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[Observatoire Gravitaire Géoazur] Tacheometry & GPS
Tacheometry & GPS

Research Studies by S. El Bedoui and T. Lebourg, Geoazur, UNS, Nice, France ; J.L. Pérez, LRPC, CETE Méditerranée, Nice, France ; P. Valty, LAREG, ENSG, Marne la Vallée, France.

The fast evolution and above all the ’mass movement’ feature have required the monitoring and the instrumentation of the site. Especially this is based upon a topographic monitoring of the slope surface displacements. An innovating and autonomous system relies on infrared tacheometric measurement (Leica tacheometer) for the three dimension positioning of prism reflectors installed on the slope. The recording stations are settled on the opposite slope of the landslide (Auron slope). The accuracy of the measurement is assessed to be around 0.5 to 1 cm for distances reaching more than 1,500 m. The recorded data are communicated for analysis to the Centre des Etudes Techniques de l’Equipement by radio waves. They are available at the OMIV at t + 1year (CETE Geoazur convention).

The CETE Méditerrannée has been in charge of the monitoring of the landslide since 1982. In 1990, a monitoring system by automatic tacheometer was installed: two stations located on the opposite slope of the landslide pointed at 47 prisms situated inside the landslide or on the estimated stable part of the slope. In 2001, one of the tacheometers was removed: nowadays only one of them remains installed and is located on the Saint Maur site. It achieves automatic measurements every four hours. Since its implementation it has shown very fast displacement velocities but floating with sudden peaks (till 10 cm per day in 1987, and 30 cm in some upper areas of the landslide), especially depending on weather conditions, but also on other more complex hazards.

The measurements are completed by displacement calculations by photogrammetry and DEM comparisons, and recently by a GPS survey. Within the framework of experiments conducted by the European project RETINA, four GPS stations were installed by Geoazur in 2003. Their first goal was to test a cheap and real time means for monitoring landslide movements. The GPS databases are stored by the permanent GPS French national network RENAG.

A warning plan ORSEC can be launched in case of movements assessed as critical. Impressive because of its dimensions and of the materials volume which threatens to collapse, the landslide has been monitored very soon, in a manual way first, then thanks to more and more sophisticated techniques. It was used as a full size experiment for some developing techniques (DORIS at the beginning of the 1990s, automatic tacheometric measurements, photogrammetric measurements, 3D scanner).

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article rubrique
Monday  26 October 2009
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GPS calculations
GPS data processing was performed with Gamit software (release 10.32) developped by MIT (Massachussetts Institute of Technology). One of the future purposes is to succeed in automatic (...) Read the whole article
Monday  26 October 2009
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Prism Data
The official monitoring of the La Clapière landslide is performed thanks to an automatic tacheometer which measures angles and distances for around forty targets located inside and (...) Read the whole article
Friday  20 November 2009
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Contribution of topometric methods to the landslide feature analysis
The velocity field obtained from targets and GPS can be used to determine the main deformation areas over the 2003.5 - 2006.5 period on the La Clapière site. For that purpose, we proceed (...) Read the whole article
Monday  26 October 2009
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Mechanisms of the landslide movements: displacement and velocity study between 1997.5 and 2006.5
The calculation of the motions of all targets of the La Clapière site between 1997.5 and 2006.5 allows to get a more global point of view concerning the whole movement of the landslide. (...) Read the whole article
Friday  4 December 2009
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Conclusion
So, the study of the La Clapière landslide movement with the help of mono frequency GPS enables to answer initial questions concerning the relevancy and quality of such monitoring. This (...) Read the whole article