This page describes the structure of the data in grid and contour forms.
The height data is presented as a raster dataset of height values, which are calculated at the centre of the pixel. Coordinate reference systems for DTMs may be used to calculate the DTM origin and coordinates of individual posts.
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 our 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 contours are presented as polyline and spot height features. The contour lines have been divided into tiles for product supply. The contour values can be viewed and analysed in a GIS.
The terms used for the feature types are drawn from the INSPIRE elevation specification. The draft INSPIRE elevation specification requires height values to be held in an attribute called ‘propertyValue’, which has been implemented in the new OS Terrain contour GML. In the shapefile format, the GML feature types and attributes have been followed but with the 10-character limit on shapefile fields, ‘propertyValue’ is abbreviated to ‘PROP_VALUE’.
The contours are named ‘master’ and ‘ordinary’, which are equivalent to ‘index’ and ‘standard’ contours in the withdrawn Ordnance Survey height product, Land-Form PROFILE.
In the contour products, the mean high and mean low water lines have been derived from our 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. Inevitably, this means that there is a small discrete step between adjacent tiles. There are no contours supplied between the mean high and low water lines.
The ‘LandWaterBoundary’ feature type has an attribute of ‘waterLevelCategory’, which has four possible sub-values: ‘meanHighWater’ and ‘meanLowWater’ or, for Scotland, ‘meanHighWaterSprings’ and ‘meanLowWaterSprings’. This attribution allows their display parameters to be changed to show individual features, as desired.
These have been created using an algorithm that selects the highest source data point (masspoint) within every enclosed contour that has a difference in height of at least 1m.
As they are from an interpolated surface of the real world, they cannot be guaranteed as summits or highest points of the feature. The GML data model lists the spot height sub-value, ‘spotHeightType’, which allows the potential for formSpot, generic, mountainPass or summit to be used in future releases of the product.
Currently, all spot height features are attributed as ‘generic’.
It is possible that there will be some instances of spot heights recording lower height values than the enclosing contour, but it is likely that these are the result of genuine depressions.
