Why georeference a building? A step that has become essential for BIM projects
Understanding the benefits of georeferencing 3D laser scan data to ensure the quality of BIM models, site inspections and future surveys.
When carrying out a 3D laser scan of an existing building, one question often comes up:
Should the building be georeferenced?
For a long time, this process was reserved for major infrastructure projects or buildings situated on complex sites. Today, with the widespread adoption of BIM, Scan-to-BIM and laser-scanner-based site inspections, georeferencing is becoming standard practice for the majority of projects.
A non-georeferenced survey is perfectly suitable for producing plans or a BIM model. However, it has no actual geographical reference. It is impossible to determine precisely the building’s position within the Swiss national coordinate system or its official elevation.
Conversely, a georeferenced building is directly linked to the national coordinate system MN95 as well as the official altimetric system NF02 (or LHN95, depending on the project requirements). All the data produced can then be used throughout the building’s entire life cycle.
⏱️Résumé in 20 seconds
The georeferencing of a building involves linking a laser scan survey to the Swiss national coordinate system MN95 and the NF02 height system. This process ensures that point clouds, BIM models, topographical surveys and site inspections all use the same geometric reference frame. It facilitates comparisons between the design and as-built conditions, improves BIM coordination and ensures the long-term viability of digital data throughout the building’s lifespan.
Why are there limitations to a building that is not georeferenced?
When the scanner is used without georeferencing, it automatically creates its own local coordinate system.
The building is accurately depicted in this reference document.
On the other hand, it is impossible to know:
- its exact location within Switzerland; ;
- its official altitude; ;
- its actual orientation; ;
- its relationship with a topographical survey; ;
- its compatibility with a future survey carried out several years later.
This situation does not usually pose any problems when it comes to producing a few shots.
However, it quickly becomes a hindrance as soon as several people are working together.
Georeferencing creates a common language
BIM is based on a simple principle:
Everyone works within the same coordinate system.
Architects.
Engineers.
Surveyors.
Businesses.
CVSE offices.
Farmers.
When each discipline uses exactly the same reference framework, all the digital models naturally align with one another.
The building then becomes a true digital twin situated at its actual location.
How is georeferencing carried out?
Georeferencing begins with the placement of several markers around the building.
These targets are then surveyed by a surveyor using a total station or a GNSS receiver.
Each target then receives:
- an X-coordinate; ;
- a Y-coordinate; ;
- an altitude Z.
These coordinates are expressed in the official Swiss system MN95 for site planning and NF02 (or LHN95) for altimetry.
The target layout plan and the list of coordinates will serve as the project’s reference documents.
Why is this essential for Scan-to-BIM?
Nowadays, it is common practice to carry out several laser scanning surveys during a construction project.
These statements enable you to compare:
- the planned BIM model; ;
- the structure as actually built.
This method is known as Scan-to-BIM or performance monitoring.
What is a Clash?
In BIM, a clash refers to a clash or incompatibility between two elements.
These may include:
- a duct running through a beam; ;
- a pipe that runs through a veil; ;
- technical equipment that encroaches on a structure.
But a clash can also arise between the BIM model and the building as actually constructed.
A laser scanner inspection can then immediately detect positional deviations, installation faults or workmanship errors before they become costly to rectify.
Is it possible to georeference a point cloud using a BIM model?
The answer is usually no.
A BIM model is not a topographical document.
Even if it contains coordinates, these do not guarantee that the modelled features correspond exactly to their actual positions.
When software such as Leica Cyclone Register 360 performs georeferencing, it assumes that the coordinates of the targets are perfectly accurate.
If these dimensions are taken from an inaccurate BIM model, the software will still attempt to adhere to them.
It will then apply a mathematical transformation which may result in:
- rotation of the point cloud; ;
- a change in altitude; ;
- a false level; ;
- an overall distortion of the geometry.
In other words, the software will give priority to the specified coordinates rather than the measurements actually taken by the laser scanner.
Common mistake
Using points extracted from a Revit or Archicad model as georeferencing coordinates is a common mistake.
The coordinates of the model generally do not have any topographical guarantee. Using them as a reference may lead to distortion of the point cloud and skew the site checks.
Best practice for checking a non-georeferenced BIM model
When the sole aim is to compare an existing BIM model with the actual status of the construction site, it is generally not There is no need to georeference the point cloud.
Best practice is to:
- keep the point cloud in its original coordinate system in order to preserve its full accuracy; ;
- import the point cloud into the BIM software; ;
- reposition the point cloud relative to the BIM model using a rigid transformation (translation and rotation), without altering its geometry; ;
- then carry out the checks and comparisons.
This alignment is carried out using strong reference points, that is to say, structural elements that extend the full height of the building and whose geometry is particularly stable.
The most commonly used references are:
- stairwells; ;
- lift shafts; ;
- reinforced concrete cores; ;
- the main lifting sails; ;
- building façades or corners, provided their geometry is simple and reliable.
These features provide a much better geometric reference than partitions, technical installations or finishing elements, which may have been moved.
The aim is not to artificially align the point cloud with the model, but to place both datasets within the same local reference frame in order to accurately compare the completed structure with the theoretical design. The point cloud thus retains its full geometric integrity.
When is it absolutely essential to georeference?
Georeferencing is strongly recommended where the project involves:
- site supervision; ;
- several laser scanning surveys; ;
- a future extension to the building; ;
- a BIM check; ;
- a large-scale project; ;
- drone surveys; ;
- a link to the topography; ;
- the construction of new structures; ;
- a digital twin; ;
- wealth management.
Case study – Tracouet cable car station in Haute-Nendaz.
As part of the replacement of the installations, the georeferencing of the 3D laser scan was essential in order to accurately position the future technical equipment within the Swiss national coordinate system (MN95/NF02). This approach ensured consistency between the laser scan data, the BIM model, the site layout carried out by the surveyor and future site inspections.
Conclusion
Georeferencing is not simply a matter of assigning coordinates to a building.
It ensures that all the data produced today will still be usable in several years’ time, regardless of how the project develops.
At a time when BIM, Scan-to-BIM and on-site inspections are becoming the norm, this is now considered best practice, as it ensures the accuracy of design work, facilitates coordination between stakeholders and maintains the geometric quality of laser scan data.
Key takeaways
A non-georeferenced laser scan can be used to produce high-quality plans and BIM models, but it is limited to a local coordinate system.
Georeferencing links this data to the Swiss national coordinate system MN95/NF02 and ensures its compatibility with future surveys, BIM projects, site inspections and topographical data.
For most projects that are set to evolve over time, this is now considered best practice, ensuring the long-term preservation of data and the reliability of comparisons between the current situation and the project.
