Determining profile and alignment using an optical technique with extrapolations

In the field of railway inspection, track alignment and profile are measured as the mid-chord offset (MCO) of a specified chord length at a known contact point on the rail. Generally, a 31-foot chord length is used. Due to developments of high speed rail standards, measuring longer wavelength MCOs, such as 62 and 124 feet are required. This is currently done using inertial means, however verification under static conditions using mechanical means is difficult. Currently the MCO is measured by stretching a string of the desired chord length between two points and manually measuring the offset from the center of the string to the rail. This is labor intensive, and the accuracy is limited, as external effects such as wind can make the string sway back and forth. While measuring a short chord length, and then extrapolating to the longer wavelengths can eliminate the sway problem, the extrapolation process magnifies any error in measurement. This paper presents the application of an optical technique, consisting of cameras and targets to measure a 31-foot mid-chord offset with very high precision, and the extrapolation technique used to obtain long chord measurements with a high degree of accuracy.