Passive NGS Control in a Subsiding Basin
Continuous GNSS at a basin station shows subsidence of 10–14 mm/yr. You are establishing control for a 6.2 km pipeline. Eight new points are observed by static GNSS in 45-minute sessions and processed to three CORS 35–55 km away; repeat baselines agree to 8 mm horizontally and 16 mm vertically. You tie to two NGS passive marks whose NAD83(2011), epoch 2010.0 coordinates and NAVD88 heights were published in 2012. At mark A the new GNSS coordinate differs from the datasheet by 0.10 m horizontally; at mark B by 0.03 m. A digital level loop between A and B closes within 4 mm, yet both marks differ from the nearby continuous station by about 0.09 m in height. The contract requires 2 cm horizontal and 3 cm vertical accuracy relative to current ground. The client asks you to hold the published NGS coordinates so the project GIS will match the state's parcel layer.
1. Diagnose the discrepancy and decide whether to hold the published NGS coordinates. Show the numerical reasoning.
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The marks are passive and their coordinates refer to an old epoch. At 12 mm/yr, 15 years is about 0.18 m of absolute motion. The 0.10 m difference at A and 0.03 m at B are not a closed-loop error; they reflect differential subsidence and possibly local disturbance. The 4 mm level closure only proves that A and B moved together; it does not prove that either is stable. Holding the published values would constrain the network to epoch 2010.0 and impose a systematic distortion on the project. Because the contract is 2 cm relative to current ground, holding the old coordinates fails. Use the CORS as the frame and compute coordinates at a common project epoch, for example 2025.0. Treat A and B as check points, not constraints. HTDP can model plate motion but does not model local subsidence, so it should not be applied blindly. The 0.09 m height difference is consistent with subsidence and/or geoid and leveling effects; verify it against a stable benchmark before using either height.
2. Design the control and verification strategy you would use, including reference frame, epoch, redundancy, and vertical control.
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Use three or four surrounding CORS and four stable bedrock or deep-rod marks outside the subsidence bowl. Observe two-hour sessions on two days, form redundant vectors and closed loops, and compute at epoch 2025.0. For vertical control, run a digital level loop from a stable NAVD88 benchmark to the project area; use GNSS plus a geoid model only after verifying against leveling. Establish two local monitoring points and re-observe quarterly to estimate velocity. Use total-station distances for short ties and as an independent check on GNSS. Set acceptance criteria such as horizontal repeatability of 10 mm + 1 ppm, vertical repeatability of 20 mm, loop misclosure within 8√K mm, and a GNSS-to-leveling height check within 2 cm. Do not constrain to passive marks unless they are proven stable by repeat observation or local evidence.
3. What will you state in the survey report and on the deliverable to prevent misuse of the coordinates?
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State the reference frame, realization, epoch, geoid model, velocity assumption, and the instability of the NGS passive marks. Provide a table of observed or assumed velocities and a clear statement that datasheet values must not be used directly without transformation. On the plat or control diagram, note that coordinates are at the project epoch and that a documented transformation is required for GIS or state parcel-layer use. Advise the client to re-observe before construction if predicted motion exceeds 2 cm. Include a limitation that the survey does not certify the marks as stable. This gives the client the information needed to use the coordinates correctly without misrepresenting the survey as tied to fixed published values.