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Height values source data

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.

Graphic depicting the difference between Digital Terrain Models and Digital Surface Models, which are the source data used to create Building Height Attribute. A Digital Terrain Model shows the bare ground surface (having extruded features such as buildings, trees, etc.) and is depicted by a solid black line in the image. A Digital Surface Model depicts the elevations of the top surfaces of buildings, trees, towers and other features elevated above the bare earth; it is depicted by a dashed black line in the image.

Height values

Absolute height values

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

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.

Illustration of absolute minimum height at ground locations. Three simplified buildings are shown and the lowest point at which the external building walls and the underlying ground surface intersect is highlighted by a red arrow for each building.

AbsHMax

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.

Illustration of possible absolute maximum heights for two buildings. A red arrow depicts the absolute maximum height of the first building, which is the top of the building's industrial chimney stacks. A red arrow depicts the absolute maximum height of the second building, which is the top of the building's roof as the residential building's chimney stacks are excluded from the absolute height maximum measurement.

AbsH2

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.

Illustration of the principal part of a building, which is highlighted by an arrow that points to a rectangle with a red dashed outline.

Relative height values

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

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

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).

Illustration of the two relative and three absolute height values. Full explanations of the five height values and how they are calculated are included in the main text. RelHMax equals AbsHMax minus AbsHMin. RelH2 equals AbsH2 minus AbsHMin.

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Last updated 9 months ago

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