Modern Techniques and Problems in Monitoring Horizontal Strain

An introductory description of the modern results, methods and instrumentation of Geodesy for the deformations of the crust of the Earth is followed by a list of problems, and a discussion concerning the needs for the survey of the physical quantities of interest in seismology, geodesy, geology, mechanical and civil engineering such as the strain invariants, the optimal network of baselines and their accuracy. Some detailed comments are made on the problem of the determination of the three-dimensional strain by using two dimensional observations.