Traverse Closure, a Direct-Only Angle Set, and a Leaning Deed Monument
You are running a closed-loop traverse to control a 12-lot short plat on 6.8 acres of bench land near Pocatello. The loop has nine stations and 7,912.64 ft of total traverse length, and it closes with ΔN = +0.286 ft and ΔE = −0.131 ft. The angular misclosure is 26 seconds after nine angles turned with a 5-second total station, each turned in the direct position only. Distances came from an EDM rated ±(2 mm + 2 ppm). Three found plat monuments at the perimeter agree with your traverse within 0.04 ft, but the fourth - the one your client's deed calls to - falls 0.31 ft from the position computed from the plat bearings and distances. The client wants the plat recorded Friday and asks you to "just adjust it." A 2019 survey of the adjoining parcel shows that same fourth monument 0.18 ft from where you found it; it is a 30-inch galvanized pipe set in 1978, and it leans noticeably.
1. Evaluate the traverse closure. Is it acceptable, and what does the data suggest about the source of error?
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Compute the actual closure first: √(0.286² + 0.131²) = √(0.0818 + 0.0172) = 0.315 ft. Precision = 7,912.64 / 0.315 ≈ 1:25,100, which easily passes the 1:10,000 conventionally required for boundary and plat control.
A passing ratio is not the end of the analysis. The angular misclosure of 26" in nine angles is the tell. A 5-second instrument in good adjustment should hold far tighter; a reasonable allowance is on the order of 5"√9 ≈ 15", and turning direct only means you cannot cancel the vertical-axis, collimation, and circle-eccentricity errors - you have doubled them instead. An angular misclosure of 26" with direct-only observations points to a systematic source: an instrument out of adjustment, a target or plumb error at one station, or a single angle turned carelessly.
Also look at the pattern. A misclosure that is mostly north (0.286 ft) with a relatively small east component is consistent with a rotation of the network rather than with a random scatter of distance errors. Do the residual analysis station by station: a systematic angular error leaves a characteristic pattern of increasing residuals away from the suspect station.
So the answer to "is it acceptable" is: the ratio passes, but the work should not be adjusted and filed until the angles are re-observed direct and reverse. Chasing the number to hit Friday is how a rotation gets baked into twelve lot lines.
2. Assuming the re-observation is clean, what adjustment method do you use and why?
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Adjustment choice should follow the error model, not habit.
If the re-observed closure is consistent with random error, the compass (Bowditch) rule is defensible for a traverse of this size and geometry - it distributes the misclosure in proportion to distance, and total-station distances are of uniform quality, so the underlying assumption is reasonable. The transit rule assumes angular error dominates and distributes by latitude and departure; it is a poor fit for a modern total-station traverse and is rarely justified.
A least-squares adjustment is better if you have redundant observations - repeated angles direct and reverse, ties to the three consistent perimeter monuments, and check measurements between non-adjacent stations. It gives you residuals, an error ellipse at each station, and a statistical basis for saying whether the 0.31-ft conflict at the deed monument is significant or is inside the noise. That is the real question on this project.
Whatever method is used, adjust only after the systematic error is removed, and keep the record of the unadjusted observations and the adjustment report in the file. Do not adjust found monuments to fit the traverse - the adjustment moves your stations, not the evidence.
3. How do you resolve the 0.31-ft conflict at the leaning 1978 pipe?
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Found monuments control over record bearings and distances, but a monument that is leaning and that two independent surveys place 0.18 ft apart is a monument whose integrity is in question. Work the evidence in order:
- Condition. Is the pipe bent, or is it straight but tilted? Does the base align with the top? A pipe driven straight and later rotated by frost or equipment leaves a disturbed collar and often a visible gap. Photograph and describe it.
- Record. Pull the 1978 plat and any filed survey that set it, and the 2019 map. Do the 2019 ties, when re-run, reproduce the found position or the record position? A consistent 0.18-ft difference in the same direction from two independent surveys suggests movement; a scatter suggests measurement noise.
- Corroboration. Test the pipe against the three consistent perimeter monuments. If the plat geometry reproduces the other three to 0.04 ft but not this one, the odds shift toward a disturbed monument.
- Occupation. Is there a fence, a walk, or a 1978-era improvement keyed to it? Occupation keyed to the original position is powerful corroboration.
Report the conflict regardless of the conclusion, and never silently move the corner 0.31 ft to make the deed close. If the monument is sound, hold it and note the record discrepancy for the title company. If the evidence shows it was disturbed, restore the corner by the best available evidence and document the basis in a record of survey.