Existing Conditions Modelling
Mô hình hiện trạng
Create a 3D model of an existing site or building using laser scanning (point cloud) or traditional survey, as a foundation for new design or renovation.
Common tools
Leica/Faro scanner, Autodesk ReCap, Revit
Value delivered
Reduces survey errors; provides accurate foundation for renovation and expansion design
Adoption in Vietnam
Common in large renovation and infrastructure projects (metro, stations, factories). Rarely needed for new civil buildings.
Level of Information Need (LOIN)
G = Geometry · A = Alphanumeric · D = Documentation (ISO 7817-1:2024 method). A blank cell means this use sets no specific requirement for that discipline/stage.
| Discipline | Concept | Schematic | Developed | Technical | As-built / Operation |
|---|---|---|---|---|---|
| Architecture | G3·A1·D0 | — | — | — | — |
| Structure | G2·A1·D0 | — | — | — | — |
| Mechanical | G2·A1·D0 | — | — | — | — |
| Electrical | G2·A1·D0 | — | — | — | — |
| Plumbing | G2·A1·D0 | — | — | — | — |
Critical stages: Concept
Rationale: Captured pre-design (ST1) by survey / laser scan. Existing building & topography modelled to as-captured precision (G3 = LOD300, industry sweet-spot for scan-to-BIM); existing structure and utilities to approximate G2. A1 = type/category of existing assets; D0 because no design documentation is needed at survey stage.
What it really is
Existing Conditions Modelling is the creation of a BIM model of a real, existing building or site from actual survey data — laser scanning that produces a point cloud, or traditional topographic survey with a total station and GNSS — rather than guessing from old as-built drawings that rarely still match reality after repeated repairs and additions. Unlike "rebuilding a 3D model from memory or from an archived drawing", this process starts from direct measurement: registering the individual scan stations into one coordinate system, verified against an independent geodetic control network, before parametric BIM objects are built on top of that registered point cloud. The end product is not a nice point cloud to present — it is a model whose positional and geometric reliability is stated explicitly, with measured registration error and modelling error attached, so design teams doing refurbishment, extension or infrastructure work can use it as a reference base without re-surveying themselves. The core value is reducing the risk of a wrong design decision because the surveyed existing condition did not match reality — a common cause of quantity overruns and disputes on refurbishment projects in Vietnam, where old as-built records are often missing or unreliable.
When to use
Most worthwhile for refurbishment, extension, or design built on an existing structure or infrastructure — where old as-built drawings are unreliable or no longer exist: factories, stations, infrastructure assets, heritage buildings, or works altered repeatedly without records. For a genuinely new building on vacant land this use is usually unnecessary; an ordinary topographic and geotechnical survey suffices. Do it early — before refurbishment design begins — because the later an existing-condition discrepancy is caught, the more expensive the design and construction rework becomes.
Prerequisites
- •EIR/BEP stating what the existing conditions model will be used for (refurbishment design, clash checking, condition record) — the purpose determines the required LOIN and registration accuracy, not modelling by feel
- •An independent geodetic control network (coordinate and elevation benchmarks) established for the site or building, not relying solely on the scanning software's automatic registration algorithm
- •Full site access, including areas still in operation, roofs, basements and plant rooms — agreed in advance on timing and safety when scanning inside an occupied building
- •Staff able to operate the scanner, register the point cloud, and model from a point cloud — three different skills; missing any one produces a wrong model that nobody catches
Inputs
Appointing party · .pdf / .docx
Survey task team · .xlsx / .pdf
Appointing party · .pdf
Survey task team · phần cứng đo đạc
Appointing party · .dwg / .pdf
Lead appointed party · .pdf / .docx
Outputs
Registered point cloud
.rcp / .e57 → Modelling task team · design task team
Accepted when: Registration error within the agreed tolerance, verified independently against the geodetic control network, not just the software's internal residual report
Existing conditions model
gốc (.rvt) + IFC → Design task team · lead appointed party
Accepted when: Meets the agreed LOIN for its intended use, coordinates aligned to the project's shared origin, not modelled beyond the level actually needed
Error verification report (registration and modelling)
.pdf / .xlsx → Appointing party · BIM Manager
Accepted when: States the registration error between scan stations, the error against independent control, and the model-to-cloud deviation — not a single blended figure
Existing-condition drawings extracted from the model (plans, sections)
.pdf / .dwg → Appointing party · design task team
Accepted when: Consistent with the accepted model, sufficient as a base for refurbishment or extension design
General workflow
Plan the survey and set out the geodetic control network
Conduct a preliminary site walk to identify areas hidden from scanner line-of-sight (equipment, furniture, occupied zones), then set out independent coordinate and elevation control benchmarks before scanning — these are used later to check registration error, not to perform the registration itself.
Survey task team · Total station and GNSS RTK → Independent geodetic control network
Scan the site
Set up scan stations per the plan, ensuring overlap between adjacent stations reaches at least about 30% so registration approaches the scanner's own ranging accuracy. Supplement with photographs or tape measurements in areas the scanner cannot reach.
Survey task team · Terrestrial laser scanner (Leica/Faro/Trimble) · Autodesk ReCap Pro (on-site quick check) → Raw scan stations
Register the point cloud
Merge the scan stations into a common coordinate system using cloud-to-cloud or target-based registration, then align to the project's shared origin. Do not stop once the software reports a low residual — an internal residual is not a substitute for the independent check at the next step.
BIM Coordinator · survey task team · Autodesk ReCap Pro → Registered point cloud (.rcp)
Verify registration error
Compare the registered point cloud's coordinates against the independent geodetic control surveyed in step 1 — this is a mandatory quality gate, not an optional step. If the error exceeds the threshold agreed in the EIR/BEP, re-scan or re-register; never adjust the figures to make them fit.
Survey task team · BIM Coordinator · Autodesk ReCap Pro (registration QA report) · control-coordinate comparison sheet → Registration error report
Build the existing conditions model to the agreed LOIN
Import the registered point cloud into authoring software and build parametric BIM objects to the LOIN agreed for the intended use — model only what affects a design decision, not every bolt, if the purpose is merely spatial clash checking.
Modelling task team · Revit (importing the .rcp point cloud) → Work-in-progress existing conditions model
Verify the model against the point cloud
Compare modelled objects against the source point cloud to catch objects that are misaligned, missing, or spuriously added. This is a second, independent quality gate from the registration gate in step 4 — low registration error does not guarantee the model itself was built correctly.
BIM Coordinator · Revit (deviation analysis against the point cloud) · CloudCompare (supplementary check, open source) → Modelling deviation report
Publish the existing conditions model for design
Publish the model that has passed both quality gates to the CDE, with the error reports attached, so the design team can use it as a reference base instead of re-surveying or guessing from old drawings.
BIM Coordinator · appointing party · Forma Data Management (CDE) → Published existing conditions model
Diagram
↺ A model failing the second gate against the agreed LOIN also loops back to modelling — never lower the acceptance criteria to save schedule
Common pitfalls
The finished model is off from reality by a few centimetres to tens of centimetres when checked on site
Cause: Registration relied only on the automatic matching algorithm, with no independent geodetic control network to cross-check — cumulative error across many scan stations went undetected
Fix: Set out an independent geodetic control network before scanning and always verify registration error against it, not just the software's internal residual report
Point cloud registration fails or shows large error in geometrically complex areas such as stairwells and plant rooms
Cause: Overlap between adjacent scan stations fell below the required threshold, or targets were missing in areas with plain, uniform surfaces that give the algorithm too few features to lock onto
Fix: Ensure at least about 30% overlap between adjacent stations and add artificial targets in plain-surface areas
The existing conditions model is modelled down to every bolt and bracket, yet the schedule slips and the file is too heavy to use
Cause: The LOIN for the intended use was never fixed before modelling; the modelling team defaulted to "more detail is always safer"
Fix: Fix the LOIN to the actual intended use (refurbishment design, clash checking, condition record) in the EIR/BEP before modelling, and model only objects that affect a design decision
The appointing party has to demand a full re-scan after discovering missing elements
Cause: The scan plan skipped a preliminary site walk and never identified areas hidden from the scanner by equipment, furniture, or occupied zones
Fix: Walk the site and plan scan station positions in advance, accounting for occluded areas that need supplementary photographs or manual measurement
The existing conditions model does not align with the new design model at coordination
Cause: The existing conditions model kept the scanning software's internal coordinate system and was never aligned to the project's shared origin
Fix: Align the existing conditions model to the origin and coordinate system agreed in the BEP before publishing it to the CDE
Measuring effectiveness
Compare RMS residual between overlapping scan stations, cross-checked independently against the geodetic control network
Benchmark: Target-based registration approaches the scanner's own ranging accuracy once overlap between adjacent stations reaches at least about 30% — per the literature review in a 2025 peer-reviewed study; not a single absolute error figure valid for every scanner
Objects passing divided by total objects, checked by comparing the model against the point cloud at the modelling-verification step
Benchmark: no independent benchmark — set an internal target
Measure the mean and maximum distance between modelled object surfaces and the source point cloud on the same elements
Benchmark: no independent benchmark — set an internal target
Person-hours from completed scanning to a model that has passed both quality gates and been published
Benchmark: no independent benchmark — set an internal target
Legal basis
There is NO specific requirement for existing conditions modelling in Decree 217/2026/NĐ-CP — this use falls under Article 8 generally. Specifically: Art. 8(1)(a) makes BIM mandatory for new-build works Grade II and above from the feasibility study or economic–technical report stage — for refurbishment or extension of Grade II+ works, existing conditions modelling is the first step toward meeting that obligation; Art. 8(2) assigns scope, content and information requirements to the contract, the basis for fixing the LOIN, registration accuracy and point density required for the existing conditions model in the EIR and BEP. The practical consequence: if the appointing party does not write the accuracy requirement into the contract, there is no legal basis to hold the surveyor to any specific error threshold — this is a CONTRACTUAL obligation, not a separate statutory one.
Sources
Only official sources (legislation, standards) and peer-reviewed academic work are cited. No vendor marketing figures or press sources. Reference only — does not replace legal advice.