RTK Check Shot Fails Under Canopy
Your two-person crew is mapping a 12-acre wooded parcel for a topographic base. You set the GNSS base on an NGS monument 3.1 miles away and run RTK with a radio link. Under heavy canopy the rover reports a fixed solution on 8 satellites with PDOP 2.1. Horizontal residuals on shots range from 0.04 to 0.09 ft and vertical from 0.12 to 0.25 ft. A check shot on a known control point 400 ft from the base returns 0.15 ft horizontal and 0.28 ft vertical. You have two hours of daylight left and the client wants the base map tomorrow. A total station is in the truck, but the control point has no intervisible backup.
1. Do you accept the RTK check shot? Why or why not?
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No. A fixed solution is an internal status, not an accuracy guarantee. Typical acceptance for topographic RTK is about 0.05 to 0.10 ft horizontal and 0.10 to 0.15 ft vertical; 0.15 ft horizontal and 0.28 ft vertical exceed that. Under canopy, multipath and incorrect ambiguity resolution can produce a fixed solution with bad coordinates. PDOP 2.1 and 8 satellites do not prove correctness. The check shot is independent evidence and it fails. Also, a base 3.1 miles away may be near the radio link limit; verify the base coordinates and antenna height before trusting the solution.
2. What is your plan for the remaining two hours?
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Stop RTK mapping in canopy. Use the remaining time to establish local control by total station from the known control point, with a closed traverse or a resection to two known points, and shoot critical features and canopy boundaries by total station. Use RTK only in open sky, with repeat occupations and a new check shot at a different time. If time runs out, deliver partial mapping with a clear note rather than unchecked data. Do not mix unchecked RTK and total station coordinates in one surface.
3. How do you document the method decision?
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Record the base station coordinates and source, antenna height, radio settings, satellite count, PDOP, residuals, occupation times, check-shot values, the failed check, and the decision to switch methods. Note the total station control closure and any assumptions. Notify the office before leaving the site so the PLS can decide whether to re-observe or accept reduced precision. This record protects the crew and supports the final map metadata.