- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 14 Dec 2020
On a construction project, a datum level is an arbitrary horizontal plane of reference from which all vertical dimensions are measured. 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.
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. 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.
However, minus signs can be easily misread, not seen or read as a dash, so can cause confusion. For this reason, a suitable fixed point (called a temporary benchmark – TBM) is assumed to ensure that all other levels are positive. 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. The levels usually refer to finished floor levels (FFL) but can also indicate other features e.g finished structural level (FSL)
Datum 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. 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.
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. Using this approach means that everything is to the same OS datum and OS grid (OSG).
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. OSG is the standard known grid across the UK.
 Other datums
A datum may also be a point that is used for setting out the building. This must be clearly marked on drawings to help the contractor’s setting out. For example, a circular building will be set out from a datum point from which all radii are generated.
Drawing 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.'
 Related articles on Designing Buildings Wiki
- At grade.
- Ordnance Survey.
- Origin point.
- Geographic information system GIS.
- Global positioning systems and global navigation satellite systems.
- Building height.
- International Property Measurement Standards.
- Location plan.
- Ordnance datum.
- Setting out.
- Site plan.
- Water engineering.
Featured articles and news
Prioritising tax considerations.
The four D creative process: discover, define, develop and deliver.
National Cyber Security Centre initiative is announced.
Reviewing trends and projections.
Legislation will establish initiatives to move towards net zero.
How to document contractor employment status.
Tech tools to help manage people and space post-pandemic.
A style that ranges from mock Tudor to arts and crafts to the 'Wrenaissance'.
Free guide from Secured by Design.
BREEAM strategy for sustainability and the circular economy.
Free tool to improve the construction programming process.
Are buildings doing what they're supposed to be doing?
Cities with quick access to everything by foot or bike.
The pressures and pinch points of global destinations.