Scattering-type scanning near-field optical microscope (s-SNOM) has become an essential tool to study polaritons—quasiparticles of light coupled to collective charge oscillations—via direct probing of their near field with a spatial resolution far beyond the diffraction limit. However, extraction of the polariton’s complex propagation constant from the near-field images requires subtle considerations that have not received the necessary attention so far. In this study, we discuss important yet overlooked aspects of the near-field analysis. First, we experimentally demonstrate that the sample orientation inside the s-SNOM may significantly affect the near-field interference pattern of mid-infrared polaritons, leading to an error in momentum measurement up to 7.7%, even for the modes with an effective index of 12.5. Second, we establish a methodology to correctly extract the polariton damping rate from the interference fringes depending on their origin—i.e., the s-SNOM nanotip or the material edge. Overall, our work provides a unified framework for the accurate extraction of the polariton momentum and damping from the near-field interference fringes.