GML Feature Types

The model is ‘feature-based’ so that ContourLine, SpotHeight and LandWaterBoundary are feature types with specific attributes.

The feature types within the contours are represented as individual features to enable easier interpretation. For example, the contours can be drawn as different colours to highlight the index contours, the high- water mark, and spot heights over a certain value.

The column names have been reformatted to comply with Esri software.

ContourLine feature type

Column name	Type	Description
+geometry	GM_LineString	The structure of the feature
+propertyValue	Distance	The length of the contour
+contourLineType	String	master ordinary auxiliary

The ContourLine sub-type value names align with INSPIRE draft elevation specification. The terms 'master' and 'ordinary' represent the more traditionally recognised terms of 'index' and 'standard' (contours), respectively.

SpotHeight feature type

Column name	Type	Description
+geometry	GM_Point	The structure of the feature
+propertyValue	Distance	The z value (height above Newlyn, or other British height datum) of the feature
+spotHeightType	String	formSpot generic mountainPass summit

At launch, only the sub-type value 'generic' has been used, but the other values provide functionality to enrich the attribution if required in a later product release.

LandWaterBoundary feature type

Column name	Type	Description
+geometry	GM_LineString	The structure of the feature
+propertyValue	Distance	The length of the boundary line
+waterLevelCategory	String	meanHighWater meanLowWater meanHighWaterSprings meanLowWaterSprings

Mean High and Low Waters apply to tidal waters in England and Wales, whereas the Mean High and Low Water Springs apply to those in Scotland.

Example of the ContourLine feature type

<os:member>
    <os:ContourLine gml:id="os.t50.sx98.691">
        <os:geometry>
            <gml:LineString srsName="urn:ogc:def:crs:EPSG::27700"
gml:id="os.t50.sx98.691.geom">
                <gml:posList>300000 87424.1 299997 87432.8 299995.9 87445.7 299997.1
87459.6 300000 87473.8</gml:posList>
            </gml:LineString>
        </os:geometry>
        <os:propertyValue uom="m">40</os:propertyValue>
        <os:contourLineType>ordinary</os:contourLineType>
        </os:ContourLine>
</os:member>

Example of the SpotHeight feature type

<os:member>
    <os:SpotHeight gml:id="os.t50.sx98.5">
        <os:geometry>
        <gml:Point srsName="urn:ogc:def:crs:EPSG::27700"
gml:id="os.t50.sx98.5.geom">
            <gml:pos>298939.2 88163.4</gml:pos>
        </gml:Point>
        </os:geometry>
    <os:propertyValue uom="m">53</os:propertyValue>
    <os:spotHeightType>generic</os:spotHeightType>
    </os:SpotHeight>
</os:member>

Example of the LandWaterBoundary feature type

<os:member>
    <os:LandWaterBoundary gml:id="os.t50.sx98.138">
        <os:geometry>
        <gml:LineString srsName="urn:ogc:def:crs:EPSG::27700" 
gml:id="os.t50.sx98.138.geom">
            <gml:posList>298703.2 80000 298695.7 80002.8 298681.6 80002.1
298662.7 80000</gml:posList>
                </gml:LineString>
        </os:geometry>
        <os:propertyValue uom="m">-0.51</os:propertyValue>
        <os:waterLevelCategory>meanLowWater</os:waterLevelCategory>
   </os:LandWaterBoundary>
</os:member>

Unique identifiers

GML 3.2.1 requires features and their geometries to have unique identifiers. For OS Terrain products, the feature identifiers have been structured as follows: os.t50.<tile name>.<sequential number>, where the second part abbreviates the product name, i.e. t50 for OS Terrain 50 and t5 for OS Terrain 5. Geometry identifiers in the GML use the same form, but with a .geom suffix.

Therefore, for a given release of the product, every feature and geometry is guaranteed to have a unique identifier. The OS Terrain products will both be updated by whole tile refresh, and there are no plans to supply feature-based change-only updates (COUs). When a tile is updated, the sequential identifiers are re-generated.

Coordinate reference system

The coordinate reference system for geometries in the OS Terrain 50 GML is expressed using an

EPSG code embedded in a URN (urn:ogc:def:crs:EPSG::27700). This is a more generic way of expressing the reference system, rather than osgb:BNG (British National Grid) which was used in previous versions of the product.