In practice, a reliable survey of existing installations usually precedes any work on heating, ventilation, plumbing and electrical systems (HVAC).
What distinguishes a reliable survey from a 3D image
A property owner or facility manager who needs to organise a refurbishment or plan maintenance often faces the same obstacle. Plans are either missing or no longer reflect the actual state of the building.
A 3D survey only solves this problem if it produces usable data, not just an impressive image to rotate on screen. An unstructured point cloud shows the geometry and position of visible elements, but it reveals nothing about their functional condition or what lies behind a partition or a suspended ceiling. It is this requirement for reliability, rather than visual rendering, that defines a digital twin of a building that is useful for operations.
Point clouds, BIM models and digital twins are not the same thing
A point cloud is the raw output of a laser scanner: a faithful but uninterpreted snapshot of the building at a given moment. It captures millions of points in a local coordinate system specific to the device, which is then re-registered by the processing software into a georeferenced coordinate system if the project requires it.
Three formats are most commonly used. E57 is a standardised open format (ASTM E2807), independent of manufacturers. RCP is specific to Autodesk ReCap software. LAS is the exchange standard published by the ASPRS (American Society for Photogrammetry and Remote Sensing), most commonly used for airborne LiDAR and geospatial data.
The BIM (Building Information Modelling) model goes a step further. It transforms this point cloud into modelled objects (walls, ducts, equipment) according to a level defined by the LOD (Level of Development) scale of the BIMForum LOD Specification, a North American standard. The SN EN ISO 19650 series, for its part, is based on the level of information required. An object at LOD 200 has only approximate geometry; however, non-graphical information may be attached to it depending on the project. At LOD 300, its geometry can be measured directly within the model.
For property management, the digital twin differs from the first two in one further respect: its ongoing connection to building operational data, maintenance schedules, CMMS (Computerised Maintenance Management System) or technical supervision. A static model, however detailed it may be, is not a digital twin if it is never updated.
The table below summarises these three objects. The BIM model is most often exported in RVT (the native format of Revit software) or in IFC (Industry Foundation Classes, an open exchange format standardised under ISO 16739-1), whilst the digital twin is based on this same IFC linked to a technical management system.
| Nature of the object | Typical formats | Main use | Frequency of updates |
|---|---|---|---|
| Point cloud | E57, RCP, LAS | Raw reference data for modelling or ad hoc verification | None; snapshot taken on the date of the survey |
| BIM model | RVT, IFC, LOD 200 to 350 | Project coordination (design, refurbishment, construction) | Ad hoc, linked to a specific project |
| Digital twin | IFC or model linked to a building information management system | Ongoing operation and asset management | Recurring, defined by a management process |
The distinction is not merely cosmetic. It determines the budget, the delivery format and, above all, the question that follows: when does each level become relevant?
At what point in the building’s life cycle does the need arise?
Four situations regularly arise in practice. The lack of reliable plans prior to a renovation project is a common occurrence, particularly in older buildings where technical records have been lost over successive renovations.
A change in management or an acquisition often results in a building being taken over without a complete technical file. Bringing a building into compliance with CVSE standards (heating, ventilation, sanitation and electricity) requires, in practice, a reliable survey of the existing installations before work begins. An asset audit prior to investment decisions rounds off the list.
These four scenarios share a common feature: they are one-off, project-specific uses. The digital twin operates on a different principle: it meets an ongoing need, updated over several years to support asset management decisions.
From laser scanning to a structured model
Data capture and pre-processing follow a common framework, regardless of the intended deliverable. A laser scanner scans the building from several fixed positions, known as setups, each producing a local point cloud.
These point clouds are then aligned with one another, cleaned of measurement artefacts (reflections, temporary furniture, people in motion) and assembled into a single point cloud. This stage sets the tone for the entire process; poor alignment affects every object modelled subsequently. However, the scan density, georeferencing and quality controls applied vary depending on whether the aim is to produce an archive point cloud, 2D plans or a detailed BIM model.
BIM object modelling only takes place afterwards, using Revit or Archicad depending on the expected deliverable and the practices of the recipient design office. The resulting modelling tolerance – the discrepancy between the modelled geometry and the source point cloud – depends on the density of obstacles and the geometric complexity of the scanned space.
A service duct-cluttered corridor requires more set-ups than an unobstructed façade, which directly affects the time and cost of the survey.
Deliverables that make a digital twin usable
The choice of format is not a matter of arbitrary technical preference. It stems from the end use, whether it be HVAC maintenance, space management or works planning. Three deliverables enable these uses to be met within a facility management specification.
- Point cloud in E57 format, for archiving or subsequent verification without reliance on proprietary software.
- An IFC model with a Level of Detail (LOD) defined per element according to the intended management use, often LOD 200 for a simply located object and LOD 300 for equipment whose geometry must be measurable within the model.
- DWG drawings (the native, proprietary format of AutoCAD software, published by Autodesk) of the existing conditions, useful for rapid coordination with an engineering practice or a main contractor.
A property management company managing a portfolio of buildings does not require the same deliverables as an architect working on a one-off refurbishment project. The format follows the decision to be made, not the other way round.
The scope and LOD, not the sophistication of the model
The scope covered, the chosen LOD and the frequency of updates determine the value of a digital twin; all three must be aligned with actual decisions regarding maintenance, refurbishment or asset management.
The most common objection relates to cost. Modelling an entire building to a high LOD, and then maintaining that model over time, represents an investment that quickly exceeds the benefit it will provide.
Modelling secondary plant rooms to a LOD of 350 without any planned maintenance work illustrates this risk of over-specification. The detail is there, but no one will ever consult it. A scope restricted to the areas actually governed by a management decision is cheaper to produce and, above all, cheaper to keep up to date.
What triggers a model update
Three events warrant an update to the model. Works that alter the building’s geometry are the most obvious cause.
A change to HVAC equipment also affects the model, in a less obvious way but one that is just as fundamental to an operational tool. A discrepancy between the model and the actual condition – often discovered during maintenance work – indicates that the model has become out of sync with the building, usually following undocumented works.
A one-off update, triggered following works, differs from a recurring process. A living digital twin follows a frequency defined by the building’s management requirements, not by a default schedule.
Quality control and acceptance criteria prior to handover
A 3D survey is accepted in the same way as any other technical service, on the basis of verifiable criteria rather than visual impression. The check first focuses on the assembly of the point cloud; the residual deviation between the setups after realignment must remain compatible with the model’s intended use.
The check then focuses on the model itself; each object must comply with the contractual Level of Detail (LOD) in terms of its geometry and with the contractual level of information (level of information need, as defined in the EN ISO 19650 series, which can be expressed as IFC property sets) for the data associated with it. A quality control report, provided upon delivery, documents these checks and may serve as the basis for the acceptance certificate provided for in the contract, signed by the parties.
This document is binding on both parties. It establishes the benchmark against which the building is to be compared should any discrepancies be identified on site in the future.
Data governance, access rights and integration with operational tools
A digital twin of a building that has no designated manager quickly falls into disrepair. Governance begins with a simple question: who has the right to modify the model, and who is only authorised to view it?
In a property management context, the technical team generally has write access to incorporate changes following works, whilst external service providers (engineering consultancies, main contractors) are granted read-only access limited to the scope of their involvement. This division of roles prevents parallel versions of the same model from arising.
Connecting the digital twin to operational tools – such as CMMS, technical supervision systems or facilities management software – cannot be imposed at the time of handover. It must be prepared in advance by selecting an exchange format (usually IFC) and an identifier structure compatible with the target software.
Where to start before carrying out a survey
A digital twin of a building is prepared before the first on-site setup, not after the model has been delivered.
Defining in advance who will accept the model, according to which quality control criteria, and who will ensure it is updated over time allows a 3D survey to be scaled to suit its future use.