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Datum in drawing series#
The Geospatial Glossary, published by the Geospatial Commission, and accessed on 17 September 2022, defines horizontal datums as: ‘…a series of data points on the surface of the earth that are used to measure the position of oneself on the Earth’s surface.’ For example, a circular building will be set out from a datum point from which all radii are generated.ĭrawing for Understanding, Creating Interpretive Drawings of Historic Buildings, published by Historic England in 2016 defines a datum line as: 'A horizontal or vertical reference line used to control height and horizontal distance measurements.' This must be clearly marked on drawings to help the contractor’s setting out. An example is a building that has a plan generated from a back wall.Ī datum may also be a point that is used for setting out the building. OSG is the standard known grid across the UK.įor more information see: Ordnance datum.Ī datum may also be a line from which all other horizontal dimensions relate to whether, on architectural or engineering drawings. Note: OS datum is taken as height above sea- level based on a known datum point at Newlyn, Cornwall if the OS datum is being used, it must be indicated on the drawings. Using this approach means that everything is to the same OS datum and OS grid (OSG). It is increasingly the case, especially for large projects, that the Ordnance Survey (OS) datum is used as a project datum as it allows work on the site to be related to other features in the area, e.g manholes and sewers, local gradients etc. They should be clearly indicated on all relevant drawings with all levels described in metres to three decimal places but always as positive numbers because they are above datum level.
The levels usually refer to finished floor levels (FFL) but can also indicate other features e.g finished structural level (FSL)ĭatum levels are useful as they provide points of reference to allow the vertical setting out of buildings and how they relate to other levels on a site. So, in the case of the basement level referred to above, it may be given 0.000m as the site datum, in which case (following the logic from above) ground level = 3.250m ground floor = 4.000m first floor = 8.000m and so on. For this reason, a suitable fixed point (called a temporary benchmark – TBM) is assumed to ensure that all other levels are positive. However, minus signs can be easily misread, not seen or read as a dash, so can cause confusion. In contrast, a basement which will be below ground will be below datum level and will therefore have a minus symbol prefixed to it e.g -3.250m. The datum on a site can be given a 0.000m designation but this does not have to necessarily be outside ground level if it is, a ground floor may be at say, 0.750m and a first floor at 4.750 and so on proceeding up the building. It can show the vertical height difference between floor levels of a building as well as differences in levels between one part of the site and another. On a construction project, a datum level is an arbitrary horizontal plane of reference from which all vertical dimensions are measured.
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