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Near-field microscopy allows for visualization of both the amplitude and phase of surface plasmon polaritons (SPPs). However, their quantitative characterization in a reflection configuration is challenging due to complex wave patterns arising from the interference between several excitation channels. Here, we present near-field measurements of SPPs on large monocrystalline gold platelets in the visible. We study systematically the influence of the incident angle of the exciting light on the SPPs launched by an atomic force microscope tip. We find that the amplitude and phase signals of these SPPs are best disentangled from other signals at grazing incident angle relative to the edge of the gold platelet. Furthermore, we introduce a simple model to extract the wavelength and in particular the propagation length of the tip-launched plasmons. Our experimental results are in excellent agreement with our theoretical …