The Project
Trilogy was engaged to undertake a 3D dilapidation survey of a heritage-listed structure as part of a pre-construction documentation program for adjacent development works. The objective was to create a defensible, spatially accurate record of the structure's pre-construction condition — capturing existing cracking, surface defects, material condition, and structural geometry — and to deliver a 3D model suitable for use by heritage consultants, structural engineers, and conservation designers.
Heritage structures present specific challenges for condition documentation. Access constraints frequently prevent close-range inspection of upper-level facades, roof structures, and parapets. Traditional photographic condition surveys capture condition at eye level but miss the geometry — they cannot produce the measured drawings, cross-sections, or as-built records that heritage conservation work requires. A UAV-derived 3D model addresses both limitations simultaneously.
A 3D digital twin of a heritage structure captures geometry and condition in a single dataset — producing the as-built record, the measured surfaces, and the condition documentation simultaneously. It is queryable, reproducible, and comparable: future surveys can be aligned to the same model to detect movement, crack propagation, or material loss.
Survey Methodology
Flight Planning. The survey flight plan was designed to achieve complete 360° coverage of the structure — nadir imagery from altitude for plan geometry, and systematic oblique/façade-parallel passes at close range for each elevation. Flight altitude and standoff distance were calibrated to the structure height and the required ground sampling distance for defect detection. All airspace notifications were lodged through CASA's Drone Safety App prior to flight.
Photogrammetric Processing. Overlapping imagery from all passes was processed using photogrammetric software to generate a dense point cloud and a textured 3D mesh model of the full structure. The model captures surface geometry at millimetre-level resolution — sufficient to detect and measure crack widths, material delamination, mortar joint condition, and structural deformation. The model is georeferenced to GDA2020, enabling direct registration with cadastral and planning datasets.
Condition Mapping. Key defects identified in the imagery were mapped directly onto the 3D model and annotated in the condition report — linking the spatial location of each defect to its photographic and dimensional record. This approach produces a condition report in which every finding can be located precisely on the 3D model, rather than described by reference to approximate elevation or a hand-drawn sketch.
Deliverables
- Textured 3D mesh model — georeferenced to GDA2020, viewable in standard 3D model viewers, suitable for import into Revit, ArchiCAD, or heritage conservation design software
- Dense point cloud — full structure coverage in LAS/LAZ format, suitable for cross-section extraction, measured drawings, and structural analysis
- Orthophoto elevations — geometrically corrected, scaled facade images for each elevation, directly usable as the base for heritage condition mapping drawings
- Condition report — annotated with defect locations referenced to the 3D model, photographic record of each finding, and dimensional measurements where accessible
- Baseline dataset — the georeferenced model constitutes a permanent pre-construction baseline; any future survey can be registered to the same coordinate system to detect movement or deterioration
Value for Adjacent Development
Where a heritage structure is adjacent to proposed development works — excavation, piling, demolition, or heavy construction — a pre-construction dilapidation survey creates a defensible baseline that protects both the developer and the heritage asset owner. If a claim of construction-induced damage arises, the 3D model and condition report establish pre-existing condition with spatial precision that a traditional photographic survey cannot match.
For Trilogy projects where geotechnical investigations are also underway — monitoring the potential for vibration or ground movement from construction activities — the 3D dilapidation survey complements the geotechnical monitoring program, providing above-ground structure condition data alongside below-ground movement data from inclinometers and settlement markers.