Etna discrete geodetic networks
The GPS network
The first GPS (Global Positioning System) measurement campaign took place in 1988. Based on the experience acquired over the previous 15 years, and in order to improve the existing networks, the 9 GPS sites observed in this first campaign were selected according to the following criteria: (1) To include stable points outside of the volcano edifice. The importance of connecting the local geodetic networks to an external frame had been mentioned in the past but hardly achievable with classical methods, especially on a routine basis. (2) To link the various existing trilateration networks by GPS measurements. The trilateration networks were poorly linked, mostly for logistical reasons due to the shape and size of the volcano. (3) To link the GPS to the leveling and micro-gravity networks. (4) To allow easy re-observation of the GPS points. In contrast with classical geodesy, GPS allowed for the first time to select sites not on inter-visibility or network geometry criteria, but more on accessibility and other logistical considerations. The nine sites were therefore established along roads and trails. This introduction of GPS techniques profoundly changed the operation and application of geodetic surveys on Etna.
Since 1988, GPS measurements have been carried out at least yearly, with sometimes more frequent observations during periods of higher volcanic activity. Since then, the GPS monitoring network has been continuously improved, by installing new benchmarks to improve the density, the geometry and the capability of monitoring particular volcanic and volcano-tectonic features.
The GPS discrete network is currently composed by 80 benchmarks and includes a reference frame and an inner network, composed of various sub-networks.
