E3S Web of Conferences 415, 03022 (2023)
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https://doi.org/10.1051/e3sconf/202341503022

Landslide Analysis combining Laser Scanning andPhotogrammetry
Pedro Pinto*1, Juliana Barros2, Marisa Pinheiro2 ** * , Roberto Tomás3, Tiago Miranda, and Eduardo Pereira1* *1ISISE, University of Minho, 4800-058 Guimaraes, Portugal
2Level, a Geotechnical Management Company, 4700-727 Braga, Portugal
3Civil Engineering Department, University of Alicante, Spain.

Abstract. After a sequence of weeks of heavy raining, a catastrophic mass movement occurred in the parishof Palmeira de Faro, Esposende (N Portugal) on November 23, 2022. The collapsed slope presented amaximum height of about 21 m and a length of 45 m. In order to document and analyse this case from thegeotechnical perspective, a general survey was devised including the use of aerial photography, laser
scanning and conventional topographic survey methods. The results obtained showed that these techniques
provide very accurate and abundant information that is essential to support a comprehensive documentationof the features and variables of the slope. The mechanisms involved in this catastrophic collapse are still
under investigation, and causes such as the excessive rainfall in the previous months or geologic conditions
are still being considered, although currently the existing information seems to indicate for a combinationof debris flow / rock slide driven event.

1 Introduction
Nowadays there seems to exist an increasing trend forthe occurrence of unexpected mass movements andextreme weather events, very likely related to climatechanges. The possibilities of current technologies are
particularly interesting in this context. Photography has
been used for quite some time as a source of data formapping and interpreting landforms, and the extractionofquantitative
elevation
information
fromstereophotography, under the umbrella term ofphotogrammetry [1, 2]. Also, spurred by developments
in traditional ground surveying, such as the advent ofdifferential GNSS and reﬂectorless, robotic totalstations, the acquisition of topographic data has beentransformed most signiﬁcantly by a new generation ofremote sensing technologies, that have transformed thedigital elevation modelling and geomorphologicalterrain analysis [3]. For instance, terrestrial laser
scanning [4] and automated digital photogrammetry [2,5] in particular, have revolutionized the quality of digital
elevation models (DEMs), extending their spatial extent,
resolution, and accuracy [3]. Moreover, since its
emergence, automated aerial and close-range digital
photogrammetry has become a powerful and widelyused tool for three-dimensional topographic modelling[3], that can be acquired from a range of cheap,
lightweight platforms on which imaging sensors aredeployed such as drones [6].
The slope under the study was located in the northregion of Portugal, in the Parish of Esposende (fig. 1).
According to Geological Chart of Portugal with scale
1:50000 – sheet 5-C (Barcelos) is composed essentiallyby a monzonitic granite predominantly biotitic, withporphyroid texture and medium grain, sometimes coarse[7]. As discussed in a previous publication [8], this slope
suffered a catastrophic landslide after a particularly
rainy autumn, and caused two casualties. The site iscomplex, with strong slope discontinuities, vegetation,trees and the presence of houses and otherinfrastructures.

Fig. 1. Rockslide location, in Palmeira de Faro, Esposende,and orthomosaic showing the original slope crest, the sliding
plane, and the area where the debris accumulated.

* Corresponding authors: id10434@alunos.uminho.pt;* * eduardo.pereira@civil.uminho.pt;* * * marisapinheiro@level-gam.com© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 03022 (2023)
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https://doi.org/10.1051/e3sconf/202341503022

2 Methodology

3 Results

Considering that the slope was mostly a heavily
weathered granitic rock mass, with visible fracturing
and residual soil involving large blocks, besides thegeometrical information it was important to document
the orientation of the discontinuity families in the
distinct parts of the cut slope. Initially, the intention was
to use photogrammetry to obtain the overall geometricrepresentation, not only of the sled slope but also of theadjacent cut slopes that remained intact, as well as thesurroundings. Laser scanning was adopted with the aimof obtaining a high-resolution representation of thediscontinuities visible at the surface of the cut slopesthat remained intact, which apparently were lessweathered.

3.1 Photogrammetry

2.1 Photogrammetry
The photogrammetric survey was performed using a DJI
drone (Inspire 2) equipped with a 20 Mpix resolution
camera (DJI MFT 15 mm, F1.7, 3.28 µm), and a GNSS
rover receiver (Unistrong, G970-II) in RTK phasedifference mode, with a reference station approximately
30 km away (Braga). The flight plan was set to ensure
70% and 80% of lateral and longitudinal photography
overlap, respectively, 55 m maximum height above
ground, a coverage area of 4.90 ac, and pixel resolutionof 1.25 cm (fig.2). For calibration of the radial andtangential image distortion of the sensor, andgeoreferencing, eight ground control points (GCPs)
were used (fig.2 b)), as a fundamental step to determine
the quality of the DEM in the absolute reference frame[6].
Regarding the 3D terrain model generation, aproprietary software (Metashape® Professional - version1.84), was used for dense cloud construction with 2D
images. The resulting 3D model was then used for
integrated geotechnical analysis of the terrain geometry,including the extraction of topographic heights andother features such as slopes and their orientations. For
this purpose two open-source tools were used, QGIS®
and CloudCompare®.

Fig. 2. Bundle adjustment of photo positions and alignment,after point-matching, for cloud densification andgeoreferencing using all pixels of the 372 images (sparse cloudin view, for clarity) a); colorized dense cloud withapproximately 94 million points b); photo texturized 3D modelin Northeast-Southwest c), and West-East d) viewing angles,
not to scale.

Based on the GCPs and two validation points (VPs)
surveyed with a GNSS receiver, it was possible to obtaina root mean square error (RMSE) of 4.19 cm (XY: 3.87cm, Z: 1.61 cm) for the GCPs and 10.77 cm ((XY: 2.71cm, Z: 10.42 cm) for the VPs. Since both RMSEs
obtained for XY (horizontal plane) were similar, andconsidering that VPs were placed in unfavourable
locations (too close to vertical walls and subject tohigher error due to light reflectance and increased
neighbour ground pixel saturation) and do not represent
the overall sliding area, it was admissible to consider anoverall uncertainty of ± 5 cm. Using Metashape®
processing workflow, it was possible to retrieve theDEM (fig.3 a)) and the ortho-rectified mosaic (fig.2 b)),which were later exported to QGIS for contour, slope,and aspect extraction (fig.3 a), c-d)). Based on theresults obtained it was possible to confirm that thetopographic height of the sliding plane varied between
97 m and 118 m, the dip varied between 35º and 40º, andthe aspect set towards SSW direction, at approximately
190º of azimuth.

2.2 Laser Scanning
The laser survey was carried out using the Faro Focus
350 Laser Scanner series equipment. In order to survey
the rocky slope, 8 sweeps were carried out. These scans
were distributed in order to guarantee the partial
overlapping of the scans and allow the subsequent
seamless stitching of all scans in a unique bundle. As
expectable, the higher the overlapping the lower thescanning error. The scans were then processed using
FARO® SCENE software. After aggregating the various
scans and cleaning up all the “noise” captured aroundthe rocky slope, the point cloud was generated.
Afterwards, CloudCompare ® and Revit ® were used toperform all the subsequent analyses, including cloudmeasurements and the automatic identification ofdiscontinuities, which was one of the goals of this study.

3.2 Laser Scanning
The aggregated point cloud showed an average error of
13.8 mm, and the maximum error was somewhat higher,
approximately 35 mm, and occurred between the scansin the extremities of the surveyed area. It should be
noted that, for safety reasons and some limitations of thein-situ conditions, it was not always possible to position

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the equipment in the most favourable locations. The
model obtained is shown in Fig.4.

Therefore, the point cloud was cleaned using
CloudCompare®, and a Matlab routine designated
Discontinuities Set Extractor (DSE) was used toperform the identification [8]. As a result, a 3D
visualization of the rock slope with the discontinuity sets
identified was obtained (fig. 5a)), as well as their poledensity in stereographic representation (fig. 5b). Table 1
presents the values of dip and dip direction of theprincipal poles (selected according to the major
representative) – (J1 to J4).

a)b)Fig. 5. Automatic identification of discontinuities: a) 3D
visualization of rock slope with Cloud Compare®, with theplanes of discontinuities identified using DSE routine; b)
stereographic representation of principal discontinuities poles(J1, J2, J3 e J4).
Table 1. Most representative discontinuities poles arepresented using dip and dip direction values and respective
colors as identified in fig. 5a.
Discontinuities
setJ1
J2J3J4Dip Dip Direction
Color of each set(º)
(º)
Blue
78179
Green
38
350
Yellow
67
60
Red
65
288

3.3 Combined analysis
Fig.6 shows the statistical analysis of the monthly
rainfall time series. The results show that, after a long
period of severe draught, two months of exceptional
rainfall have preceded the catastrophic event. Therefore,
precipitation has very likely influenced the mass
movement observed. The visual inspection of the site
has revealed that large blocks and soil in large amounts
were forming the moved mass. Although the accident
seems to show characteristics of both debris/landslide
and rock movement, the final causes are still underinvestigation. The results allowed, in general, todistinguish the most favourable characteristics andweaknesses of both techniques used in the present casestudy. Regarding the photogrammetry approach using
the drone, limitations were related to the use of passive
sensors that depend on lighting and weather conditions,as well as sensor spatial, spectral, and radiometricresolution. The number and quality of ground control
points also influence the quality of results, since theaccuracy of the positioning system of the drone istypically insufficient to attain the best results. However,drone-based
photogrammetry
presents
severaladvantages, such as the great cost-effectiveness, thepossibility to access difficult or high-risk areas, fastresults, high spatial and temporal resolutions, highaccuracy scale, aerial view, automated data collection,
open-source software availability and easiness of use.

Fig. 3. Digital elevation model (DEM) with a spatial resolutionof 5 cm/pixel, and ± 5 cm of uncertainty, with contour lines at
0.50 m spacing a); ortho-rectified photo mosaic based on DEM
and cartographic projection ETRS89/PT-TM06, where X+ at
East, Y+ at North and Z+ the height above mean sea level, withground control points (GCPs) and validation points (VPs) for
reference b); terrain slope c) and aspect d).

The point cloud obtained by laser scanning wassubsequently used to automatically identify thediscontinuity sets of the slope, in order to help
understanding the possible causes of the landslide
incident.

Fig. 4. 3D visualization of the coloured point cloud obtainedby laser scanning on CloudCompare (x – red, y – green, z –blue) on CloudCompare®.

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Fig. 6. Monthly rainfall statistics in the region since 1980.

In this particular case study, due to the presence ofnumerous terrain obstacles such as houses, walls, andvegetation, these advantages were essential to allow theproduction of an overall high-resolution model of thearea, very useful to extract the slope features alreadyidentified.
Conversely, the effectiveness of ground-based laser
scanning was limited by the ground obstacles that often
didn’t allow to position of the scanner in the optimal
locations, leading to the creation of shadow areas andresulting in less-than-optimal beam orientations withrespect to the scanned area. However, excluding
extreme conditions, the laser scanner showed to be lesssensitive to light and weather conditions, delivering
more consistent results. Due to this, laser scanning
results of the slope cut were the ones selected for theautomatic identification of the discontinuities.

This study was funded by COMPETE2020 and FEDER funds
in the scope of the GeoCrit project (POCI-01-0247-FEDER047173).

4 Discussion and conclusions

5.
6.

References
1.

2.
3.

4.

The combined interpretation of the results obtained
allowed to understand that the orientation of thediscontinuity sets with respect to the orientation of theslope cut was highly unfavourable, and very likely
related to the landslide occurred. The photogrammetry
results clearly helped to identify the formation of analmost perfectly flat sliding surface at the top part of theslope, which was determining the sliding kinematics.
The dip and orientation of this flat surface perfectly
matched the dip and dip orientation identified in thestereographic representation of the discontinuity sets,obtained by automatic identification using the laser
scanning results. Therefore, the combined use of bothtechniques showed to be particularly advantageous in
the description and analysis of the landslide that
occurred. The analysis of the monthly rainfall timeseries also showed that exceptional rainfall preceded bya long period of severe draught may also have caused
the observed mass movement. Therefore, a mix of debris
flow and rockslide may be considered as well, as one
form to describe the instability observed.
The use of multiple sensors and platforms to acquire andmeasure sites or structures were highly complementary.
For obtaining a more precise identification of the
discontinuity plans and movements, laser scanningshowed to be the best option. The combination of bothmethods was quite opportune, in this case, because it
allowed obtaining good documentation of the scenario
while taking the most out of both approaches.

7.

8.

4

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