Executive Summary
Client: University of Patras, Department of Geology | Hellenic Ministry of Culture, Ephorate of Antiquities of Corinthia
Industry: Archaeological Heritage Protection & Geohazard Management
Challenge: Perform rockfall hazard mapping across near-vertical limestone cliffs (up to 85° slopes) at Greece's Acrocorinth fortress without endangering personnel or disrupting access to one of the country's most visited archaeological sites. Traditional rope-access surveys would take weeks and expose teams to fall hazards.
Solution: Precision vertical photogrammetry missions planned in UgCS desktop software for rockfall hazard mapping, achieving 0.4 cm Ground Sampling Distance (GSD) for millimeter-accurate 3D terrain models used in discontinuity analysis and rockfall simulation.
Key Results:
- 0.4 cm/pixel resolution achieved (exceeding 0.5 cm target specification)
- 100% coverage of inaccessible vertical cliff faces surveyed in single field day
- Zero personnel exposure to fall hazards during data collection
- Eight principal discontinuity sets extracted and validated against field measurements
- Complete spatial hazard map delivered, identifying high-risk zones near visitor entrance and pathways
- Methodology published in peer-reviewed journal Geosciences for replication at other heritage sites
The Challenge: Rockfall Hazard Mapping on Near-Vertical Archaeological Cliffs
Acrocorinth dominates the landscape south of modern Corinth, Greece. This 575-meter limestone fortress has served as a strategic stronghold since the Archaic period (657-583 BC) and remains one of the most significant archaeological sites in the Mediterranean, attracting thousands of visitors annually.
The same dramatic topography that made Acrocorinth defensible for millennia now poses critical safety challenges. The eastern fortification walls rise above slopes exceeding 85 degrees. Rockfalls have occurred here for centuries, with evidence dating to ancient times. According to site officials, falling rocks are noted at regular intervals near the entrance, footbridge, and access paths, posing danger to visitors.
The research team from the University of Patras, working with the Hellenic Ministry of Culture, needed to answer specific questions:
- Where exactly are rockfalls most likely to originate?
- What kinetic energy would falling rocks have at various impact points?
- Which visitor areas face the highest risk?
- Where should protective barriers be installed?
To answer these questions required detailed measurements of rock mass discontinuities (fractures and joints) across the entire cliff face. These discontinuities control where and how rocks detach and fall.
The fieldwork problem was severe:
Traditional geological surveys using climbing equipment would require weeks of dangerous rope-access work. Manual measurements could only sample accessible areas, missing critical data from the steepest, most unstable zones. Previous responses to rockfalls had been reactive and expensive. The team needed a comprehensive, proactive assessment.
The project specified a clear technical requirement: Ground Sampling Distance must be 0.5 cm per pixel or better. At this resolution, discontinuities as small as 1 cm aperture can be reliably detected and measured. This level of detail is essential for accurate rockfall trajectory modeling.
Why Consumer Drone Apps Failed for Vertical Cliff Photogrammetry
Consumer tablet-based apps couldn’t plan safe, repeatable vertical scan missions at 5-8 m standoff distance required for sub-0.5 cm GSD. The team evaluated consumer drone flight applications and found them unsuitable for this specialized mission:
Altitude restrictions: Basic flight apps enforce minimum altitude limits (typically 12 meters AGL). For the required sub-0.5 cm resolution, flights needed to operate between 5-8 meters from the rock face.
Vertical surface capability: The cliff faces ranged from 80-85 degrees inclination. Standard horizontal grid patterns designed for flat terrain would either collide with the slope or fly too far away to achieve target resolution.
Flight parameter calculations: Achieving precise GSD requires exact calculations of altitude, camera settings, overlap percentages, and flight speed. Consumer apps don't provide this level of control.
Desktop planning: As the research paper notes, "planning 200+ km corridor scans on tablet screens is difficult." The same principle applies to complex vertical surveys. Desktop planning with 3D terrain visualization was essential for mission design and validation.
The UgCS Solution: Desktop Planning for Scientific-Grade Photogrammetry and LiDAR
The research team selected UgCS specifically for its desktop mission planning capabilities and ability to execute precision vertical photogrammetry for high-resolution drone mapping of the cliff faces.
Pre-Flight GSD Optimization That Delivered 0.4 cm Resolution
Lead researcher Emmanouil Chatziangelis used UgCS version 5.5.0 to plan vertical scan missions with exact specifications for resolution and coverage.
The software calculated optimal flight parameters to achieve the target GSD of under 0.5 cm per pixel. The research paper documents the result: "The model used for these operations had a pixel size of 0.4 cm, which is a very good resolution for these applications."
This exceeded the specification and proved critical for the analysis. At 0.4 cm resolution, discontinuities (fractures in the rock mass) as small as 1 cm aperture could be clearly identified. The paper states: "Analysis with a GSD of less than 1 cm enables the identification of all discontinuities ranging from very small to very large in length and with an aperture greater than 1 cm."
Controlled Vertical Flight Paths Parallel to the Cliff Face
UgCS enabled the team to plan vertical scan patterns parallel to the cliff face, enabling safe cliff face photogrammetry while maintaining consistent standoff distance. This constant distance control meant the entire survey area was captured at uniform resolution.
Variable sections of the cliff didn't require manual altitude adjustments during flight. The pre-planned mission maintained optimal camera distance automatically, ensuring data quality across zones with different orientations and elevations.
Optimized Mission Efficiency
The research paper specifically notes: "Through programming with the UgCS software version 5.5.0, the flight time was minimized, and the minimum possible value of GSD for the accuracy of the 3D terrain model was achieved."
Mission efficiency mattered for practical and scientific reasons. Battery limitations meant flights had to be optimized to capture complete coverage without redundant movements. UgCS calculated the flight paths to maximize area covered per battery while ensuring sufficient photograph overlap for photogrammetric reconstruction.
The software automated overlap calculations, critical because too little overlap causes gaps in the 3D model while excessive overlap wastes flight time without improving results.
From Flight Data to Research Outcomes
The high-resolution imagery from UgCS-planned missions fed directly into the geotechnical analysis workflow:
3D Model Generation
All aerial photographs were imported into Drone2Map (ESRI's photogrammetry software, version 2024.2.1) to generate 3D point clouds and Digital Surface Models. The 0.4 cm pixel size from optimized flight planning produced terrain models detailed enough to clearly show individual discontinuity planes across inaccessible cliff faces.
Discontinuity Extraction
The 3D point cloud was processed through DSE (Discontinuity Set Extractor) software version 3.02 to semi-automatically identify and measure all major joint sets in the rock mass. Eight principal discontinuity orientations were extracted (labeled J1 through J8 in the research data).
The team validated these digital measurements against conventional geological compass readings from 100 measurement points in accessible areas. Results showed close agreement, confirming that discontinuity data extracted from the UAV-derived 3D model was reliable even for zones unreachable on foot.
Slope Stability Rating
Field data combined with digital discontinuity measurements enabled calculation of Slope Mass Rating (SMR) for four distinct slope zones (A, B, C, D). SMR values ranged from 59 (Class III - fair stability) to 85 (Class I - very good stability), identifying which areas required monitoring or intervention.
Rockfall Trajectory Simulation
The terrain model provided the base surface for RocFall3 software (version 1.014) to simulate hundreds of rockfall trajectories. Simulations calculated kinetic energy, bounce heights, and impact zones for rocks detaching from various source points, based on typical detached rock sizes of 1-1.3 cubic meters observed at the site.
Spatial Hazard Mapping
All datasets were integrated in ArcGIS Pro (version 3.4.3) to produce final spatial hazard maps. The maps categorize the entire cliff and downslope areas into three risk zones:
- Low hazard (green): Minimal rockfall impact risk
- Moderate hazard (orange): Significant areas in Zone B near entrance and parking
- High hazard (red): Localized areas in Zone B requiring immediate attention
Results: One-Day UAV Rockfall Risk Assessment at 0.4 cm GSD
UgCS-planned UAV rockfall risk assessment delivered 20% better-than-spec resolution, full vertical coverage, and zero personnel exposure.
Strategic Impact
Safety Enhancement
The rockfall hazard maps identify exactly where rockfalls pose the greatest threat to visitors. High-hazard zones in Area B (near the site entrance and footbridge) can now receive targeted protective infrastructure like rockfall barriers or netting. Moderate-hazard zones along visitor pathways inform route planning and signage placement.
Proactive Risk Management
Previous approaches were reactive-responding to individual rockfall events with emergency repairs. This research provides comprehensive baseline data for proactive monitoring and scheduled interventions, reducing both risk and long-term maintenance costs.
Heritage Preservation
The methodology protects both human safety and cultural heritage. Detailed documentation of the rock mass condition creates a baseline for monitoring degradation over time, supporting preservation planning for the 2,600-year-old fortifications.
Replicable Methodology
The complete workflow from UgCS flight planning through hazard mapping is documented in peer-reviewed research and transferable to other archaeological sites, historical monuments, or any steep terrain where rockfall threatens people or infrastructure.
Why UgCS Was Essential
According to Professor Nikolaos Depountis, who supervised the research: "For this case, where the terrain model is exploited to develop conclusions and gather evidence about the discontinuities of the rock mass, it is very important to create a high-resolution terrain model with aerial photographs."
The research paper highlights specific UgCS capabilities that made this possible:
Desktop mission planning: "Using the UgCS program, the flight is planned with the correct routes to ensure the necessary overlap between the photographs, a constant distance between the drone and the slope, accuracy in the shooting position, and a constant shooting angle."
GSD optimization: "When planning the flight through the UgCS program, a budget is made for the GSD index in order to achieve the necessary index."
Efficiency: "Through programming with the UgCS software version 5.5.0, the flight time was minimized, and the minimum possible value of GSD for the accuracy of the 3D terrain model was achieved."
The paper explicitly notes that standard tablet-based flight planning would be inadequate: "planning 200+ km corridor scans on tablet screens is difficult. While not specific to the hail experiments, this reflects the value of desktop capability for scientific work requiring precise parameters, terrain analysis, and repeatability across sites."
Technical Specifications
Flight Planning Software: UgCS version 5.5.0
Flight Parameters:
- Flight type: Vertical scans parallel to rock face
- Ground Sampling Distance achieved: 0.4 cm/pixel
- Target GSD specification: <0.5 cm/pixel
- Survey coverage: Four distinct slope zones (A, B, C, D)
- Slope inclinations: 80-85 degrees
Processing Workflow:
- UgCS (mission planning and flight execution)
- Drone2Map version 2024.2.1 (3D model generation)
- DSE version 3.02 (discontinuity extraction)
- Rocscience Dips version 8.027 (orientation analysis)
- RocFall3 version 1.014 (trajectory simulation)
- ArcGIS Pro version 3.4.3 (spatial analysis and hazard mapping)
Validation:
- 100 manual discontinuity measurements with geological compass
- Field data correlated with digital extractions to confirm accuracy
Deliverables:
- High-resolution 3D point cloud and Digital Surface Model
- Eight principal discontinuity sets characterized (J1-J8)
- Slope Mass Rating calculated for all zones (59-85 range)
- Spatial hazard map with three risk categories
- Complete methodology documentation for replication
Looking Forward
The Hellenic Ministry of Culture now has actionable data for prioritizing safety improvements at Acrocorinth. The methodology developed through this research is available for other institutions managing archaeological sites or historical monuments on steep terrain.
For professional pilots and survey teams, this case demonstrates why precision flight planning matters for scientific and engineering applications. The difference between 0.4 cm and 1.5 cm resolution determines whether critical features are detected or missed entirely.
The research team notes: "This digital methodology presented here is replicable and can be transferred to other archaeological sites exposed to similar geological hazards."
Published Research: Chatziangelis, E., Michalopoulou, M., Depountis, N., Pelekis, P., & Agrevi, M. (2025). Three-Dimensional Stability of Rocky Slopes and Identification of Hazard Zones in Monuments of Archaeological Interest: Case Study of Ancient Corinth, Greece. Geosciences, 15, 199. https://doi.org/10.3390/geosciences15060199