| Line | Reversal | Corrected PV | Edge err PV | Status |
|---|
180° reversal. Each line is read twice: 0° normal, then 180° with the straight edge reversed so its own straightness error contributes with opposite sign at the same station. The surface deviation is the average of the two sets — the straight-edge error cancels — and the half-difference is that straight-edge error, shown live as a check that you reversed correctly.
Chord reference. With the straight edge supported at both ends, every reading is a height relative to the chord through those ends (what a slip gauge under the edge gives). Line ends therefore carry zero local deviation.
Reduction. The eight lines share nine points (four corners, four edge mid-points, the centre). Absolute heights are solved by least squares so those shared points agree — the software equivalent of Moody’s closure — then referenced to the chosen datum. The reduction is invariant to any added plane, so setup tilt drops out.
Reversal registration. Default assumes the reversal flips the edge’s error sign in place. If your reversal also swaps ends (mirrors the profile), tick “end-swapped” in the line editor and the half-difference check confirms it.
Reported flatness is datum-referenced peak-to-valley, not strict minimum-zone. Verify against your reference before accredited use — this is a calculation aid, not a substitute for your UKAS procedure.
Diagrams are on by default — untick any to leave it out of the PDF.