Difference in fringe depth depending on what section of the ramp is used to determine the slope. The right column shows the results from this work (top), the current pipeline (middle), and the original, uncorrected ramp (bottom). The raw ramps are presented in the panel on the left, where the regions used to determine the slope per line style (corresponding to the line styles in the right column) are shown in grey. The fringe depth should be constant if the ramp is linearised.