3D laser scanning and scan-to-BIM
When laser scanning pays: scan before MEP prefabrication
Prefabricating MEP spools, risers and ductwork off site saves site time only if the pieces fit when they arrive. The fit depends on the real building, not on the drawings. A laser scan is one way to capture the real building. It is not always the right one, and this article explains when it pays and how to ask for it.
This is a capability note. It describes a method, not a record of delivered projects.
The cost of a spool that does not fit
Prefabrication moves the measuring risk upstream. A pipe spool made to a drawing dimension is cheap to build and expensive to correct. If the structure is out of position, if a slab is lower than shown, or if an earlier trade installed something that is not on the drawings, the error shows up only when the spool is lifted into place.
On new buildings the gap between drawn and built is usually small, but it is rarely zero. On existing buildings it can be large, and drawings may be missing or out of date. The question to ask is not "is scanning good?" but "what does an error cost us, and how likely is it?"
When scanning usually pays
Scanning tends to be worth the cost when several of these are true:
- Tight services zones. Ceiling voids and shafts with little spare room leave no margin for deviation.
- Large repeated prefabricated elements. An error in one module is repeated in the others.
- Existing buildings. Refurbishment, extension and plant-room replacement often lack reliable as-built drawings.
- Late access. If the site will only be open for a short window, you cannot go back to re-measure.
- Structure built before the MEP design was final. Concrete cast ahead of the services design is more likely to deviate from it.
- Heavy or costly components. Chillers, large ducts and pre-insulated pipework are costly to re-make.
Scanning usually does not pay when the work is small, the as-built information is reliable, and the tolerance is generous. A tape measure and a total station may be enough.
Start with purpose, then specify accuracy
A scan has no value in itself. Its value depends on the decision it supports. Write the purpose down first, for example "confirm clear height and beam positions in the Level 2 corridor before prefabricating ductwork." That purpose tells you the area to cover, the elements that matter and the accuracy needed.
The USIBD Level of Accuracy (LOA) Specification, version 3.0 published in 2019, gives teams a common way to state accuracy requirements. It includes a statement-of-intent section, so the specifier can explain why the accuracy is required, and a validation section that describes how accuracy is checked. Using it moves the conversation from "high accuracy" to a stated, checkable requirement. It is free to download.
Two practical points:
- Specify accuracy for elements, not the entire building. Structure in the services zone may need a tighter specification than a blockwork partition that is not affected.
- Specify how it will be verified. A requirement that cannot be checked will not be met.
Plan the scan
A scan plan covers:
- Areas and elements to capture, and the ones to leave out
- Control: how the scans are tied to the project coordinate system and levels
- Access: working hours, permits, and what has to be cleared first
- Scanner positions and overlap, so that the registration is reliable
- Deliverable formats and the date
Control is the part that gets forgotten. A point cloud that is accurate in itself but not tied to the project grid cannot be used for coordination. Agree the origin and levels with the BIM coordinator before scanning.
Process and register
After the survey, the individual scans are registered into one point cloud. Check the registration report and the overlap between neighbouring scans before the team leaves site. Cleaning removes noise, temporary works and people. Do not delete anything that might be relevant to the purpose; mark it instead.
For exchange, E57 is a safe choice. ASTM E2807 defines the E57 file format for 3D imaging data. An E57 file can store points, attributes such as colour and intensity, and 2D images. It is a vendor-neutral format, so the point cloud can move between survey, modelling and coordination software without being locked to one product.
Scan-to-BIM: model for the purpose
Modelling everything in the cloud costs time and creates a heavy model. Model what the decision needs. For ductwork prefabrication, that may be structure, slabs, beams, openings and existing services in the zone, and not furniture or finishes.
Decide these points with the modeller:
| Question | Why it matters |
|---|---|
| Which elements are modelled? | Keeps the model focused and light. |
| What LOD or detail for each? | Avoids modelling what no one will use. |
| How are deviations treated? | Walls that are not plumb can be modelled as measured or as idealised. State the choice. |
| How is the model checked against the cloud? | A deviation check shows where the model departs from the scan. |
| Is the cloud kept as reference? | Linking the cloud in the coordination file lets people check the model against it. |
A model that departs from the cloud in a critical place is worse than no model, because it gives false comfort. Run a deviation check on the elements the prefabrication depends on, and record the result.
Use it: clash the as-built against the design
The real payoff comes next. Combine the as-built model with the MEP design model and run the coordination process. Clashes found at this stage are fixed on screen: reroute a duct, move a support, change a spool length. Found on site, the same problem becomes rework.
Then keep the as-built reference for the prefabrication drawings. Spool drawings can state the dimensions taken from the coordinated model, and the fabricator can see which dimensions rest on measured data and which on design.
Questions to ask a scanning provider
- What is the stated accuracy, and how is it verified?
- How are scans tied to the project control?
- What formats will we receive, and can we open them?
- Who is responsible if the cloud is later found to be off?
- What is left out of the scan, such as ceilings or areas not accessible?
If you are weighing a scan for a particular package, send us the area, the element types and the access window. We can help define the purpose and the specification before you request prices from survey providers.