A 1:17,500 Closure, a Perfect Pipe, and a 0.14-ft Conflict
Your six-sided closed traverse around a 12-acre parcel has a linear misclosure of 0.10 ft north and 0.10 ft east over a total length of 2,470.00 ft. The angular closure after six angles is 12 seconds. Distances were measured with a total station rated 2 mm + 2 ppm; angles were turned in four sets with a 5-second instrument. The record for the parcel is a 1968 record of survey with ties stated to 0.01 ft. After a compass-rule adjustment you find that an original 1-inch iron pipe at the northeast corner, stamped with the license number of the surveyor who prepared the 1968 map and set in an uncracked concrete curb, lies 0.14 ft from your adjusted position.
1. Evaluate the closure and state the relative precision.
Read the worked answer
Misclosure magnitude = sqrt(0.10^2 + 0.10^2) = 0.1414 ft. Relative precision = 2,470.00 / 0.1414 = 1:17,500.
That is a respectable closure and passes a typical 1:10,000 standard, but precision is not accuracy:
- A 12-second angular closure over six angles is essentially noise for a 5-second instrument; nothing suggests an angle blunder.
- The linear misclosure direction is N 45 E. That direction diagnoses the error. A single bad distance produces a misclosure along that line's azimuth; a single bad angle produces a misclosure roughly perpendicular to the bisector of the angle's two legs. Compare N 45 E to each leg's azimuth and each angle's bisector - the strongest match is the prime suspect.
- A misclosure this close to exactly equal north and east components is suspicious of something systematic rather than random, such as a scale error, an uncorrected prism constant, or distances measured on the ground but reduced on the grid.
Do not treat 1:17,500 as proof the work is right; treat it as evidence that no gross blunder survives.
2. Do you hold the pipe or your adjusted coordinate?
Read the worked answer
Hold the pipe. The adjustment distributes random error in your traverse; it does not create a better position than an original monument that is physically undisturbed.
Reasons:
- The pipe is stamped with the license number of the surveyor whose map you are retracing, is set in uncracked concrete, and shows no sign of having been moved. That is strong evidence of original position.
- Your traverse is a connection between monuments, not a determination of where the corner belongs. A 0.14-ft offset is within the plausible accumulation of your own work plus the 1968 ties.
- Ask the diagnostic question first: is 0.14 ft consistent with random error in a 2,470-ft traverse, or is it systematic? If your distances were reduced incorrectly for scale factor or if an instrument constant is off, the mismatch will appear at every monument you compare, not just this one. Compare to a second found monument before concluding anything.
If the offset is systematic and repeats, fix the traverse; if it is isolated, accept the monument and report the discrepancy.
3. What must the Record of Survey show about this conflict?
Read the worked answer
Disclose it fully; a map that shows only your adjusted position misleads the next surveyor.
Under the Professional Land Surveyors' Act (Bus. & Prof. Code §8762), the record of survey must show monuments found, set, reset, or replaced, the manner in which the survey was performed, the basis of bearings, and the relationship of the survey to the record. Show:
- The pipe as found, described physically, with the stamp and the curb.
- Your measured position for it - the raw, unadjusted measurement - as well as the adjusted value, so a reader can reproduce your reasoning.
- The 0.14-ft difference, stated as a number, not implied by a line weight.
- A note identifying the 1968 map as the record and stating that the found monument was held over the traverse adjustment.
Also record the closure and the basis of the adjustment in your field book, and mention the adjustment method on the map. If a later surveyor disagrees, the note tells them exactly what you did and why.