10.2 Monitoring Surveys
Key Takeaways
- USGS reports that by 1970 more than one foot of land subsidence had occurred over about 5,200 square miles of the San Joaquin Valley, locally as much as 28 feet.
- USGS documents about 9 meters of land-surface lowering from 1925 to 1977 at the approximate point of maximum San Joaquin Valley subsidence.
- NGS Horizontal Time-Dependent Positioning (HTDP) estimates modeled velocities and earthquake-related displacements between dates; it is not a field measurement.
- A monitoring scheme holds reference points outside the deforming body and repeats geometry, equipment class, and reduction between epochs.
- Every reported displacement must carry the two observation epochs and the reference frame or datum; a coordinate without an epoch is not a monitoring result.
Domain III, professional activity 5 on the January 2025 California PLS test plan is perform monitoring surveys. The knowledge list that sits under Field Operations also names earth movement in the research domain (earthquakes, landslides, subsidence, plate tectonics) and, later, analysis of how earth movement affects boundaries. This section stays in the field: how you detect change, how you keep the signal larger than the noise, and how you label what you measured. Independent OpenExamPrep material here does not assign ownership consequences to a moving mark; it teaches you to produce a displacement that can be re-computed.
What "monitoring" means on a California job
A control survey answers "where is this point now?" A monitoring survey answers "how did this point move between epoch A and epoch B, relative to a stated reference?" That second question fails if you change the question between visits. Typical California signals include:
- Interseismic crustal motion as the Pacific and North American plates strain the fault system
- Coseismic offsets when an earthquake ruptures, plus afterslip that can continue after the main shock
- Inelastic aquifer-system compaction and land subsidence, especially in the San Joaquin Valley and other groundwater basins
- Landslides and slow slope creep in coastal bluffs, canyon fills, and reservoir rims
- Structural deflection of dams, levees, bridges, tanks, and buildings under load or settlement
These mechanisms can operate at the same site. A GNSS monument on a valley floor can show tectonic velocity plus seasonal hydrologic loading plus long-term subsidence. The field product is a time series and a displacement vector, not a single cause. Causes are interpreted after the measurements exist.
USGS land-subsidence summaries are the scale you should have in mind for valley work, not a prediction for your project. By 1970, significant subsidence of more than one foot had occurred in about half of the San Joaquin Valley, about 5,200 square miles, and locally some areas had subsided by as much as 28 feet. USGS also documents about 9 meters of lowering from 1925 to 1977 at the approximate point of maximum subsidence (the well-known telephone-pole photograph). Later drought years have again produced measurable annual lowering in parts of the valley; quote the campaign that actually covers your site rather than recycling the 28-foot figure as a current rate.
Design rules that keep the signal honest
Monitoring is a repeated experiment. Write the experiment before the first observation.
| Design choice | Why it matters in California | Field practice |
|---|---|---|
| Reference outside the deforming mass | A valley CGPS station inside the subsidence bowl cannot prove absolute lowering | Hold bedrock, a deep-referenced mark, or a network of external CORS with documented stability |
| Same geometry | Different setups alias centering and refraction into "movement" | Repeat standpoints, target heights, sight lengths, and GNSS session length |
| Same reduction | Mixing NAD 83 (2011) epoch 2010.00 with a later realization looks like motion | Name datum, realization, epoch, geoid model, and projection (CCS83 zone) on every campaign |
| Error budget vs expected signal | A 15 mm GNSS repeatability cannot certify a 5 mm structural limit | Set the specification tighter than the decision threshold, then prove it with repeats |
| Redundancy | A single delta can be a busted height of rod | Independent backsight, cross-bracket, or a second sensor class (level vs GNSS vs total station) |
Do not invent a Horizontal Time-Dependent Positioning displacement and write it in the notes as if the crew measured it. NGS HTDP is software that estimates horizontal velocities from crustal-motion models and estimates displacements associated with modeled earthquakes between two dates. It can also update coordinates or observations from one date to another. Use it to compare a historic NAD 83 position to a modern epoch, to plan whether tectonic motion is large enough to care about, or to discuss a published earthquake model. Then go measure. If HTDP and the field disagree, the field campaign still stands as a measurement; the model is a model. This chapter does not quote site-specific HTDP millimeter values because those outputs depend on the version, the coordinates, and the two dates you enter.
Vertical motion is outside the original HTDP horizontal emphasis. Valley subsidence, peat oxidation in the Sacramento–San Joaquin Delta, and landslide settlement are measured with leveling, GNSS ellipsoid-height time series plus a geoid, extensometers, InSAR as a reconnaissance layer, or a combination. InSAR can show you where to densify monuments. It does not replace a monument you may have to testify about.
Earthquake offsets in the field
After a damaging California earthquake the monitoring question is concrete: which marks moved, by how much, and relative to which still-standing reference? Practical steps:
- Recover pre-event control and photo-document disturbed or sheared monuments before you "fix" them.
- Reobserve the network with the same scheme if the monuments still exist; add new points if the rupture destroyed the old ones.
- Reduce both epochs into one named frame and epoch; report the measured delta with both dates.
- If you consult HTDP for a modeled coseismic offset, keep that number on a separate line labeled as a model between specified dates and software version.
- Watch for afterslip and aftershocks: a campaign two days after the main shock is not the same experiment as a campaign two months later.
Do not convert a published magnitude into a displacement with a rule of thumb. Magnitude is not a field measurement of your corner.
Sensors and what they actually sense
- Digital/optical leveling remains the cleanest local vertical for dams, walls, and valley profiles when you can run a line. Record temperature, rod type, and collimation checks.
- Total-station deformation sets (multiple faces, forced-centering pillars) catch 3-D structural motion at close range. Keep the instrument in the stable block, not on the moving crest, unless that is the point of the experiment.
- Static or rapid-static GNSS on deep rods or braced monuments tracks crustal and regional subsidence. Log antenna type, ARP height method, and the exact processing engine/orbits.
- Real-time kinematic GNSS can monitor construction and landslides at high rate but needs a stable base and a radio/cellular path; treat it as a different specification than a dual-occupation static campaign.
- Extensometers and piezometers (USGS-style valley networks) separate compaction at depth from surface lowering. A PLS may not install the well, but the surface monument next to it must share a documented height relationship.
- Terrestrial or mobile laser scanning is excellent for faces and cracks if you register to stable control each epoch. Cloud-to-cloud "movement" without control is a picture, not a survey.
Campaign notes that later analysis can use
- Monument condition, witness photos, and whether the mark is in fill, pavement, or undisturbed ground
- Start and stop time, antenna heights, rod readings, and any instrument change
- Weather, pumping, construction loading, reservoir stage, or tide if those loads could move the point during the session
- Adjustment residuals and which points were constrained
- A plain-language statement: "Between 2022-03-14 and 2024-11-02, mark DH-4 moved 0.042 m down and 0.011 m toward azimuth 245° relative to bedrock marks R1–R3, NAD 83 (2011) epoch 2010.00, NAVD88 (GEOID18)." That sentence is a monitoring result. "The valley is subsiding" is not.
If the same marks also serve a cadastral retracement, keep the monitoring displacement table separate from the boundary decision. Domain IV will ask whether earth movement moved the lines. Domain III asks whether you measured the movement with a scheme that can be repeated.
Which network design produces a defensible California monitoring displacement?
What is the correct role of NGS Horizontal Time-Dependent Positioning (HTDP) in a monitoring survey?
A valley GNSS mark shows downward motion over three years while a nearby fault-block mark shows mostly horizontal secular motion. How should the PLS report the results?