Understanding accuracy is the key to success in building documentation
Accuracy seems to be a term that is used quite often when describing what is needed with an Existing Conditions Survey; commonly referred to as an “As-built.” Before we can commit to an accuracy, we must first understand whether the term refers to a measured accuracy or a represented accuracy.
A measured accuracy refers to the accuracy of the actual measurements themselves. In the case of laser scanning, this would be the results achieved in a single scan, or in a set of registered scans. In the illustration provided the red dots represent the measured values returned when the laser strikes the true surface of the object. Due to reflectance, distance, angle of incidence, environmental factors, and the measurement instrument there is always some degree of error, so the measurements don’t always land exactly on the true surface of the object.
A represented accuracy refers to the accuracy achieved when representing the measurements (i.e., line work or a modeled element). The illustration shows a blue line for the actual represented surface of the object. In this instance, the true surface of the object is not perfectly flat, but the representation of it is. Thus, once again, we see a degree of error in the represented surface.
Because the real-world true surfaces are often not plumb or square, although designers often prefer them that way, as-builts are usually drawn orthogonally. Doing so also adds more error. You can begin to see that creating accurate 3d as-built models can be challenging. Understanding the difference between measured and represented accuracy is important when specifying accuracy on your project. In addition, if you are specifying a represented accuracy, it is important to also state whether you prefer things to be modeled in an orthogonal fashion or real world. The U.S. Institute of Building Documentation (USIBD) has published a Guide to specifying accuracy called the Level of Accuracy (LOA) Specification. If you are uncertain how to go about determining and specifying your accuracy needs for reality capture services it is best to consult with a building documentation professional.
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