PRECISION FOR THE FUTURETwo different questions: LOD and LOA
A model element answers two separate questions:
- How developed is it? Is this wall a generic placeholder, a wall of a specific type and thickness, or a fully detailed assembly? That is the Level of Development (LOD), defined in the BIMForum LOD Specification.
- How accurately does it match the real building? Is the wall within a few inches of where it actually stands, or within a fraction of an inch? That is the Level of Accuracy (LOA), defined in the U.S. Institute of Building Documentation (USIBD) LOA Specification.
For new design, LOD alone is often enough, because the model is the intent. For existing conditions captured from a laser scan, the building came first, so an as-built model also needs a stated accuracy. A beautifully detailed LOD 300 wall can still be a foot out of place.
LOD 100 to 400, in plain terms
| LOD | The element is… | Scan-to-BIM example (a wall) |
|---|---|---|
| 100 | A symbol or generic representation; it shows that something exists, not its size or shape. | A massing block for the building. |
| 200 | A generic system or object with approximate quantity, size, shape, location and orientation. | A generic wall of approximate thickness, roughly placed. |
| 300 | A specific system or object whose quantity, size, shape, location and orientation can be measured directly from the model. | A wall of a defined type and thickness, placed to the scan. |
| 350 | LOD 300, plus interfaces with adjacent or dependent elements that can be measured. | The wall with its openings, connections and penetrations resolved. |
| 400 | Represented with detail sufficient for fabrication, assembly and installation. | Studs, layers and fabrication-level detail; rare for existing conditions. |
LOD 500 is the one most relevant to Scan-to-BIM, and the most misread. In the 2025 specification it means an existing or as-constructed condition represented through observation, field verification or interpolation, with its level of accuracy noted. It is not a step above LOD 400: it describes where the geometry came from (observation rather than design), and BIMForum points to the USIBD LOA Specification for stating its accuracy. That is exactly why an LOD 500 element needs an LOA.
LOD is set per element category, not for the whole model: structural walls at LOD 300 and furniture at LOD 100 is a normal specification.
LOA10 to LOA50: the accuracy bands
The USIBD Level of Accuracy Specification expresses accuracy as a range at a stated confidence level, and it treats two kinds of accuracy separately:
- Measured accuracy: how accurate the scan or survey data is.
- Represented accuracy: how closely the modelled element follows that data.
Represented accuracy can never be better than measured accuracy. A model built from an LOA30 point cloud cannot honestly be called LOA40.
| LOA | Upper range | Lower range | Typical use |
|---|---|---|---|
| LOA10 | see current spec | 5 cm (about 2″) | Massing, rough planning, space programming |
| LOA20 | 5 cm | 15 mm (about 5/8″) | Floor plans and general as-builts |
| LOA30 | 15 mm | 5 mm (about 3/16″) | Design and renovation documents, MEP coordination |
| LOA40 | 5 mm | 1 mm | Fabrication interfaces, equipment and steel tie-ins |
| LOA50 | 1 mm | 0 | Precision industrial and metrology work |
A renovation spec might read: walls, floors, structure and openings at LOA20 represented, from LOA30 measured data; MEP mains at LOA20; finishes and furniture at LOA10. The right bands depend on what the model will be used for.
How to write the scope
The clearest Scan-to-BIM scope is a small table: one row per element category, with its LOD, its represented LOA and any notes. For example:
| Category | LOD | LOA (represented) | Notes |
|---|---|---|---|
| Walls, floors, roofs | 300 | LOA20 | Model walls plumb unless deviation exceeds the stated tolerance |
| Doors and windows | 300 | LOA20 | Family by type; sizes from scan |
| Structural columns and beams | 300 | LOA20 | Visible members only |
| MEP mains | 200 | LOA10 | Pipes and ducts above a stated diameter |
| Casework and fixtures | 200 | LOA10 | Generic families |
Alongside the table, state:
- Modelling rule for out-of-plumb or out-of-level elements: model the as-built geometry, or model ideal geometry within a tolerance.
- What is hidden: scanners only capture visible surfaces, so anything above ceilings or inside walls needs separate access or is out of scope.
- Software and template: Revit version, the client template, family and naming standards, and whether IFC is also required.
- Registration report: the scan registration accuracy that sets the measured LOA.