When a project requires an as-built BIM model of an existing building, project teams face a choice: commission a laser scan and derive the model from point cloud data, or hire a drafter to manually produce the BIM from field measurements and existing drawings. Both approaches can produce a Revit model: but the differences in accuracy, speed, and long-term value are substantial.
How Scan-to-BIM Works
In a scan-to-BIM workflow, a laser scanning team captures the building with a terrestrial scanner, producing a dense point cloud of every visible surface. The point cloud is then registered, cleaned, and loaded into Revit as a reference layer. A BIM specialist models the building elements: walls, floors, ceilings, structural members, MEP systems: by tracing the scan data, ensuring that every modeled element matches the physical reality of the building.
How Manual BIM Drafting Works
In a manual drafting workflow, a field crew takes measurements on site using tape measures, laser distance meters, or a total station. Those measurements are then used by a BIM technician to construct the model in Revit. If existing drawings are available, the technician may also use them as a starting reference, noting discrepancies found in the field.
Accuracy
Scan-to-BIM delivers significantly higher accuracy than manual drafting. The point cloud: captured at ±1–2 mm precision: is the authoritative reference, and the modeler works directly from measured data. Walls that aren't plumb, floors that aren't level, and structural elements that deviate from drawing geometry are all captured accurately.
Manual BIM drafting depends on the quality and completeness of field measurements. Experienced surveyors can achieve good results, but cumulative measurement error, missed dimensions, and reliance on assumptions make manual models inherently less precise: particularly in complex or irregular spaces.
Speed
For large or complex buildings, scan-to-BIM is typically faster from field capture to final deliverable. A laser scanning crew can document an entire floor in hours; a manual measurement crew may take several days for equivalent coverage. Modeling from a point cloud is also faster than manual BIM drafting because the drafter doesn't need to interpret ambiguous field notes or return to site for missed dimensions.
For small, simple spaces, manual drafting may be comparable in speed and lower in cost: the mobilization overhead of a scan crew may not be justified for a single small room.
Completeness
Point cloud data captures everything the scanner can see. Modelers working from scan data have full visibility into the space: they can check any dimension, verify any condition, and model elements that a manual survey might have overlooked. Manual documentation is selective by nature: crews record what they think is relevant, and gaps only become apparent when the model is used in design coordination.
Rework and Error Costs
When a BIM model contains errors (whether from measurement inaccuracy or missed conditions), the cost of correction shows up downstream as design rework, RFIs, or field conflicts. Scan-to-BIM models, grounded in dense measured data, produce fewer of these downstream surprises. Manual BIM models are more susceptible to errors that don't surface until coordination begins.
When Each Approach Makes Sense
- Scan-to-BIM: Best for moderate-to-large projects, complex or irregular buildings, renovation and adaptive reuse, and any project where accuracy and completeness are critical to coordination success.
- Manual BIM drafting: May be appropriate for small, simple projects where the scope is limited, existing drawings are reliable, and the cost of a scan mobilization isn't justified by project scale.
Laser Scan Chicago: Scan-to-BIM Specialists
Laser Scan Chicago provides end-to-end scan-to-BIM services: from field capture through final Revit model delivery: for projects across the Chicago metro area. Our models are built from verified point cloud data and delivered to your specified Level of Development. Contact us to discuss your project and get a quote for scan-to-BIM documentation.