Filtering effects of mid-cord offset measurements on track geometry data

The filtering effects of mid-cord offset measurements on the track geometry data are evaluated. A common method of measuring track geometry is using track measurement railcars that record the track deviations from the nominal measures at the mid cord of two points that are 62 feet apart. Alternatively, the track measurements can be made at discrete points, commonly one foot apart, independent of any other measurements. This method is referred to as space curve measurements. This study shows that special attention must be paid to the distortion effect of 62-foot mid-cord measurements on the data. A comparison of the gain and phase response of a space curve and mid-cord offset data in the frequency domain shows that mid-cord offset measurements greatly attenuate the low-frequency contents of the data and also introduce a ripple or scalloping effect at the higher spatial frequencies.