The Center Gap option comes into effect if there is a gap across the center of the rail head. The center of the head is the point at which the centerline of the rail intersects the top surface of the head. If the gap is no greater than the value of center gap, then the software connects the points across the gap in order to find the center point of the top of the rail. If the gap is greater than center gap, the center point of the top of the rail is not found, and vertical wear will be reported as unknown.
The Calibration Gap is the maximum allowable vertical separation between overlapping contour fragments on the rail head. A rail measurement head projects two light stripes, one on each side of rail. The two stripes are observed independently by two cameras. Each camera-projector pair produces half of a rail profile. The two half-profiles are then joined together to form a continuous contour.
Normally, there is a region on the rail head where the two half-profiles overlap. The length of the overlap varies depending on the height of the measurement head above the rail. It is usually around 0.5". If the measurement head is well calibrated, the two half-profiles will almost coincide in the overlap region. If the head is out of calibration, the two half-profiles will separate, so that one contour appears above the other in the overlapping region. The calibration gap option is the maximum allowable separation between the overlapping contour fragments.
The next three options affect head-only modeling. A rail profile is modeled as a "head-only" profile if head-only alignment is enabled and the rail type cannot be identified because the base portions of the rail cannot be seen or are a poor fit to the list of selected rail types. Although wear measurements cannot be derived from a head-only model, it may be used for rail grinding purposes.
Head-only Top is the Y-value of the highest point of the head-only rail after it is modeled.
Head-only Width and Head-only Height are clipping values used to remove excess contours from a head-only model. Head-only modeling is often used when rail is embedded in a roadbed, as in crossings and some transit lines. In such cases, the measurement system picks up contour lines from the road surface on either side of the rail head. All measured points that are horizontally separated by more than head-only width from the highest point on the rail head are clipped. Measured points more than head-only height below the highest point on the rail head are also discarded.