Geometry

This section defines the geometric data types used by features of the OS MasterMap Sites Layer. A UML diagram is used to support the data type descriptions.

Features and spatial data types

The following table details the data type of the geometric attributes of OS MasterMap Site Layer features. Each feature type has a spatial attribute called ‘geometry’. The data type of this attribute is given in the third column of the table.

Feature type	Spatial attribute	Data type of geometry attribute	For details, see:
functionalSite	geometry	Polygon	Functional site extent overview
accessPoint	geometry	Point	access point overview
routingPoint	geometry	Point	Routing point overview

Geometric data types

Point

A point is used to specify a single horizontal location by a coordinate pair in a given spatial reference system.

Example

A point defined in the BNG reference system has easting and northing coordinates in units of metres, where the easting is in the range 0 to 700000 and the northing is in the range 0 to 1300000.

Point defined in the BNG reference system

Example class model for point (from BNG).

Example class model

Polygon

A polygon is a single closed region on the spatial reference system projection plane, defined by a set of geometric rings that represent the boundaries. A polygon has one outer boundary and zero or more inner boundaries (holes in the polygon). The inner boundaries must not cross each other or contain other inner boundaries. Coordinates in outer boundaries are oriented in an anticlockwise direction; coordinates in inner boundaries are oriented in a clockwise direction. In the Geography Markup Language (GML) data, these are implemented as a Surface containing a single Polygon Patch, with the exterior and any interior boundaries represented as a LinearRing.

Example

Polygon defined in the BNG reference system.

Example class model

Example class model for polygon in BNG.

Multi-polygon

A multi-polygon is used where a single functional site consists of separate areas, such as a school on both sides of a road. Each polygon is as described above. In GML, this is represented with a gml:MultiSurface having a number of ‘surface members’ each of which is a single gml:Surface.