From Nestor Cardozo:
SSPX is a program to calculate strain/strain rate from displacement/velocity data in two and three dimensions. SSPX is a full fledged inverse modeling program to calculate best fitting deformation tensors given displacement or velocity vectors at a minimum of three points in two dimensions, or four points in three dimensions.
SSPX works equally well on small deformation problems such as computing strain rates from Global Positioning System (GPS) data, and large deformation problems such as computing the strain in a highly deformed discrete element model (DEM). SSPX was made for geologists, but it can be used by anyone interested in computing strain/strain rate from displacement/velocity data.