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Last updated
Last updated
OS MasterMap Topography Layer – Building Height Attribute data is generated from photogrammetrically derived Digital Terrain Models (DTMs) and Digital Surface Models (DSMs). That is, information gathered from aerial surveys as part of our cyclical revision programmes. DTMs show the bare ground surface, having extruded features such as buildings, trees, etc, whereas DSMs depict the elevations of the top surfaces of buildings, trees, towers and other features elevated above the bare earth.
Absolute height values represent absolute heights against Ordnance Datum Newlyn (ODN) at three distinct points within the building structure. The absolute heights require the use of additional terrain height information to provide context to these heights, such as by using them in conjunction with the OS Terrain 5 DTM product. The absolute heights are denoted with the prefix ‘Abs’. The unit of measurement used for absolute heights is metres.
AbsHMin (Absolute Height Minimum) represents the lowest point extracted from the DTM within the footprint of the building as represented in OS MasterMap Topography Layer. This aims to represent the lowest absolute height of the intersection of the external building walls and the underlying ground surface.
AbsHMax (Absolute Height Maximum) represents the highest point of the building extracted from the DSM within the footprint of the building or structure as represented in OS MasterMap Topography Layer. The highest point could be represented by any structure on top of the building, provided it is of sufficient size to be captured in the DSM source data. This may include industrial chimney stacks, machinery and any substantial structures that may be present on top of the building. Smaller structures like chimney stacks as found on residential buildings will be excluded from the AbsHMax measurement.
AbsH2 (Absolute Height 2) is a calculated value which aims to represent the lowest point where the roof intersects the alignment of the external vertical walls of the principal part of the building. This is usually referred to as the building eaves.
The principal part of the building is defined as the main structure, excluding elements at higher or lower elevations. In buildings with multiple heights (for example, a main two-storey building with a smaller single-storey extension), the eave height is calculated from the largest building extent.
The two relative building height values are generated from the differences between the absolute elevation values. The relative heights can be used in isolation to provide a third dimension to buildings, while all other features will be flat as they refer only to the height of the building, rather than the height above Ordnance Datum Newlyn (ODN). Relative heights are denoted with the prefix ‘Rel’. These attributes are also applicable to the Tank, Glasshouse and Chimney Stack structures included in the data. The unit of measurement used for relative heights is metres.
RelHMax (Relative Height Maximum) is the derived value from the calculation of AbsHMax, the calculated absolute value for the building height, minus AbsHMin, the calculated absolute value for ground height (i.e. RelHMax = AbsHMax – AbsHMin).
RelH2 (Relative Height 2) is the derived value from the calculation of AbsH2, the calculated absolute value for the eave height, minus AbsHMin, the calculated absolute value for ground height (i.e. RelH2 = AbsH2 – AbsHMin).
