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Know The Accuracy Of CAD Models Built From Scan Data - Before You Begin Work.

Working with existing, versus new, structures usually starts with the often mind-numbing experience of measuring and modeling the actual building. These days LIDAR scans are becoming common and can be used to capture an existing environment.  Then a team (often offshore) uses the scan data to build a 3D model that can be used by the Architectural, Mechanical and Planning teams. But who is responsible for the quality of the 3D model, and who takes responsibility is the model is inaccurate?


A large office campus was beginning a project to update several of its buildings for seismic and mechanical upgrades, while also looking to creating accurate working models for Building Information Management software (BIM). The General Contractor began by scanning two buildings and sending the scans to a team in India for conversion to CAD models. These models were then verified by a survey team and subsequently approved for production.

And almost as soon as the retrofit began it became evident the models were good or good enough in certain areas but critically inaccurate in others. And one-by-one, each designer, engineer, and sub-contractor made it clear they wouldn’t, couldn’t, be held responsible for work done based on inaccurate models. This left the GC with little recourse.

SKUR was then used to match the veracity original LIDAR scans to the models built offshore and to test the final production model. The results showed the models contained a variety of errors, most likely due assumptions made while interpreting the LIDAR data. The result was an immediate work stoppage and nine weeks delay while the design and engineering teams reworked and resubmitted plans for client and local planning approval.

The cost of performing SKUR Diffs to compare the 27 original LIDAR scans to the models would have been about $5,000. Additionally, professional fees to investigate discrepancies SKUR revealed are estimated to have been about $7,000. And since the contract with the model maker allowed for corrections before final sign-off, there would have been no additional cost for reworking the models at this early stage.

The cost of work stoppages can be difficult to calculate. For this case study, we evaluated design and engineering fees, rental costs for idle heavy equipment, wages for Subcontractor teams held onsite for a week after the initial error discovery, legal fees, and planning department fees.  In all, about $108,000 in unanticipated costs were easily identifiable. Likely the number is double.

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