In this paper, an efficient methodology to assess Rotor Angle Stability (RAS) in parameter-space has been proposed. This methodology maps deformations of the power system RAS region under operational changes to a security region in parameter-space that can be assessed on-line. In order to choose the proper parametrization, security boundaries have been constructed using polynomial regression models with coefficients obtained from Ordinary Least Squares (OLS). The identification of suitable parametrization has been carried out, and the newly introduced sensitivity of a SIngle Machine Equivalent (SIME) has been employed to describe the behaviour of a power system along different parameter-space directions. For the chosen parametrization, Constrained Least Squares (CLS) optimization set up as a Quadratic Programming (QP) problem has been used in order to keep the estimates inside the security region. The case study has been carried out using small test systems in two and three-dimensional parameter-spaces.