Automated Tacheometers. Since the early 90es, motorized tacheometers have been widely used for sequences of measurements on a possibly very large number of corner-cube reflectors. Considering the quality of available ATR (automated target recognition) and the extraordinary accuracy of tacheometers, sub-millimeter measurements have often been achieved, with typical sequences every hour or tens of minutes. With a proper data communication equipment, entirely automated networks have been set up, e.g. to monitor large landslides. If the distances involved are large, the limit precision is due to atmospheric refraction. But, the main difficulties to solve have always been linked to the power supplies, the data management, and the protection of the various equipment against rain and all meteorological hazards.
Satellite Radar Interferometry. Since 1992 when this methodology was published by D. Massonnet at CNES, the required (extremely complex) softwares have been more and more available and easy to use, and this methodology is now classical. Its limitations on slope instabilities or seismic areas are mainly linked to the presence of vegetation, the atmospheric index anomalies, and the availability of radar satellite passes over the surveyed area. The minimal surveyed area is large, hundreds of square meters, but when the situation is optimal, a millimetric precision may be obtained, even without any equipment on the site.
Diachronic correlation of images.
This methodology, proposed in the early 90es, became available with the commencement of digital photography, at the very beginning of the century. It allowed to detect extremely small horizontal ground movements from aerial or space imagery, with an accuracy ranging to the 1/100th of pixel, without any equipment on the surveyed site. The same methodology has been used also with terrestrial images, its main limitation being that it detects only the movements that are perpendicular to the optical axis. Pre-, co- and post-seismic movements have been successfully detected by this way. And a completely unexpected re-use of historical aerial images has been found, e. g. to monitor slope instabilities or seismic zones, since possibly more than one century ago.
Combination of laser scanner and photogrammetry.
In order to survey slope instabilities, or even cliff rock falls hazards, it has been proved that such a combination was able to provide an accurate and efficient possibility of measurement, at the cm level, even in the presence of vegetation.
GNSS disposable stations.
In recent years, the availability of high precision GNSS microchips have led to the development of cheap equipment, combining GNSS, data storage and management, solar panel, batteries, smart radio communication, and possibly other environmental sensors. The equipment being very compact and not costly, it may be abandoned in the field if the hazard grows at an excessive level, and the time series of positions helps a lot to understand the dynamics of the ground movements. The accuracy achieved may go below the cm, and the measurements may be continuous if necessary.
A very large panel of solutions is available, but each situation deserves a specific analysis. The pertinent parameters to consider are : the requested frequency and accuracy of the measurements, the cost, the environmental conditions, etc.