Traverse Misclosure and the Decision to Adjust
You close a 12-course traverse around a 40-acre parcel. The total traverse length is 9,860 feet. After reducing field data, the linear misclosure is 0.42 feet. The angular misclosure is 38 seconds against a closed polygon with 12 angles. The client needs a plat within 30 days, and the county requires a positional tolerance of 1:7,500 for rural parcels. Your field notes show that two instrument setups were made in gusty wind, and one backsight was taken on a rod that may have settled. The record deed distances for three courses differ from your measured distances by 0.15 to 0.30 feet, which is within the range you would expect from the original 1970s survey.
1. Compute the relative precision and determine whether the traverse meets the county tolerance.
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Relative precision (linear precision) = total traverse length / linear misclosure = 9,860 / 0.42 ≈ 23,476, expressed as 1:23,476.
The county requires 1:7,500. Since 1:23,476 is a larger ratio (better precision) than 1:7,500, the traverse meets the county positional tolerance on its face.
However, relative precision is a scalar measure of closure, not a guarantee of positional accuracy at every point. A traverse can close well overall while individual points carry larger errors, particularly where the misclosure is distributed unevenly. The angular misclosure of 38 seconds over 12 angles should also be compared to the allowable angular misclosure for the instrument and procedure used; if the allowable is, say, 30 seconds, the 38-second misclosure signals a possible systematic problem (the gusty-wind setups and the suspect backsight) that closure alone does not reveal.
Conclusion: the traverse passes the numeric tolerance, but the field evidence of two compromised setups and a suspect backsight warrants review before adjustment.
2. How would you decide whether to adjust, re-observe, or accept the traverse, and what adjustment method would you use?
Read the worked answer
Because the misclosure is within tolerance but the field conditions are suspect, the decision should be evidence-based rather than automatic:
- Isolate the suspect setups. Recompute the traverse excluding the two gusty-wind setups and the suspect backsight. If removing them materially changes the misclosure or the position of key corners, re-observe those setups rather than adjusting around them. A 0.42-ft misclosure that is concentrated in one leg is a red flag even though the overall ratio looks good.
- Check angular consistency. Compare the 38-second angular misclosure to the allowable for the procedure. If it exceeds the allowable, re-observe angles before distributing anything.
- Compare to record. Differences of 0.15–0.30 ft from 1970s deed distances are plausible survey error and do not by themselves indicate a blunder, but they should be evaluated against the monument positions, not used to force the traverse.
If re-observation is not feasible before the deadline, adjust using the compass (Bowditch) rule, which distributes the linear misclosure proportionally to course length, after first distributing the angular misclosure equally among the angles. For a survey of this nature, the transit rule is an alternative, but Bowditch is standard where both angle and distance errors are present. Document the misclosure, the adjustment method, and the residual uncertainty on the plat, and note the compromised setups.