# Past, Present and Future Monitoring at the Vallcebre Landslide (Eastern Pyrenees, Spain)
## 3 Discussion  
As the different systems were applied to certain common points, the resulting displacements could be compared, and practical data, including advantages and drawbacks, could be derived. For the superficial methods, we observed that the GPS measurements showed better trends and stability than the Total Station measurements, at least with the Valleebre conditions and equipment. An example is given in Figure 21. Moreover, the GPS surveying is \"all-weather\" in practice, which is an additional advantage when working in mountainous areas. A theoretical composition of errors, along with independent checks of the GPS-RTK results obtained at fixed stable points, let us establish the following standard deviations [5]: 16 mm in the horizontal plane and 24 mm in elevation. The GPS network in Vallcebre is still in use. Figure 21: Apparent change in the behavior of point G2 when monitored with classical surveying (dots 1 to 12) or with GPS (dots 12 to 18). The insets with the error ellipses for both systems show a more balanced and better precision for the GPS measurements. Blue arrow points towards the Total Station. Figure 20: L.O.S. displacements measured with noninterferometric GBSAR between February 2010 and September 2011 at eight small CRs. The CR14, in a stable zone, acts as reference CR. The GPS measurements were also used to check inclinometer and wire extensometer displacements at several boreholes [4]. As an example, in Figure 22, a cross-check is presented. In Valleebre, the inclinometric readings were possible only until 200 mm of total displacement for the top of the boreholes, due to the casing deformation in the failure zone. Nevertheless, the wire extensometers and GPS measurements have been continued without trouble beyond these figures. Compared with the standard GPS (campaign by campaign), the extensometric technique has the advantage of being an automatic and continuous measuring system which can be correlated with rainfall and piezometric heads. On the other hand, the GPS yields the direction of the movement and the \(\Delta\)Z as well as the \(\Delta\)Distance. It is worth noting that the GPS can also be installed and operated in continuous (see [18] for instance). Cross-checking was also useful for the radar measurements, validating or improving the remote sensing techniques. Every GPS campaign included the CR and the borehole heads (Figure 23). For instance, in Figure 24 a phase unwrapping problem is recovered. The figure shows two displacement time series of CR_4. The continuous blue line represents one phase unwrapping solution, which has an accumulated displacement of 6.9 cm and which shows an upward displacement of 1.1 cm between April and June 2008. Considering the kinematics of the landslide, this type of movement can be considered very unlikely. For this reason, a second solution was chosen, depicted with a dashed line and which shows an accumulated displacement of 11.9 cm in 11 months. The same figure shows the displacement measured by the wire extensometer S5 (green curve), which is located relatively close to CR_4. It is interesting to compare the time series of CR_4 (dashed line) and S5. They display quite a similar temporal evolution, which includes a strong deformation gradient between June and July, followed by a stationary period until September and a second, less strong deformation gradient. This similarity confirms the one phase shift applied to the CR_4 curve. The curves do not fully overlap because one point is 84 m from the other. Figure 22: Comparison of the corrected wire extensometer displacement (D\({}_{\text{h}}\)) against the GPS and the inclinometric measurements of the Valleebre borehole S2. The GBSAR amplitude technique presented in Section 2.6 was also validated against the continuous monitoring given by the wire extensometer system. Prior to performing a proper comparison, the CR5 and CR7 Line-Of-Sight (L.O.S.) results (Figure 20) has to be projected onto the total displacement vector (estimated from GPS measurements in the area).