5m resolution digital terrain model (DTM) of Great Britain. The elevation values are calculated at the centre of the cell and they are given to the nearest 0.01 m.
Due to local tidal conditions, the height of the mean high and low water mark varies continuously around the coast of Britain. The mean high and low water lines have been derived from large-scale mapping and assigned constant height values, based on the average for each tile. This average value has been determined from local tide tables. The mean high and low water lines were used as heighted breaklines when creating the grid to ensure the grid product is consistent with the contour product. This means that there may be a small discrete step in the height of tidal water between adjacent tiles. For areas of permanent tidal water the height of the mean low water has been extended out to the tile edge to ensure that the tile is complete. Heights in the foreshore area are interpolated between the mean high and low water heights.
The DTM is calculated as a triangulated irregular network by editing with mass points and break lines and/or automated techniques within a photogrammetric environment. The height values for each grid cell and are calculated as a mean of the heights across the whole cell. The elevation values of the source TIN data have to meet positional accuracy requirements according to their geographical location. 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 minimum coverage of the data extends out to the low water mark, defined by Hydrographic Office tables with a height value for each 5 km by 5 km 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.