In order to approach experimental accuracy in ro-vibrational calculations for polyatomic molecules one needs to empirically refine even a high accuracy ab initio potential energy surface (PES). This is most efficiently done through a least-squares fitting of theoretical energies to the available experimental data by varying potential parameters in a given analytical representation. The PES resulting from such a fitting is then referred to as a 'spectroscopic' PES. In the present work we report a new approach to the construction of 'spectroscopic' PESs of polyatomic molecules. We represent the refinement as a perturbation to the initial PES, which is diagonalized in a basis of eigenfunctions of the unperturbed Hamiltonian. We apply this method to construct a new 'spectroscopic' PES for (NH3)-N-14 using literature values for observed spectroscopic data for J <= 8 and covering the energy range below 10300 cm(-1). We impose the constraint that the resulting PES remain close to the ab initio surface. The new 'spectroscopic' PES of NH3 (called NH3-Y2010) reproduces the selected experimental term values with a root-mean-square deviation of 0.2 cm(-1).