Why do our client question our survey? Did we make a mistake? Was the scans inaccurate? Did the modeling go wrong?
These questions arise naturally, and understandably, they can shake confidence in our work. To find out why this happens let's explore the tolerance in scanning.
What is Tolerance in Scanning?
Tolerance refers to the small, acceptable range of deviation or error during the scanning and interpretation of data. Scanners like the Leica BLK360 G2, for instance, have an accuracy of ±4mm at 10 meters. As the distance increases, the accuracy decreases slightly. But tolerance isn’t just about the scanner.
Once the scan data is captured, it needs to be manually cleaned, aligned, and registered in a technical process that introduces further tiny variations. Typical tolerances for good registration are in the range of ±2mm to ±10mm.
Additionally, real-world buildings are rarely 100% straight or perfectly aligned, especially older buildings in and around London. Walls bend, floors sag, and angles deviate from an ideal 90 degrees.

How we handle bent walls, sagging floors, and angles that are deviating?
When drafting, we rely on averaging and the technical judgment of our CAD, Revit technicians. For example:
A wall might measure +15mm thicker at the end, +2mm in the middle, and -15mm at the top. Instead of creating jagged or wavy lines, we take an average to create a clean, practical representation of the structure.
Why Scan Data Matters?
Without scan data, manual measurements can differ significantly depending on the location, height, angle the measurement was taken and tools used. Scans allow us to see and analyze the true variations in a structure.
While scanners provide a good reference, the final quality of As-built Drawings or a 3D model depends on the discretion of the drafter's experience. Thanks to this scan data, which serves as a reference, enables an independent person to conduct quality control for each submitted project. In SpaceSurvey they compare it against the scan data before signing it off and delivering it to the client to ensure accuracy.
Summary
Scanners are highly accurate, but tolerance exists due to equipment limitations and manual processes.
Real-world buildings introduce additional variability due to construction imperfections and natural wear over time.
Final drawings and models strike a balance between precision and practicality. However, overly precise wavelike lines and walls in the planning stage may cause confusion and delays, ultimately having the opposite effect.
The drawing and models are prepared for coordination purposes. And not intended for manufacturing purposes (ie windows, kitchens wardrobes etc) It is always advisable for the supplier/installer to visit the site and carry out their own survey suitable for the intended purposes.
Walls are displayed as straight lines and angles averaged to fit the 90 degrees where designed this way.
The As-built Drawings and a 3D models tolerance is in the regions of ±30mm as the walls, floors and ceilings are almost never 100% even or straight.
In the end, scan data enhances trust and accuracy in as-built plans and models, speeds up the data collection process on site, and provides an objective reference that can be revisited at any time when questions arise, without visiting the site. However, interpreting it always involves skilled human judgment, and therefore we have a quality control process in place. And more detailed drawings/models can be created from any given part of the building if requested, following precisely the shapes of each curve in the wall, floor sag, and angle deviations for manufacturing purposes.
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