Datum
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.
For more information see: Ordnance datum.
[edit] Other datums
A datum may also be a line from which all other horizontal dimensions relate to whether, on architectural or engineering drawings. An example is a building that has a plan generated from a back wall.
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.'
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.’
[edit] Related articles on Designing Buildings
- At grade.
- Data.
- Ordnance Survey.
- Origin point.
- Geographic information system GIS.
- Global positioning systems and global navigation satellite systems.
- Building height.
- International Property Measurement Standards.
- Invert.
- Location plan.
- Measurement.
- Ordnance datum.
- Setting out.
- Site plan.
- Water engineering.
Featured articles and news
Retrofit 25 – What's Stopping Us?
Exhibition Opens at The Building Centre.
Types of work to existing buildings
A simple circular economy wiki breakdown with further links.
A threat to the creativity that makes London special.
How can digital twins boost profitability within construction?
The smart construction dashboard, as-built data and site changes forming an accurate digital twin.
Unlocking surplus public defence land and more to speed up the delivery of housing.
The Planning and Infrastructure Bill
An outline of the bill with a mix of reactions on potential impacts from IHBC, CIEEM, CIC, ACE and EIC.
Farnborough College Unveils its Half-house for Sustainable Construction Training.
Spring Statement 2025 with reactions from industry
Confirming previously announced funding, and welfare changes amid adjusted growth forecast.
Scottish Government responds to Grenfell report
As fund for unsafe cladding assessments is launched.
CLC and BSR process map for HRB approvals
One of the initial outputs of their weekly BSR meetings.
Architects Academy at an insulation manufacturing facility
Programme of technical engagement for aspiring designers.
Building Safety Levy technical consultation response
Details of the planned levy now due in 2026.
Great British Energy install solar on school and NHS sites
200 schools and 200 NHS sites to get solar systems, as first project of the newly formed government initiative.
600 million for 60,000 more skilled construction workers
Announced by Treasury ahead of the Spring Statement.
The restoration of the novelist’s birthplace in Eastwood.
Life Critical Fire Safety External Wall System LCFS EWS
Breaking down what is meant by this now often used term.
PAC report on the Remediation of Dangerous Cladding
Recommendations on workforce, transparency, support, insurance, funding, fraud and mismanagement.
New towns, expanded settlements and housing delivery
Modular inquiry asks if new towns and expanded settlements are an effective means of delivering housing.
Comments