> For the complete documentation index, see [llms.txt](https://docs.os.uk/os-downloads/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.os.uk/os-downloads/products/maps-and-imagery-portfolio/os-mastermap-topography-layer/os-mastermap-topography-layer-technical-specification/geometric-data-types.md). # Geometric data types All feature types are presented in British National Grid (BNG) as one of the four data types specified below: ## GM\_Point (Point) A point is used to specify a single x,y location by a coordinate pair in a given spatial reference system. ### **Example** A point defined in the BNG reference system has easting and northing ordinates in units of metres, where the easting is in the range of 0 to 700000 and the northing is in the range of 0 to 1300000. Coordinates are output to millimetre precision and are output in the data as float64 real types.
An example of a point data type.

An example of a point data type.

#### **Example class model**
An example class model for a point data type.

An example class model for a point data type.

## GM\_Curve (Polyline) A polyline is an ordered set of points that are interpolated linearly. A polyline may not intersect itself or contain repeated points. ### **Example**
An example of a GM Curve (polyline) data type.

An example of a GM Curve (polyline) data type.

**Example class model**
An example class model for a GM Curve (polyline) data type.

An example class model for a GM Curve (polyline) data type.

## GM\_MultiCurve (Multiline) A multiline is a collection of polyline geometries. There are no specific semantics or rules applied to the collection. ### **Example**
An example of a GM MultiCurve (multiline) data type.

An example of a GM MultiCurve (multiline) data type.

**Example class model**
An example class model for a GM MultiCurve (multiline) data type.

An example class model for a GM MultiCurve (multiline) data type.

## GM\_Surface (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. ### **Example**
An example of a GM Surface (polygon) data type.

An example of a GM Surface (polygon) data type.

**Example class model**
An example class model for a GM Surface (polygon) data type.

An example class model for a GM Surface (polygon) data type.

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