Determining the wavelength of a wave propagating in a medium nonuniform along the wave path implies using a measurement technique that provides enough spatial resolution to permit following up the wavelength variation resulting from such a nonuniformity. This paper presents a theoretical analysis of the respective merits of two phase sensitive methods used for that purpose, under both weak and strong wave attenuation conditions. Modifications to these methods are recommended which assure both accuracy and spatial resolution of the wavelength measurements. These conclusions are substantiated by numerical simulations of phase measurements along surface wave-sustained plasma columns used as an example of axially nonuniform and lossy media.