A definition that survives contact with vendors
A digital twin is a digital representation of a physical asset that is kept current by data flowing from the real thing. The test is the feedback loop: if the model only describes design intent and nothing updates it from the field, it is a model — valuable, but not a twin. A construction twin earns the name when site reality (progress, as-built geometry, quality records, sensor data) continuously updates the digital side, so decisions made against it are decisions made against the truth.
The maturity ladder
It is more useful to talk in levels than in absolutes. Level 1: a coordinated 3D design model (BIM) — design truth, static. Level 2: the model plus linked field records — progress, photos, inspections tied to elements, updated on a cadence; most real "twins" on active projects live here. Level 3: near-live state from reality capture (scans, 360° imagery) reconciled against design — the as-built diverges from the as-designed and the twin shows where. Level 4: sensor-fed operational twins used to run the completed asset. Each level costs more to sustain and pays differently; the mistake is buying Level 4 language for a Level 1 artefact.
What a twin costs to keep alive
The model is the cheap part. The expensive part is the discipline that keeps the twin truthful: capture routines that actually happen, element-level record keeping, someone reconciling scan-to-model divergence, and governance over who updates what (ISO 19650’s real subject). A twin that stops being updated silently becomes the most dangerous artefact on the project — an authoritative-looking picture of a site that no longer exists.
Where the value actually lands
On live projects the proven value concentrates in three places. Dispute defence: an element-level record of what was built, when, with photographic evidence, is the spine of an EOT or variation claim. Progress truth: reconciling claimed against observed progress before certifying payment. Handover: an owner who receives a live-maintained twin instead of a box of PDFs starts operations months ahead. All three are records problems more than graphics problems — which is why teams with disciplined site records get twin value cheaply, and teams without them buy expensive software that decays.
The honest starting point
Do not start with a platform purchase. Start by making element-level site records live: progress confirmed against the drawings, photos and QA tied to locations, survey and scan data on a cadence the project can sustain. That data layer is 80% of a working twin, it pays for itself through claims and progress certainty immediately, and every twin platform you might buy later feeds on it.