BIM, Laser Scanning and Level of Accuracy

As the adoption of BIM (Building Information Modeling) and Laser Scanning accelerate, an inevitable conflict between the two will take place pitting the real world accuracy of laser scanning against a parametric modeling tool that prefers to work in an ideal orthographic projection.

Building Information Models are intelligent by nature. Consider them a graphical database of information about a building and its systems. More than just a three dimensional representation of the physical elements, the BIM actually contains information about those elements (also referred to as families). For instance, a wall is more than just a series of geometric lines. In a BIM, a wall has properties that allow the various materials that make up the wall assembly to be quantified and analyzed. The wall knows it is comprised of 3 5/8, 20 gauge metal studs at 16 inches on center, with R-11 batt insulation and 5/8 Type X gypsum board on each side. As a result, quantities for all of these materials can easily be obtained when needed.

Laser scanning captures real world elements in three dimensions and represents them with millions of data points (a point cloud). There is no real intelligence in a point cloud. However, these points represent the real world coordinates of the surface of the object the laser scans across. As a result, the laser scanner is capturing elements as they exist. If something is not straight or plumb or level, the scanner will capture it that way.

When laser scanning is used to capture and document real world conditions from which its point cloud data will be translated into a BIM, we begin to see the conflict arise surrounding the LoA (Level of Accuracy).

BIM software is, by nature, an orthographic modeling tool. These tools like to represent walls as being straight, parallel and orthogonal, which is also how architects and engineers typically like to design. If you are a surveyor who has been contracted to provide an as-built BIM and your using laser scanning to document the real world conditions how will you resolve the inevitable discrepancies of what the BIM modeling tool will allow you to represent and what your point cloud data tells you exists?

For example, above (right) is an image of a building that was modeled from 2D design drawings. It was later laser scanned while under it was under construction. The point cloud data and the building model were brought together for comparison (left).

Upon close examination we can see where the real world conditions captured in the laser scan data and the model do not match up. Below is an image of the bottom chord of one of the roof trusses. You can clearly see the point cloud data shows there is an upward camber to the truss. This camber is actually manufactured into the truss to counteract the loads that will be acting upon the truss after installation. However, if you are contracted to document the real world conditions, how will you represent this truss in your model? Is the client expecting this Level of Accuracy to be incorporated into the model or are you to introduce error into your model by modeling things in an ortho fashion? If not, you will spend a tremendous amount of time trying to get a BIM tool to represent this real world condition (in many instances it just is not possible).

These are the types of things that need to be discussed and defined up front. The current BIM authoring tools do not like to represent real world conditions such as this. Until functionality is built into the BIM tools to model directly to the point cloud data, it is important to understand the limitations of both technologies and accept the fact that an acceptable Level of Accuracy may not be defined by an exacting unit of measure, but rather whether it meets the fidelity of intent to represent the actual real world conditions.