Data provenance

Data creation

Data was created from Ordnance Survey large-scale data with additional input from recognised bodies.

In OS MasterMap Topography Layer, watercourses with less than specified widths are captured as single line features for cartographic clarity purposes. These dimensions are 1m in urban areas, 2m in rural areas and 5m in mountain and moorland areas.

For watercourses captured as area features, algorithms were used to create a network, including generating topological connections if appropriate. Additional connecting geometry has been added to ensure topological connectivity where this can be deduced by inspection.

Watercourses represented by line features were used to generate network lines and these were extended both automatically and manually to indicate connectivity.

Manual improvement was undertaken where validation identified issues with flow and connectivity, and as a result of customer feedback.

Derived attributes

This section describes how attributes are derived from other information contained within Ordnance Survey large-scale data.

Width attribute

The width attribute is calculated to provide an average width of the Topographic Area feature(s) used to create the network links. There may be significant variation along the portion of the watercourse represented by a WatercourseLink with a single width attribute.

Topographic line and area features

The majority of links in the data were sourced from Ordnance Survey detailed topographic data. Different depictions within Ordnance Survey detailed topographic data are adopted for cartographic clarity purposes, and watercourses may be represented as lines or areas. Watercourses with less than specified widths are captured as single line features, while watercourses with greater than specified widths are captured as area features.

Parameters used by OS for topographic line and area features on watercourses:

Locality	Width	Topographic Representation	Width Attribute
Urban	<1.0m	line	null
Urban	+>1.0m	area	calculated
Rural	<2.0m	line	null
Rural	+>2.0m	area	calculated
Mountain and moorland	<5.0m	line	null
Mountain and moorland	+>5.0m	area	calculated

Advances in data capture techniques mean that the cartographic constraints on capture as area features are now less pronounced. As a result, some features below the minimum widths in the table above may have been captured as areas.

LevelOfDetail

This is a calculated value indicating the relative cartographic importance of a WatercourseLink to aid selection and depiction when styling the data. Currently, this has been populated with a single value pending the development of algorithms to create more usable attribution.

Primacy attribute

The Primacy attribute is derived by assessing a combination of the flow, width, gradient and length of the network WatercourseLink feature to identify the most likely course of the main flow of a watercourse at bifurcations.

Gradient attribute

Gradient is a calculated attribute that indicates the rate of descent of the surface of the water over the entire length of the WatercourseLink.

Gradient is expressed as a percentage or drop in metres per 100metres of WatercourseLink length.

Gradient = (Height on first vertex – Height on last vertex * 100) / Planar length of link

For more detailed analysis of the WatercourseLink, height is provided on each vertex that can allow a user to calculate gradients within sections of the WatercourseLink.

Heighting the network

The water network was heighted by interpolation from a bare earth terrain comprising mass points and breaklines. The highest resolution height information available was used in this process. This terrain was captured by Ordnance Survey recently and is the basis for new products that include a height component.

The data used to height the network is of a resolution and quality that in some cases results in height attribution that contradicts the flow attribution. This is typically in areas with little change in elevation. In these cases, the calculated gradient attribute will not be supplied.

Future developments will reduce occurrences by improving the terrain data to reflect the influence of watercourses on the terrain surface.