Prism Data
Monday 26 October 2009 by Thomas Lebourg

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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 outside the landslide. The geographic locations of the targets are indicated on the first figure. From 2003.5 to 2006.5 (the latest year for available data), we dispose of raw data we analyzed in order to obtain reliable results about displacements. Thus we calculated for each target and each time step the Lambert coordinates and the corrected distance at the tacheometer.

From these data, this is possible to calculate a target average velocity over the 3 measurement years. This value is plotted on the second figure. All the velocity values allow to get a whole view of the recent kinematics of the landslide. In a general way, more the spots are distant from the bottom of the landslide more their velocities are high. The faster area is the one called North-East lobe. Some areas (lower landslide and main landslide) are homogeneous with regard to the norm and the direction of the velocity. Some others (upper parts of the landslide) much more heterogeneous (from the nearest points) show velocities which directions and norms are rather different.

Repartition of the targets on the landslide (background: DEM provided by the CETE, 2002)


Annual plane average velocities (North and East), calculated between around 2003.5 and 2006.0, for all the targets and GPS stations of the landslide. Comparative scale: C29 velocity = 30.5 cm/year, C67 velocity = 56 cm/year

The comparison between targets (prisms) and GPS is performed first concerning close sites. Thus, CLP1 and the C29 target, less than 10 meters separated, point out very close motions, that are nearly superimposed. Their velocities are very highly correlated, especially long-term ones.

Comparison of the velocities (in meters per year with regard to the reference) of CLP1 (in blue color) and of the C10 and C29 targets (respectively in green and red colors) with regard to the tacheometer

Except the corrected distance (which is sufficient enough by itself for monitoring), the coordinates calculated thanks to the tacheometer are less accurate and present much more interferences than the ones obtained with the GPS (deviation between 15 and 20 mm in planarity and 25 to 30 mm vertically). This moderate accuracy is first of all due to natural conditions (harsh weather, sighting angles sometimes very high, shadows). Concerning robustness, the targets are properly aimed at on average 70% of the days, which is well inferior to the GPS system robustness rate (more than 90% of the calculated days).