Scan-to-BIM accuracy: what tolerance actually means on a retrofit project.

Every scan-to-BIM brief we receive includes some version of the phrase 'sub-centimetre accuracy'. It is the right aspiration. The problem is that without a defined tolerance standard and a clear specification of what is being measured, sub-centimetre accuracy can mean almost anything — and on a retrofit project, that ambiguity has real consequences.

Accuracy in a point cloud survey means the deviation between a measured point in the cloud and the true position of the corresponding point on the physical surface. That deviation is affected by the scanner hardware, the scanning methodology, the number of scanner positions, the registration process, and environmental conditions. Specifying 'sub-centimetre accuracy' without specifying which of these parameters are being controlled does not actually constrain the quality of the output.

The tolerance question is separate. Tolerance is the maximum allowable deviation that the downstream design team can work within without the design being compromised. On a straight corridor in an office building, a tolerance of ±10mm between the as-existing model and reality is typically acceptable. In a plant room where pipework clearances are measured in millimetres, or in a heritage building where geometric deviations have conservation implications, the required tolerance is much tighter — and the scanning methodology and modelling approach need to be specified accordingly.

We define accuracy and tolerance for each project before the first scan is taken, in a survey specification document that is agreed with the design team lead. That document records the target accuracy for each building element, the modelling level of detail for each system, and the survey methodology that will achieve it. The point cloud deliverable is then assessed against that specification rather than against a generic standard.

The downstream benefit is that the design team has a documented basis for the as-existing model that they can reference in their own documentation. On a procurement-sensitive project, that matters — the contractor is making assumptions about buildability based on the retrofit model, and those assumptions need to be grounded in something more defensible than 'we scanned it'.