LOA in plain English
"Accurate" means nothing in a proposal until it has a number on it. The Level of Accuracy (LOA) scale gives architects and scanning firms a shared way to write that number down. Here is what LOA20, LOA30 and LOA40 mean on a real remodel, how LOA differs from LOD, and which level to ask for.
What LOA is
LOA comes from the U.S. Institute of Building Documentation (USIBD), a nonprofit that writes standards for documenting existing buildings. Its LOA specification sorts accuracy into five bands, from LOA10 (coarsest) to LOA50 (finest). Each band is a range of how far a measured point or a modeled surface may sit from its true position, stated at about 95% confidence. In plain terms, nearly every check should land inside the band.
| Level | Allowed range (approx.) | What it suits on a remodel |
|---|---|---|
| LOA10 | Coarser than 2 in | Massing studies and rough area takeoffs |
| LOA20 | About 5/8 in to 2 in | Architectural models: walls, openings, floor levels, roof form |
| LOA30 | About 1/4 in to 5/8 in | Point clouds you measure from directly; tight interior layouts |
| LOA40 | About 1/16 in to 1/4 in | Critical tie-ins: steel connections, prefab, millwork against existing walls |
| LOA50 | Finer than 1/16 in | Specialist work, such as industrial or fabrication checks |
Ranges are rounded to inches for readability. See the USIBD specification for exact values and definitions.
Measured is not the same as modeled
The LOA scale makes one distinction that most proposals skip. The measured accuracy is how close the point cloud is to the real building. The represented accuracy is how close the Revit model is to that point cloud. They are different numbers, and the model is almost always looser than the scan.
That is not a flaw. A real wall bows, leans and changes thickness. A Revit wall is straight, plumb and one thickness from end to end. Every time a modeler turns a wavy surface into a clean element, some accuracy is traded for a model you can design in.
What it means for you: ask for both numbers. Our default is a point cloud at LOA30 and an architectural Revit model at LOA20. When a wall is far enough out of plumb to matter, you can still measure it in the point cloud.
The modeling method moves the number
A 2021 study in Automation in Construction (Esnaashary Esfahani et al.) compared scan-to-BIM models built from the same data in different ways. Some were modeled by hand and some with software help. The researchers found that the method itself changed how closely the finished model matched the scan.
What it means for you: a LOA target belongs in the modeling scope, not only the scanning scope. We write the target into the instructions our modelers work from, and we check the finished model against the scan before it reaches you.
The scanner spec is not the job accuracy
Scanner brochures list accuracy under ideal conditions. A 2022 study by Kersten and Lindstaedt in Applied Geomatics tested several terrestrial laser scanners both in a lab and in the field. Their results show that real job accuracy depends on more than the spec sheet. Distance to the surface, the angle of the beam, the surface material and how well the scans are tied together all play a part.
What it means for you: judge a scan by its registration report, not the scanner's brochure. The report shows how well the scans were tied together on your building, on the day it was scanned.
LOA and LOD answer different questions
Architects already know LOD, the level of development that says how much detail a model element carries. LOD answers "what is modeled, and in how much detail?" LOA answers "how close is it to the real building?" A wall can carry a high LOD (layers, finishes, openings) and still sit 2 inches from its true position. Good scopes state both.
Which level to ask for
- Remodel or addition, permit set: LOA30 point cloud, LOA20 Revit model. This covers most single-family and small commercial work.
- Kitchens, built-ins or tight interior fits: the same defaults, plus field-verified dimensions taken from the point cloud at the critical walls before cabinets are ordered.
- Structural tie-ins, steel or prefab: LOA40 on those elements. That needs survey control, which we can add to the scope.
- Early massing or feasibility: LOA20 or looser is usually enough, and it keeps the fee down.
Higher accuracy costs more field and modeling time, so ask for it only where a decision depends on it. Our cost guide shows how scope moves the price, and our first post, Decide this before you scan, covers how to set the scope in the first place.
Sources
- U.S. Institute of Building Documentation. Level of Accuracy (LOA) Specification. usibd.org.
- Esnaashary Esfahani, M., Rausch, C., Sharif, M., Chen, Q., Haas, C. and Adey, B. (2021). Quantitative investigation on the accuracy and precision of Scan-to-BIM under different modelling scenarios. Automation in Construction. doi:10.1016/j.autcon.2021.103686.
- Kersten, T. and Lindstaedt, M. (2022). Geometric accuracy investigations of terrestrial laser scanner systems in the laboratory and in the field. Applied Geomatics. Open access: doi:10.1007/s12518-022-00442-2.
- BIMForum. Level of Development (LOD) Specification. bimforum.org.
Summaries are our own reading of each source. We do not quote the standards; see the originals for exact definitions.
Not sure which level your project needs?
Our free As-Built Risk Check is a one-page checklist that helps you decide what to ask for before you spend anything on a scan. You can forward it to your principal or the owner.