mainland. If joined, they are generally included in the local government and parliamentary areas. If separate, they are not part of the local government and parliamentary area unless they are placed therein by legislation of some kind; for example, Plymouth breakwater is included in the local government area, whilst the centre portion of the Portland Harbour breakwater is not.
Permanent or solid structures in the sea, such as the forts in the Solent off Portsmouth Harbour and Brighton Marina, are usually included in local government and parliamentary areas.
