Source data

The source DTM for the two 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.

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 which we endeavour to achieve in the source data.

The grid and contour forms of the products are both interpolated from this source TIN model. This is because the TIN model is not widely supported by GIS software. As OS Terrain 50 is designed to work with small-scale products, the feature modelling will have a more generalised representation in the product.

Coverage

The minimum coverage of the data extends out to the low water mark, defined by Hydrographic Office tables with a height value for each tile. For England and Wales, the low water mark is Mean Low Water (MLW); for Scotland, it is 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.

Positional accuracy requirements

The z values of the source TIN data must meet positional accuracy requirements according to their geographic location. The terrain has been divided into the following three classifications to ensure that modelling reflects customer requirements:

• Urban and major communication routes

• Rural

• Mountain and moorland

The accuracy of the height value above Newlyn Datum must achieve the root mean square error (RMSE) set for each area.

Modelling of features in source data

Representation of the surface

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 Survey Newlyn Datum. 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 (those expected to remain until the next revision period or longer) are modelled.

Underground and overhead features

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.

Terrain smoothness

The DTM will be free of spikes and wells that do not reflect the real-world terrain. A surface that is smooth is one that consists of a regular plane (which may be angled). For example, a road carriageway or railway track bed will appear smooth in the data.

Edgematching

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 tile edges in tidal areas due to local tidal differences.

Supported structures

Supported structures include bridges, viaducts, cranes, elevated buildings, and jetties or piers on legs. All supported structures will be removed from the data where the structure departs from the bare earth surface and an air gap exists.

Vegetation

Areas of vegetation, such as hedgerows and trees, are removed to ensure that the bare earth surface is correctly recorded.

Vertical features

Locations with a vertical change in height or an 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 the positional accuracy requirements of the feature but is offset from its real-world planimetric position to ensure that there is only one z value present in the same location.

Major communication routes

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 will be smooth, flat (not necessarily horizontal) and free from undulations.

Manmade landforms associated with mineral workings and landfill

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.

Contained water bodies greater than 0.7ha

In order to respond to the Flood and Water Management Act 2010, the extent of all flat-water bodies that are greater than 0.7ha in area (that is, greater than 7000 cubic metres capacity) must 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.