# Wireless Sensor Network-Based Rockfall and Landslide Monitoring Systems: A Review
## 3 Rockfall and Landslide Monitoring Based on WSNs
### Examples of Non-WSN-Based Monitoring Systems  
WSNs are not the only possible method for monitoring rockfalls and landslides. Generally, we can identify two main categories of monitoring methods: remote sensing and ground-based methods. Synthetic-aperture radar (SAR) and interferometric synthetic-aperture radar (InSAR) are used to achieve models with accuracy up to a millimetric scale [52]. Infrared thermography (IRT) remote sensing methodology is based on the measurement of the radiant temperature of a surface from a long distance and can provide important parameters such as temperature maps, emissivity, and the humidity of an area. Those are all useful in landslide and rockfall prediction systems [53]. Other physical quantities like temporal displacement, soil moisture content, and type of material may be measured with high resolution using the terrestrial laser scanning (TLS) sensing technique [54; 55; 56]. The usage of unmanned aerial vehicles (UAVs) has been studied in the literature in many applications. We also found contributions to natural disaster prevention, as the authors of [57] report. These devices can be successfully employed in management strategies for evacuation plans and in risk assessment, but as suggested, UAVs can be used in coverage enhancement for wireless networks. The use of optical very high-resolution (VHR) data in conjunction with analytical methods enables monitoring systems to discriminate between different water contents and vegetation covers [58]. Satellite interferometry [59] and laser scanning are other technologies used for remote land-movement monitoring [60; 61]. These solutions offer good performance for movement assessment but, in some cases, are not accessible and have moderate costs. Geographic information system (GIS) data can be used for a preventive risk assessment. In [62], a best-fit method was used to generate the probability maps for landslides by combining different GIS data layers. Optical fiber (OF) can be used locally for geohazard monitoring in strain and temperature sensing. The possibilities include underground deployment [63] and overground cable installations [64]. The accuracy assessment shows that the models obtained using OF could detect potential rockfalls with an accuracy greater than 90 percent. Sensory systems can be developed locally in an area subject to risk using inexpensive instrumentation, also in wired implementation. In [65], a low-cost infrastructure wired-sensor network was presented. The nodes were accelerometer-based and acted as a warning when predetermined acceleration values were exceeded. A similar hybrid wireless-wired \begin{table}
\begin{tabular}{c c} \hline \hline
**Pros** & **Cons** \\ \hline Energy efficiency & Limited range (relative to the application) \\ \hline Flexibility and scalability & Some implementations can be subject to interference \\ \hline Installation accessibility & Security \\ \hline Wiring constraint reduction & Power constraints \\ \hline Cost effectiveness & Possible data amount and speed limitations \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pros and cons of WSNs. system for rock collapse forecasting was presented in [66], where a controlled area network (CAN) bus was used for wired communication with a base station and Wi-Fi [67] for communication between the base station and a control room. Sensor-less networks were also studied to assess if the soil conditions could be analyzed for early warning purposes using the reception parameters of a radio transmission [68], as those vary with the characteristics of the soil in sub-ground installations. The diagram in Figure 3 shows a list of remote and local monitoring systems for landslide and rockfall geohazards. Structural health monitoring (SHM) is a connected aspect of geohazard events. The authors of [69] reported a review of WSNs applied to SHM. Several works were investigated in detail regarding the hardware architecture, in particular, that of the sensing interfaces and radio communication units.