Traverse Closure Looks Good, Diagonal Check Does Not
An engineer is checking a five-point closed traverse used to establish column-line control for a two-storey building. The total station has a current calibration certificate. The field book shows an angular misclosure of 22 seconds and a linear misclosure of ΔE = +0.035 m and ΔN = −0.032 m over a perimeter of 318.6 m. The specification requires relative precision of 1:5,000 and an angular closure no worse than 15″√n. The surveyor applied the compass rule and computed coordinates for grid intersections A1 through C3, then set 24 anchor bolts. A diagonal check between two grid intersections measured 42.318 m, while the adjusted coordinates give 42.336 m. The foreman wants the anchor bolts grouted this afternoon because the concrete supplier arrives at 06:00 and the base plates are due on site tomorrow.
1. Compute the angular and linear closure and judge the traverse against the specification.
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Angular closure. Allowable = 15″√n = 15″ × √5 = 33.5″. Actual = 22″, so the angular closure is acceptable.
Linear closure. Misclosure = √(0.035² + 0.032²) = √(0.001225 + 0.001024) = √0.002249 ≈ 0.0474 m. Relative precision = 318.6 / 0.0474 ≈ 6,720, i.e. about 1:6,700. That is better than the specified 1:5,000, so on the traverse statistics alone the control network passes.
But this does not close the question. A traverse can pass its overall closure and still contain a local blunder: the compass rule spreads the misclosure proportionally across all legs, which can mask a single bad leg or a setting-out error at one intersection. The diagonal check contradicts the good closure, so the network cannot be accepted for setting out until that contradiction is resolved.
2. The diagonal check is 18 mm short over 42.3 m. What does that indicate, and what independent check would you require before any grouting?
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What it indicates. 18 mm over 42.336 m is a relative discrepancy of about 1:2,350, worse than the 1:5,000 required for the control. Because the overall traverse closure passed, the most likely explanations are local rather than global:
- a blunder in one leg that the compass rule has smeared across the network;
- a setting-out error at one or both grid intersections (instrument centring, target/prism constant, or a misread offset);
- the diagonal measurement itself being wrong (tape temperature/tension/sag, or the wrong points measured).
Independent check. Re-measure the diagonal with a different method - a calibrated steel tape with tension and temperature correction, or a second instrument setup using resection from three control points. Re-observe both grid intersections from a third known point and compare coordinates. Check the instrument and prism constants, the height of instrument, and the recorded point numbers. Confirm the 24 bolts by offsets from two independent control lines, not from the same adjusted coordinates.
Only when two independent methods agree within the specified tolerance should the control be released for setting out.
3. Grouting is scheduled this afternoon. What is the decision, and what are the options if the anchor bolts are out of position?
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Decision: stop the grouting. Grouting locks in the bolt positions. If they are out of tolerance, the fix moves from adjusting a loose bolt to chopping concrete, epoxying new dowels, or modifying base plates. The cost and schedule consequence of a short delay is far smaller than the consequence of locked-in error.
If the bolts are confirmed out of position, options to put to the engineer of record and steel fabricator include:
- slotted or oversized holes in the base plate, sized for the actual deviation and checked for bearing and nut/washer coverage;
- a redesigned base plate or adapter plate;
- post-installed anchors designed for the actual loads, subject to embedment and edge-distance checks;
- local chopping and re-setting of the affected bolts, with a concrete repair method accepted by the engineer.
Record the as-built deviation, the chosen option, and the design check. Release the remaining bolts only after the diagonal and offsets agree within tolerance, and issue a signed setting-out certificate so the grouting crew has a written basis to proceed.