User Guide - Digimap

Figure 17 shows a sample of real-world objects in the water theme, including a pond, a river, flow arrows,
sluices and drains.
Figure 17: Examples of features within the water theme
The non-physical features shown in the water theme are:
• the highest point in a river to which normal tides flow is described as normal tidal limit (NTL). The point is
shown and annotated by text;
• low water level (LWL) is the point to which mean tides (or mean spring tides in Scotland) flow at low
water. The point is shown and annotated by text;
• the text descriptions of all water features; and
• flow arrows, which are symbols used to indicate the direction of flow of non-tidal moving water.
As water is a dynamic element within the landscape, certain survey principles and constraints are imposed
on the representation of water within OS MasterMap Topography Layer.
Rivers, streams and drains are shown at their true scale width. A single line is used where their width is less
than:
• 1.0 m urban areas; and
• 2.0 m rural, and mountain and moorland areas.
OS MasterMap Topography Layer does not contain polygons of the open sea. Where inland water bodies
meet the sea the following principles are applied:
• Ordnance Survey shows high and low water marks of a mean average tide, that is an average tide
halfway between spring and neap tides in England and Wales, and of average spring tides in Scotland.
• In tidal rivers the point to which mean tides (or spring tides in Scotland) flow at high or low water is
included.
Lakes and ponds are surveyed at normal winter level; reservoirs are shown at top water level, that is
spill-over level. All water features are described. Continuous topographical water features that extend into
private gardens are shown. Where a river flows under another object, typically a bridge, the part of the river
beneath the object is not supplied. This is why there are gaps in rivers when the theme is viewed on its own.
This is shown in figures 18 and 19 below.
OS MasterMap Topography Layer user guide chapter 4 v1.3 – 07/2007 © Crown copyright Page 36 of 140