What ±2mm actually means in a scan-to-BIM model

Every scan-to-BIM quote leads with an accuracy figure, and ±2mm is the one you'll see most often — it's the figure we quote too. But the number on its own doesn't tell you much unless you know what it's describing. Accuracy in a scan-to-BIM workflow isn't one number; it's a budget that gets spent in three places.

Where the tolerance budget goes

The first spend is the scanner itself. A survey-grade instrument like the Trimble X7 has a stated range accuracy of a couple of millimetres under good conditions — reflective surfaces, dust, and extreme ranges all eat into that. The second spend is registration: individual scan positions have to be stitched into one point cloud, and every join introduces a small alignment error. Good field technique — sensible scan spacing, strong overlaps, checked registration reports — keeps this contribution tiny; rushed scanning lets it grow invisibly.

The third spend is the one nobody talks about: modelling. A point cloud is millions of measurements; a BIM model is a decision about which geometry to draw through them. A wall that bows 8mm along its length has to be modelled either as it really is or as the flat plane the design assumed. That choice — tracing reality versus idealising it — affects the usefulness of the model far more than the scanner's spec sheet.

Accuracy versus level of detail

Accuracy is also not the same thing as level of detail (LOD). A model can be highly accurate and deliberately simple — structural steel traced to ±2mm with no attempt to model every bracket — or detailed and loose. When we model existing steel directly in Tekla from a point cloud, the accuracy matters because the next step is fabrication: new steel has to meet old steel, and the tolerance left over for the fabricator is whatever the survey didn't spend.

The questions worth asking

Before commissioning a scan-to-BIM survey, ask three things. What instrument and registration method will be used, and can you see the registration report? Is the quoted accuracy a scanner specification or a verified model deliverable? And what happens where reality deviates from design geometry — does the model follow the building, or the drawings? The answers tell you more than the headline number ever will.