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Validating measurements
June 2, 2009

Measuring

Although elevation plans exist for the building (see Backgrounder entry), the accuracy of those plans is unknown. Normally, a number of physical measurements can be made. However, there are many situations where it can be very helpful to have point cloud data for a site, especially for very large sites. For example, the 210 King lobby is two storeys in height and would be difficult to measure with traditional methods. Also, non-flat objects, such as curved walls or cylindrical ventilation ducts can be challenging to measure and place within spaces. In some cases, contractors may discover additional measurements are needed once a project has begun requiring additional site visits. However, with a detailed 3D building scan, enormous numbers of measurements can be taken without ever revisiting the physical site.

Measuring

To build confidence that the point cloud accurately reflects the real dimensions, we went through a validation phase of comparing a number of physical measurements against virtual distances measured from the point cloud data. We ensured that comparison measurements were made in all three dimensions both within a single scan and, more importantly, across combined scans.

Comparison

Shown here in color are measurements from the physical site together with distances queried from the point cloud data, shown in grayscale. Although measuring modern machined elements (like drywall or metal posts) was a trivial task, we found it to be challenging to exactly measure physical dimensions for brick or wood materials, like interior walls and beams. The same challenge existed in taking virtual measurements where the width of a wooden beam could be measured from any number of (non-planar) samples, for example. Given these issues, we still found a stable average difference to be .06 inches which is equivalent to a precision value of 1.524 millimeters.

We found Autodesk Navisworks (building information modeling [BIM] project review software for construction simulation, scheduling, and coordination), to be a good tool for 3D navigation around the point cloud data. Using the Walk tool in the Steering Wheels (shown below), we could more easily move to the correct location in the data set. Also, the measurement tool in Navisworks was easy to use and we could confirm distances quickly.

Navisworks

Overall, this process gave us great confidence in the point cloud scan data set. Finally, the accuracy of the elevation plans could now be seen to be quite low. The real physical building plan was significantly different from the original drawings we had so this data set was very helpful in the accurate creation of our 3D BIM model of the 210 King building.

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