The source DTM for OS Terrain products is captured as a triangulated irregular network (TIN) by editing with mass points and breaklines and/or automated techniques within a photogrammetric environment.
The TIN is a superior model for three-dimensional data as it uses triangles, which can retain the edges of features more accurately than a grid, for example.
The source data capture is subject to demanding rules defined by the height capture specification. Particular attention is paid to communication routes and features significant to height applications. This section describes some of the key capture requirements from the detailed capture specification that we endeavour to achieve in the source data.
The grid and contour products are both interpolated from this source TIN model.
The minimum coverage of the data extends out to the low water mark, defined by Hydrographic Office tables with a height value for each 5km-by-5km tile. For England and Wales, the low water mark is mean low water (MLW) and for Scotland, mean low water (springs) (MLW(S)).
All land wholly within inland water bodies that is represented by topographic area features is captured according to the positional accuracy requirements of the area. The minimum requirement is to capture the outer edge of the feature. The surrounding water will remain flat.
Any other land within inland water bodies captured by automated processes will be removed from the data.
The z values of the source TIN data must meet positional accuracy requirements according to their geographic location. The terrain has been divided into three classifications – urban and major communication routes, rural, and mountain and moorland – to ensure that modelling reflects customer requirements. The accuracy of the height value above Ordnance Datum Newlyn must achieve the RMSE set for each area, which are not the same as the stated product accuracy.
The height of the bare earth surface is recorded as a series of points with three-dimensional coordinates.
The X and Y coordinates are eastings and northings in OSGB36; the Z coordinate is height in metres relative to the datum for the area. Most areas will record a height relative to Ordnance Datum Newlyn. For a small number of offshore islands, a local datum has been used.
The bare earth surface excludes buildings, supported structures, and vegetation. Structures that form an obstruction at ground level – such as dams, breakwaters, and groynes (wide enough to affect the positional accuracy requirements), bridge revetments and earthworks – are considered to be part of the bare earth surface. Only permanent terrain features are modelled.
Underground and overhead features are, by definition, not the ground surface and are thus not included in a DTM. Underground features are those that are obscured and require excavation to construct.
Underground features are not recorded, and overhead features are removed from the data.
The DTM will be free of spikes and wells that do not reflect the real-world terrain. A surface that is smooth; that is, one that consists of a regular plane (which may be angled); for example, a road carriageway or railway trackbed, will also appear smooth in the data.
Most data will present without visible tile edges or discernible height differences between tiles. In places, there may be small edges present or a difference in feature modelling between new and older content. There will also be small edges in tidal areas due to local tidal differences.
Supported structures include bridges, viaducts, jetties or piers on legs, cranes, elevated buildings, and so on.
Supported structures are removed from the data where the structure departs from the bare earth surface and an air gap exists. All supported structures will be removed from the data.
Areas of vegetation, such as hedgerows and trees are removed to ensure the bare earth surface is correctly recorded.
Locations with a vertical change in height, or overhang, have the height of the top of the feature recorded at the correct planimetric location according to the positional accuracy requirements.
The height at the lowest point of the vertical feature is recorded according to positional accuracy requirements of the feature but offset from its real-world planimetric position to ensure that there is only one z value present in the same location.
Major communication routes are major road and rail networks identified in our core database.
The limits of a road carriageway or railway track bed are modelled to ensure that the route reflects its real-world shape. Modelling is required for changes in height to meet the positional accuracy requirements, to smooth the surface, and to remove extraneous features such as road furniture and bridges. Any associated slopes and embankments along the length of the route are also modelled.
In all other cases, the surface must be smooth, flat (not necessarily horizontal) and free from undulations
The outer limits, shape and depth of mineral extraction and landfill sites are captured to meet positional accuracy requirements. Temporary features that do not represent the terrain at the time of capture, for example spoil heaps, are removed from the data.
To respond to the Flood and Water Management Act 2010, the extent of all flat-water bodies greater than 0.7ha in area (that is, greater than 7,000 cubic metres capacity) will have their limits captured to ensure that the presence of the water body can be inferred from the data.
The height of the water recorded is that at the lowest height of the surrounding data. The surface of the water will be flat.
The following measurables are used to determine the accuracy and coverage of modelling in the source TIN data.
Any height value extracted from any point within the coverage and above the high-water mark will meet the positional accuracy requirements.
The DTM will be free of spikes and reflect the general texture of the terrain.
Measurable
The absence of spikes or wells in the data that cause a false height value on a data point or interpolated surface.
A surface smooth in the real world will appear smooth in the data.
Major communication routes will be smooth longitudinally, with no ‘steps’ in the data.
Conformity
There must be no spikes or wells in the data larger than the value required to meet the positional accuracy requirements of the geographic area.
Data added to model specific features will reflect the correct relationship to itself and immediately surrounding data.
All features with parallel limits that require modelling will have parallel data points recorded in the data. The relative heights of features in proximity will be in sympathy.
The coordinate position of any point on the surface as represented by the TIN data compared to the true elevation of the same point.
The RMSE of a selection of points from the real world compared to the data will be within the values stated.
No single point will exceed the 99% confidence level.