The measurement of track irregularity has long been based on the principle of versed sine method. But the waveform measured by this method is different from its original one corresponding to the true track irregularity on ground. In this paper a theory and practice of restoring an original waveform of a railway track irregularity from measured data is described. The digital inverse filter for a restoration is designed to have a complete linear phase characteristic and so a restored waveform usually has no distortions as derived from the phase characteristic. From the design of restoring digital inverse filter to the filtering operation, FFT( Fast Fourie Transform ) algorithm is utilized. Data processing in the restoration can be executed very quickly and accurately. So far the effectiveness of restoring method has been proved successfully in some direct or indirect ways as compared with the results of experimental measurements and analysis. A restored data exposes the actual state of the presence of track irregularity along the railway track and so it can be applied to the maintenance management of long-wavelength components of track irregularity necessary for high speed trains such as Shinkansen, the estimation of shifting quantities of track in the repairing work by a multiple tie tamper and the execution of miscellaneous simulations of vehicle dynamic motion, etc.
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