Brief
A photogrammetric and topographic survey of part of the Pyrgos Ragiou fortification, carried out to produce a complete, geometrically faithful record of its standing masonry: elevation orthomosaics, a plan orthomosaic, a digital elevation model, a 3D mesh, and CAD drawings, all on the Greek national grid (EGSA87).
Fieldwork took place on 20 September 2026; processing in the office followed from October 2026.
Method
Control and total-station survey
Two instrument stations (S1, S2) were established in EGSA87, and the survey was carried out from S1 with a Topcon GM-55 reflectorless total station. Ten photo-targets were observed at distances of 15–31 m. Six of them served as control points in the photogrammetric solution and one as an independent check point; the remaining three were measured as auxiliary heights and kept out of the final solve. Theoretical precision of the target coordinates is about 2–3 mm, relative to station S1.
UAV capture
Flights used the wide-angle camera of a DJI Mavic 3 Enterprise (RTK). Three automated missions — nadir at 23 m and 12 m, and a 45° oblique block at 12 m — gave the solution high overlap and a robust geometric base. Manual flights at a median altitude of 6.8 m, at angles from 4° to 90°, covered the wall faces, sections, and fine detail. A separate overview flight at 37 m (529 images, about 1 cm/pix) provided site-wide context. The final solution used 1,706 images.
A Nikon Z6 III was also used on the ground for hard-to-reach spots (291 images). Those frames were not needed for the solution and are archived.
Photogrammetric solution
Images were processed in Agisoft Metashape Professional. 1,705 of 1,706 images aligned, producing 694,739 tie points. Absolute orientation came from the surveyed photo-targets, entered at 5 mm accuracy, while the UAV’s own RTK positions were kept at low weight only to support the solution. This was chosen because Metashape’s built-in WGS84 → EGSA87 transformation is an older three-parameter model, whereas the current official transformation is a seven-parameter model with a correction grid.
The control points reach an RMSE of 3.2 mm (2.5 mm horizontal, 2.0 mm vertical), and the independent check point shows an error of 1.3 mm. The software’s image-position deviation from the UAV’s RTK measurements averages 1.69 m, and is not a measure of model accuracy.
Mesh, orthomosaics, and DEM
Depth maps were computed at high quality with aggressive filtering and used to build a shaded mesh of 47.5 million faces. The mesh was sliced to produce the elevation orthomosaics, each in its own local reference frame at 2 mm/pix and cleaned by choosing the best source images locally. Two elevations were produced: elevation 1 (about 34.4 × 4.9 m) and elevation 2 (about 6.2 × 3.9 m). The plan orthomosaic covers about 40.4 × 21.6 m at 3 mm/pix, and the overview flight gives a site-wide orthomosaic at 1 cm/pix.
Plan orthomosaic of the surveyed section at 3 mm/pix, georeferenced in EGSA87.
Elevation 1 — about 34.4 × 4.9 m at 2 mm/pix.
Elevation 2 — about 6.2 × 3.9 m at 2 mm/pix.
Deliverables
- 3D mesh (.obj) and a tiled 3D model (.tls)
- Elevation orthomosaics at 2 mm/pix, with world files and local reference-frame definitions
- Plan orthomosaic at 3 mm/pix and a site-wide orthomosaic at 1 cm/pix, both georeferenced in EGSA87
- DEM at 2 mm/pix and contour lines
- Plan, elevation, and section drawings in CAD
- Coordinate register of stations and control points, raw total-station measurements, the Metashape project, and the processing report