Canopy RTK Failures and a Traverse That Looks Acceptable
A crew surveys a 22-acre wooded parcel bounded by a meandering creek. Under heavy canopy, 14 of 30 RTK shots show HRMS between 0.08 and 0.15 ft, but repeated occupations of the same points differ by 1.2 to 2.5 ft. The base is 9 miles away and the radio link drops intermittently. The crew then runs a closed traverse with a 5-second total station and eight prism poles. Total traverse length is 4,180 ft; unadjusted misclosure is delta N = +0.16 ft and delta E = -0.29 ft. Angular misclosure is 26 seconds for eight angles. The field book shows one backsight distance 0.31 ft different from the previous setup's foresight distance for the same line. The party chief proposes to apply the compass rule and set the boundary corners the same day.
1. Evaluate the traverse and the proposed compass rule. Compute linear misclosure and precision. What is the likely problem?
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Linear misclosure = square root of (0.16 squared + 0.29 squared) = square root of (0.0256 + 0.0841) = square root of 0.1097 = 0.331 ft. Precision = 4,180/0.331 = 1:12,630, which looks acceptable in isolation, but the angular misclosure of 26 seconds for eight angles is the warning. A common criterion is 5 seconds times the square root of n to 8 seconds times the square root of n, or about 14 to 23 seconds; 26 seconds exceeds both. The 0.31-ft backsight/foresight discrepancy on the same line is a direct red flag: wrong prism constant, target height, centering error, or a misidentified point. Applying the compass rule would distribute a blunder and could produce a smooth-looking but wrong plat. First re-observe the suspect angle and the 0.31-ft line, check the EDM and prism constant, check instrument and target centering, and compare the two distances. Only after the blunder is eliminated should a least-squares or compass-rule adjustment be considered; even then, the adjustment is an estimate, not boundary evidence.
2. What should the crew do about the RTK results, and how can the total station traverse be tied to the state plane?
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HRMS is an internal precision estimate, not an accuracy check. Repeated occupations that differ by 1.2 to 2.5 ft show the fixes are not repeatable even if each reports 0.08 to 0.15 ft. Under canopy, multipath and cycle slips can produce a 'fixed' solution with misleading precision. Accept only fixes with fixed ambiguity status, adequate satellite geometry, and long enough occupation times to average; repeat at different times; check against an independent baseline or a total-station tie. Establish control in the open, then traverse or run a local baseline into the woods. Do not use the RTK shots as boundary evidence until they agree within project tolerance. Tie the traverse to the state plane by occupying two points with known coordinates, compute the grid-to-ground factor, and state how the traverse coordinates were transformed. If RTK and traverse disagree by about 1 ft at common points, the traverse tied to stable control should govern.
3. The creek is called for in the deed as a boundary. How should the crew handle the creek line, and what field evidence should be recorded?
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A deed call to a creek is a natural monument and generally controls over bearings and distances, but the surveyor must determine what the call means: the thread of a non-navigable stream, the bank, or the ordinary high-water mark, depending on the deed language and Indiana law. Do not set the line from scattered RTK shots. Map the creek's actual location at the date of survey, the ordinary high-water mark or thread as appropriate, and enough cross-sections and bank evidence to support the location. Record the water level, bank shape, vegetation, and any avulsion or accretion indicators. Show the meander line as a representation, not as the legal boundary, unless the deed uses it that way. Because riparian boundaries can shift by accretion or avulsion, note the date and conditions. Advise the client that a court may need to resolve an ambiguous water boundary.