Brief
The acropolis of Polyneri is a Byzantine fortification whose walls and towers were captured at high spatial resolution to support preservation and restoration studies. The objective was a complete and geometrically faithful record of the monument: a 3D mesh, plan and elevation orthomosaics, sections, and a topographic plan — all on the Greek national grid (EGSA87) with millimetre-level internal accuracy.
The work was structured into three field days at the monument and roughly thirty office days for processing, mosaicking, CAD integration, and packaging deliverables.
Method
Reference network
Five reference stations were established on rock with steel nails and red spray markers, then measured by RTK GNSS (60 epochs each) against the Hexagon SmartNet network. Recorded RMS error per station was a few millimetres in EGSA87.
Ground control
Photo-targets were distributed across the site for absolute orientation of the photogrammetric model. Targets and auxiliary points were observed by total station from four occupations; 137 detail points were captured in total, including geometric features of the fortification (wall edges, levels) and check points used to validate the network. Internal errors on the targets were below 1 cm, with theoretical precision in the 5–10 mm range.
Aerial capture
Two automated UAV missions covered the site at different scales. A site-wide overview at 37 m altitude (≈ 1 cm/pix) provided context. The high-resolution mission flew at 12 m altitude with 80 % forward and 70 % side overlap, combining nadir and 45° oblique imagery to produce a robust geometric base. Manual flights at roughly one-metre standoff provided façade-level imagery.
Terrestrial capture
A Nikon Z6 III with a 24–70 mm f/4 S lens, hand-calibrated at the 24 mm focal length, was used for surfaces that the UAV could not reach safely.
Photogrammetric solution
Image alignment was performed in Agisoft Metashape Pro on a workstation. After relative orientation against the on-board RTK metadata, the model was geo-referenced from the surveyed ground-control points; the UAV’s RTK metadata was deliberately discarded for the final solve, since the targets carry better absolute accuracy and are tied to the same reference network as the rest of the project.
The bundle adjustment converged with a control RMSE of 1.05 cm and a check-point RMSE of 0.35 cm — comfortably below the 1 cm target for this class of monument. Reprojection error was 2.43 px on 17.9 million projections from 6,682 images.
Mesh and orthomosaics
Depth maps were generated at high quality with aggressive filtering, producing a shaded mesh of approximately 292 million faces and 146 million vertices. The mesh was sliced for elevation and section processing; per-face local frames let the orthomosaics resolve at 2 mm/pix without distortion. Each orthomosaic was cleaned by re-projecting alternative source images where coverage or shadowing degraded the result.
CAD integration and topographic plan
Cleaned orthomosaics were brought into CAD with their photogrammetric scale preserved on X and Y, then vertically aligned using the surveyed elevations. A plan-view orthomosaic at 3 mm/pix and a DEM at 10 cm/pix were converted to contour lines, combined with the 137 surveyed points, and rendered as a complete topographic drawing.
Lightweight 3D tiled model
A 3D tiled model at 5 mm/pix was generated as a portable summary of the site: full georeferenced geometry plus colour from the source imagery, suitable for stakeholder review without the weight of the full mesh.
Deliverables
- Full 3D mesh exports in
.obj,.ply, and.tls - Elevation and section orthomosaics at 2 mm/pix
- Plan orthomosaic at 3 mm/pix
- DEM at 10 cm/pix and contour-derived topographic plan in CAD
- 3D tiled model at 5 mm/pix in EGSA87
- Coordinate registers and Metashape processing report