RTK Under Canopy at a Boundary Corner
You are locating a corner of a 6.2-acre NH lot in mixed hardwood. The corner is a 5/8-inch rebar with a plastic cap found at the base of a stone wall. Your GNSS receiver on an 8-km NTRIP baseline reports a fixed solution with horizontal RMS 0.07 ft and vertical RMS 0.12 ft; PDOP is 3.8, but 11 of 14 satellites are below 25° elevation. You take three 60-second occupations 20 minutes apart; the horizontal positions differ by 0.18 ft east-west and 0.05 ft north-south. A total-station traverse from two published control monuments closes 0.04 ft over 1,850 ft. The client wants the corner coordinates on the NH State Plane to record on a plan and asks if the RTK coordinates are 'survey grade.' The wall and rebar are the only physical evidence at the corner; no original bearing tree is found.
1. Interpret the RTK results. Would you accept the RTK coordinate as the final position of the rebar? Explain.
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A fixed solution and low RMS values are internal receiver estimates, not a guarantee of positional accuracy. The low-elevation satellite geometry and 8-km baseline in canopy increase multipath and atmospheric error. Three occupations separated by 20 minutes that spread 0.18 ft east-west are not consistent enough for a boundary corner; that spread is larger than the reported 0.07-ft RMS and suggests the coordinate is not stable at the 0.05-0.10-ft level. I would not accept a single RTK coordinate, and I would not simply average the three. I would reobserve with longer occupations and better satellite geometry and, more importantly, check the rebar by total station from the two control monuments. The 0.04-ft closure over 1,850 ft shows the total-station control is suitable for the check.
2. Describe the field procedure you would use to produce defensible coordinates for the rebar and its relationship to the stone wall.
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I would occupy one control monument, backsight the other, and measure the rebar and wall from at least two setups, closing the traverse or measuring redundantly. I would record the cap, the rebar condition, the wall face, and ties from the corner to the wall. If GNSS is used, I would observe multiple long occupations, use a local calibration to NH State Plane with at least four control points around the site, and verify scale and rotation. I would compare the total-station coordinate to the RTK coordinate; if they disagree by more than the project tolerance, I would resolve the difference before using either. The boundary position should be held by the rebar and wall evidence; the coordinates are a reference to that position, not a replacement for it.
3. What should the plan and report say about the coordinate basis and accuracy?
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The plan should state the coordinate system and datum, units, epoch if applicable, method, control used, and the check results. It should say that the boundary corner is located from the rebar and stone wall and that the coordinates are derived from a total-station traverse checked by GNSS, or vice versa. I would not print 'RTK accuracy 0.07 ft' as a positional accuracy claim. If the coordinates are suitable only for GIS or mapping, I would label them that way. The report should include the observation log, occupation times, satellite and PDOP information, and the comparison between RTK and total station.