TERRESTRIAL LASER SCANNING AND DIGITAL PHOTOGRAMMETRY
TECHNIQUES TO MONITOR LANDSLIDE BODIES
G. Bitelli, M. Dubbini, A. Zanutta
DISTART - University of Bologna, Viale Risorgimento 2 — 40136 Bologna, Italy
e-mail: (gabriele.bitelli, marco.dubbini, antonio.zanutta)@mail.ing.unibo.it
Commission V, WG V/2
KEY WORDS: LIDAR, Classification, Correlation, Registration, DEM/DTM, Orthoimage, Rendering, Accuracy
ABSTRACT:
Photogrammetry and laser scanning, thanks to significant development in last years, are comparable surveying techniques to
generate - without object contact and with a precision commensurate with scale - Digital Terrain Models (DTMs), a fundamental
tool to detect, classify and monitoring landslides. The traditional way to survey the territory for landslide detection purposes is aerial
or, in some cases, terrestrial photogrammetry, that permits to carry out 3D models of the terrain and, by means of DTMs
comparisons, to realise multi-temporal studies. The massive introduction of modern digital photogrammetric workstations, with
automatic matching procedures, allows for a rapid DTM production for landslide monitoring activities. On the other hand, the 3D
reconstruction of the terrain with terrestrial laser scanning methods is another modern way to reproduce the natural surface of the
ground with high accuracy and high automation. There are however some open problems concerning the elaboration of the data and
the procedures to generate DTM starting from Digital Surface Models (DSMs), taking off vegetation, buildings, etc. The present
work describes the terrestrial laser scanning and photogrammetric surveys realised on a small landslide body located on the Northern
Apennines in Italy (municipality of Vergato, Bologna), an interesting case in order to test the laser scanning capabilities and the
procedure of laser data processing, also in comparison with photogrammetry.
1. INTRODUCTION
The multitemporal monitoring of a landslide is a fundamental
tool for its knowledge and the prediction of its possible spatial
or temporal evolution. Several surveying methodologies are
used to investigate the activity of existing landslides. Two main
classes are identifiable: point based (Total Station, GPS) and
area based techniques (Photogrammetry, Laser Scanning and
Remote Sensing, in particular spaceborne radar interferometry).
It is well known that the geodetic methodologies like
triangulation and distance measurements with electronic
instruments permit a very high accuracy. By means of GPS
(Global Positioning System) or Total Stations it is possible to
detect movements on the order of mm/yr or cm/yr, and estimate
the boundary of the landslide area. The three dimensional
reconstruction of the surface of the landslide can be done
interpolating dense profiles realised walking inside the body of
the landslide; this operation requires however long on-the-field
procedures and in some cases is not allowed due to local
accessibility problems.
There are also several promising Remote Sensing techniques
used to generate DEMs for landslide monitoring and
characterized by high level of accuracy, in particular thanks to
the new generation of high-resolution satellite imagery and
mostly Interferometric SAR (Synthetic Aperture Radar).
Photogrammetry is the technique most commonlv used for this
kind of survey, permitting to reconstruct the three dimensional
landslide shape with great wealth of information and to study its
3D evolution over time. The development of digital
photogrammetry offers today new possibilities and innovative
procedures, like the creation of DSMs in automatic mode for
the reconstruction of surfaces and the generation of
orthoimages.
Laser Scanning is a surveying method conceived more than ten
years ago that in a short time can supply DSM (Digital Surface
Model) and good quality DTM as a result of elaborations with
specific procedures that apply specific filtering and
interpolation algorithms; only nowadays this technique is
having growing development and applications thanks to
information technology improvements. A telemeter laser
obtains the 3D reconstruction of objects; the measure of the
distance from the ground derives generally from the time
employed by the laser beam to go and to come back (time-of-
flight principle). While airborne laser scanning (using airplanes
or helicopters) constitutes one well established method
landslide surveying (Barbarella, Lenzi, Gordini, 2003),
terrestrial laser scanning (TLS) can be investigated as an
alternative or complementary methodology.
In the present research a traditional photogrammetric technique
and a Terrestrial Laser Scanning method have been applied and
compared to investigate Ca di Malta landslide (figure 1) located
in the municipality of Vergato, near Bologna (Italy). The area is
located in northern Apennine on the eastern slope of Reno
River Valley. The average slope is about 13 degrees and the
area about 40,000 square meters. The landslide affect strongly
deformed units known in literature as argille scagliose (Pini,
1999). This melange is widespread in the entire Apennines
mountain range. The composition in the area is mainly clayey.
The area of study was adopted because of some reasons:
- the landslide is monitored from some years by GPS
measurements, evidencing in many cases movements on the
order of 1-2 cm/month (Mora ct al., 2003);
- historical photogrammetry and geological photo interpretation
studies of the area are yet in progress from the authors (Baldi et
al., 2002);
- the lacking in vegetation makes the site suitable for
photogrammetry and one-echo laser surveys.
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