In gas-liquid stratified flows, pressure drop and transport across the interface are strongly influenced by the interfacial wave structure, making the determination of interfacial topography in this kind of flows very important. An objective way of characterizing the wave pattern present in the interface is proposed here. The method consists in analysing the spectra of the signal obtained from Laser Doppler Velocimetry (LDV) measurements of fluctuations occurring close to the air-sheared interface. Transitions are defined by the appearance and disappearance of peaks in the frequency spectra. The method was applied to study the transition regimes of a stratified air-water flow in a square-cross section channel. A flow pattern map for air water channel flow is presented and compared with the maps available from the literature. (c) 2006 Elsevier Ltd. All rights reserved.