Scan to BIM and as built drawings: a practical workflow for construction

What are as built drawings and why do they matter?

As built drawings (also called as-builts, as built plans or record drawings) are the final set of drawings that show a project as it was actually constructed. They capture the real position of walls, columns, openings, pipes, ducts and cables, including every deviation from the design drawings.

Design drawings describe intent. On site, dimensions shift, routes for services change, and contractors resolve clashes in the field. If those changes are not recorded, the owner inherits a building whose documents no longer match reality.

Accurate as-builts are used for:

• Handover and facility management – locating shut-off valves, cable routes and access panels.
• Renovation and extension – designing new work that fits the existing structure.
• Final account and dispute resolution – proving what was installed and where.
• Regulatory approval – many authorities require a final as-built survey before occupation or before utilities are adopted.

What is an as built in construction: drawings, surveys and models

In construction, "as built" can mean three related deliverables: marked-up drawings, a measured survey, and a 3D model. Knowing which one the contract requires avoids expensive rework later.

An as built site survey on infrastructure or land development usually records levels, kerbs, manholes, pipe inverts and building footprints against the site control. Typical as built survey requirements specify the coordinate system, the tolerance for each feature type, the survey date and the surveyor's certification. A 3D as built model adds geometry and data for every element, which is where BIM comes in.

What is BIM and how does BIM work?

BIM (building information modelling) is a process for creating and managing a shared digital model of a building or asset, in which each element carries both 3D geometry and data. A wall in a BIM model is not just lines; it knows its type, materials, fire rating and the level it sits on.

The BIM information model is the combination of geometry, attributes and documents for the whole project. Drawings, schedules and quantities are generated from the model rather than drawn separately, so a change in one place updates every view.

BIM is used across the AEC (architecture, engineering and construction) industry:

• BIM in architecture – design development, visualisation and coordinated drawing sets.
• BIM in construction – clash detection, 4D sequencing, quantity take-off and construction management.
• BIM for infrastructure – roads, bridges, tunnels and utilities, often combined with GIS and survey data.

The open exchange standard IFC (Industry Foundation Classes) allows BIM data to move between different BIM software platforms without losing element information. Ask for IFC deliverables when you need vendor-neutral as-built models.

How does the scan to BIM workflow work?

Scan to BIM is the process of capturing an existing building or structure with a 3D laser scanner and converting the resulting point cloud into a BIM model. A typical point cloud to BIM project follows six steps.

1. Define the scope. Agree the area, the elements to model (architecture, structure, MEP services), the level of detail and the accuracy tolerance.
2. Establish control. Set out survey control with a total station or GNSS so scans can be placed in the project coordinate system and checked.
3. Scan on site. Place the scanner at planned stations with enough overlap (commonly 30 % or more between neighbouring scans), plus targets where required.
4. Register and clean. Combine the scans into one point cloud using targets or cloud-to-cloud matching, then remove noise, people and vehicles.
5. Model. Trace walls, floors, beams, pipes and equipment in BIM modelling software, using the point cloud as the reference.
6. Check and deliver. Compare the model with the cloud (deviation analysis), then issue the model, 2D as built drawings and the registered cloud.

The method behind step 3 is explained in our guide to terrestrial laser scanning, and step 4 in point cloud processing and registration.

Which equipment do you need for laser scanning as-built surveys?

An as built laser scanning job needs a scanner for geometry and a control instrument for coordinates. The right combination depends on the size of the site and the tolerance.

3D laser scanner

A terrestrial scanner records millions of points per station. Our terrestrial 3D laser scanner measures up to 2 million points per second, with a standard range of 0.5–70 m or 0.5–150 m on white surfaces and a 13 MPx HDR camera for colourising the cloud. Its 4.4 kg weight makes it practical for indoor plant rooms and stairwells.

Scanning total station

A scanning total station combines total station measurement with 3D scanning in one instrument, which suits sites where control and scanning are done by one person. Our scanning total station covers a 360° × 282° field of view and is Class 1 eye-safe under IEC 60825-1.

Total station or GNSS for control

Control points are usually measured with a total station indoors and GNSS outdoors. An Android instrument such as the STSA9 total station, with 2" angle accuracy and 1500 m reflectorless range, works well for traversing through a building and coordinating scan targets.

How accurate are 3D as built drawings from scanning?

The accuracy of 3D as built drawings depends on the whole chain: scanner, registration, control and modelling. The model is never more accurate than the weakest step.

• Scanner range noise – terrestrial scanners typically measure to a few millimetres at short and medium ranges; noise grows with distance and on dark or shiny surfaces.
• Registration error – small errors accumulate across many stations; tying scans to total station control limits that drift.
• Modelling tolerance – modellers usually fit idealised elements (straight walls, round pipes), so real out-of-plumb walls may be simplified unless the specification asks for true geometry.

Specify the tolerance per element group, for example tighter for structural grids than for ceiling finishes. A level of development (LOD) or level of accuracy specification in the contract makes the expectation measurable. Always ask for a deviation report that compares the final model with the registered point cloud.

BIM software, file formats and BIM data management

Scan to BIM software falls into three groups: registration tools, point cloud viewers and BIM authoring tools. Many teams use one package for registration and another for modelling.

Good BIM data management means a common data environment where models, clouds and drawings are versioned and access-controlled. Agree naming conventions and coordinate systems before the first scan, not at delivery.

Scan to BIM services or in-house: sourcing the equipment

Firms that deliver as-builts regularly often find it more economical to own a scanner than to outsource every job to scan to BIM modelling services. Outsourcing still suits one-off projects or peak workloads.

Mars Geosystems supplies terrestrial scanners, scanning total stations and Android total stations from Shanghai with a minimum order of one unit, OEM options for distributors and one-year warranty with remote support. Compare scanner classes in our 3D laser scanner buying guide, then request a quotation with your project scope.

Frequently asked questions