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This paper describes a method for analyzing undulation and off-center of a flat product in a web inspection by a linear camera. The longitudinal undulation in the plate appears to the high speed of the line and different tensions across the strip section as it is pulled to the end of the line. If the amplitude and/or frequency of the undulation is too high it will produce an increase of breakage probability and new defects may appear. In addition, it introduces errors in width measurement if it is not taken into account. We propose a method to identify undulation characteristics based on time-frequency analysis of the projection of the strip edge. We recover the position of the edge and undulation by assuming that band off-center is a slower process than undulation. Simulations with different undulation waveforms and off-centers, as well as experimental results are shown.