Global Positioning Systems and digital photogrammetryfor the monitoring of mass movements:application to the Ca’ di Malta landslide(northern Apennines, Italy)

A combination of digital photogrammetry and Global Positioning Systems (GPS) surveying has been used to measure landslide surface displacements and to estimate the volume involved in the movement. Ninety-eight percent of landslides mapped as active in the Emilia-Romagna region of northern Italy are reactivations of dormant mass movements. The Ca’ di Malta landslide, south of Bologna, was chosen to test this integrated system. A 0.5-m cellsize Digital Elevation Model (DEM) with vertical accuracy of 0.1 m was generated using digital photogrammetric techniques. We have used a combination of digital photogrammetry and three GPS techniques to monitor landslide movement. Rapid static surveying in which the survey observations are made for a period of minutes yielded sub-centimetre positions for several marked points distributed on a longitudinal axis along the landslide. Kinematic surveying, in which the observations are carried out continuously, in this case by a walking person, provided models of the surface (11 m grid) by measuring the position of irregularly distributed points. Continuous observations over 7 months were made between two GPS receivers. One positioned within the moving landslide body and the other at a fixed location outside the landslide. An automatic procedure was developed to download, process and compute relative movement at constant time intervals. The accuracy achieved with the GPS measurements ranges between several millimetres to a few centimetres for static and kinematic observations, respectively. This integrated survey technique is a cost-effective method that could be applied to other structural and morphological settings. The real-time monitoring could be coupled with a warning system for landslide hazard management and the repeated kinematic GPS surveys derive precise DEMs of a landslide, providing information on geometry, volumes and evolution of the phenomenon