BIM Basics: What is Level of Accuracy?

The 3D BIM modeling software (most often AutoDesk Revit) that we use here at Factory Reality is very versatile. If you are familiar with the software, you know that it can be used for both new construction projects and existing conditions projects. For new construction, architects and engineers design and test the feasibility of the proposed structure as well as use these 3D BIM models to order supplies for construction. When it comes to modeling existing conditions, however, the structure's information has to be gathered efficiently.

The most common way to gather this information is through some form of laser scanning.

It’s important to know that laser scanning produces a 3D “point cloud” that represents the structure. The point cloud consists of millions or even billions of points. Each point represents a location where the laser scanner registers a physical object. The clarity and density of these points determines the quality of the point cloud.

When it comes to converting these point clouds into 3D BIM models, the quality of the point cloud is very important. What this allows is for our team to accurately place objects according to the corresponding points. That said, the BIM model can only be as accurate as the point cloud. With 3D BIM models, the value is really in the information that can be gathered from them. The reliability of this information depends on the accuracy of the model.

What is Level of Accuracy?

Similar to Level of Detail/Development, the Level of Accuracy (LOA) is a scale that is used to determine how precise the model should be to the point cloud. No building is perfectly orthogonal. This means that, as a modeler, you have to decide how close to stay to real-world conditions. What the LOA scale does is define the standard deviation allowed.

LOA Scale

The LOA scale was developed by the United States Institute of Building Documentation (USIBD) to standardize specifications industry-wide. You can find a great deal of information about LOA on their website. The chart below was developed by the USIBD.

An important note when considering LOA is that we are talking about very small deviations, centimeters, and millimeters. It may seem like a lower level of accuracy could be used but consider how a deviation of a few millimeters could add up when expanded over a wall that is 40, 50, or 100 ft in length.

As you can see from the chart,the higher the LOA the lower the deviation. Meaning, the higher the LOA the tighter the modeler should hold to the point cloud. This is where the quality of the point cloud comes into play. With lower-accuracy scanners, higher levels of LOA may not be possible.

This post is meant to serve as merely a brief introduction to LOA and how it is used. For greater detail on the topic, check out the USIBD website.