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Cities detect land subsidence by combining satellite interferometric synthetic aperture radar (InSAR) with ground measurements from survey-grade GPS/GNSS, precise leveling and, in some areas, borehole extensometers. InSAR shows where the surface is moving across a wide area; ground stations provide reference elevations and independent checks at selected points. Agencies then compare the movement with groundwater levels, pumping, geology, construction and land-use records to determine what is changing and where further investigation is needed.
No single sensor explains a sinking city. Radar measures displacement in the satellite’s viewing geometry, while ground instruments are sparse but provide stable reference points and, with extensometers, information about compaction below the surface. The combined network supports water-management decisions, infrastructure planning and targeted engineering studies.
What land-subsidence monitoring measures
Land subsidence is the lowering of the ground surface relative to a reference frame. It can be gradual and widespread, concentrated around a pumping well, or uneven across neighborhoods. Uplift can occur in the same metropolitan area, so a citywide average can hide important local behavior.
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- Where is the surface moving? InSAR maps movement over broad areas, often revealing gradients and localized bowls that point measurements would miss.
- How much has a known point moved? GPS/GNSS and leveling establish elevations at monuments and benchmarks that can be revisited.
- What is likely causing it? Movement records become diagnostic only when paired with groundwater, pumping, geology, construction and land-use information.
A displacement map is therefore an observation, not a diagnosis. The same pattern can have different causes in different basins.
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How satellite InSAR detects motion
Repeat radar observations
A radar satellite passes over the same area repeatedly and records the phase of the returned signal. Processing compares observations from different dates to estimate the change in distance between the satellite and the ground along the radar line of sight. As the USGS explains, “By bouncing radar signals off the ground surface from the same point in space but at different times, the radar satellite can measure the change in distance between the satellite and ground (range change) as the land surface uplifts or subsides.”
Interferograms and time-series products turn those phase differences into maps of relative displacement. Under suitable conditions, USGS demonstrations show measurements at a scale of a few millimeters. That figure is not a universal accuracy guarantee: vegetation, atmospheric effects, radar geometry, surface changes, processing choices and the observation period all affect the result.
Line of sight is not automatically vertical motion
InSAR records motion toward or away from the spacecraft. A mostly vertical sinking signal can dominate in a city, but horizontal movement also contributes to the measured line-of-sight change. Analysts use the satellite’s viewing geometry, observations from different orbital directions and ground data to interpret vertical and horizontal components. A single color scale on a map should not be read as a direct measurement of vertical lowering without that context.
What a city receives from InSAR
- A spatially continuous map over the radar-covered area rather than measurements only at monuments.
- Localized deformation patterns that can show a moving neighborhood, a subsidence bowl or a stable zone inside a broader affected region.
- Changes between acquisition dates and, with a suitable time series, acceleration, slowing or reversal.
- A way to prioritize where survey crews, wells or extensometers should be installed.
Radar coverage is broad, but the result remains sensitive to ground conditions. Dense vegetation and rapidly changing surfaces can reduce coherence. Newer long-wavelength radar can help in difficult environments, yet every product still requires quality checks and ground interpretation.
Ground sensors that anchor the satellite map
GPS and GNSS benchmarks
Survey-grade GPS/GNSS receivers occupy stable geodetic monuments and determine their positions relative to a reference frame. Repeated occupations show how a benchmark’s elevation changes over time. Continuous stations can provide a different temporal record from campaign-style surveys, but the sources do not prescribe one universal observation schedule.
Precise leveling
Conventional spirit leveling measures elevation differences between benchmarks along carefully controlled routes. It is a point-and-line method rather than an areal map, but it can provide highly consistent relative elevations for a designed network. Repeated leveling identifies movement between benchmarks and helps test whether an InSAR pattern is real.
Borehole extensometers
An extensometer measures compaction or expansion at depth in a borehole. That depth information helps distinguish surface movement associated with compressing aquifer materials from deformation occurring elsewhere in the geological column. Houston-area monitoring has combined extensometers, GPS, leveling and InSAR for this reason.
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Why the methods are complementary
| Method | Primary coverage | What it contributes | Main interpretation issue |
|---|---|---|---|
| InSAR | Broad area with dense spatial detail | Relative surface-displacement patterns between radar acquisitions | Measures line-of-sight change; atmospheric, vegetation and geometry effects require screening |
| GPS/GNSS | Selected monuments or continuous stations | Reference-frame positions and repeat point elevations | Stations are spatially sparse and must be tied to stable references |
| Precise leveling | Routes connecting benchmarks | Accurate relative elevation differences along a designed network | Provides route and point information, not a continuous citywide surface |
| Borehole extensometer | One or more instrumented boreholes | Compaction or expansion at depth | Represents the instrumented location and geological interval |
Ground observations can support, verify and add temporal detail to InSAR. Conversely, a satellite map can show where a ground network needs more benchmarks or when a repeat survey is most useful.
A practical municipal monitoring workflow
1. Define the decision and reference frame
Start with the decision the network must support: regulating groundwater withdrawals, protecting a runway, checking a levee, designing a pipeline or screening a new well field. Select a vertical reference frame and identify monuments expected to remain stable. Without a consistent reference, separate surveys and satellite products cannot be compared reliably.
2. Assemble the context layers
Compile aquifer and confining-layer maps, groundwater levels, pumping records, well construction, oil and gas activity where relevant, reclaimed-land boundaries, landfill history, construction records and infrastructure locations. These layers do not prove a cause, but they allow analysts to test whether movement follows a plausible mechanism.
3. Process repeat radar scenes
Analysts select compatible scenes, form interferograms and remove or flag areas with poor coherence and atmospheric contamination. A time series can separate persistent trends from one-off changes. The output should include quality indicators and the observation dates, not just a colored deformation image.
4. Establish and reoccupy ground benchmarks
Install or adopt stable GPS/GNSS monuments and leveling benchmarks in areas with different radar signatures, near critical infrastructure and at suspected deformation boundaries. Reoccupy them on a schedule appropriate to the decision and the observed rate. Where depth is important, add extensometers in representative wells.
5. Compare the independent measurements
Check whether the sign, magnitude, timing and spatial trend of satellite displacement agree with benchmark and borehole records within their respective uncertainties. A mismatch may indicate radar decorrelation, an unstable benchmark, an unmodeled horizontal component or a real depth-dependent process; it is a reason to investigate, not to force agreement.
6. Update the network and publish qualified results
Use the combined evidence to add stations, alter survey routes or focus hydrogeologic studies. Reports should state the observation period, processing method, reference frame, line-of-sight geometry and known limitations. Maps should distinguish measured displacement from interpretations about cause.
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What causes cities to sink
Groundwater withdrawal is a documented driver in several urban basins. Lower pore-water pressure can allow compressible aquifer and clay layers to compact, lowering the surface. The effect depends on the local geology and on whether compaction is recoverable or permanent.
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- Oil and gas extraction can alter subsurface pressure and deformation.
- Reclaimed land, landfill and artificial fill can consolidate under their own weight or when wetted.
- Natural geological processes, including post-glacial adjustment, can produce sinking or uplift.
- Urban construction and changing loads can create localized settlement.
Because these mechanisms can produce similar surface patterns, a movement map alone cannot identify the cause. Time alignment with pumping and groundwater records, depth measurements from extensometers and geological information are needed for a defensible attribution.
Case studies show why cities combine methods
Houston–Galveston, Texas
A USGS study analyzed ERS-1 and ERS-2 radar scenes and produced 27 interferograms for the 1993–2000 investigation using four overlapping scenes. GPS data supported and verified the InSAR results and added temporal resolution. Historical records in the study reported up to 3 meters of accumulated subsidence in parts of the region by 1979. During the study period, the earlier downtown Houston and Ship Channel subsidence area had stabilized, while newer subsidence appeared farther west and north of Galveston Bay. Those figures describe the historical study period and boundaries, not present-day rates.
The USGS describes the regional network as using “borehole extensometers, Global Positioning System (GPS) and conventional spirit-leveling surveys, and more recently, an emerging technology—Interferometric Synthetic Aperture Radar (InSAR).” The combination supplied area-wide patterns, surface reference points and depth information.
San Diego area, California
For a network associated with new groundwater-production wells, USGS, the City of San Diego and Sweetwater Authority used earlier InSAR deformation maps to guide survey design. Static GPS measurements established baseline elevations at 14 benchmarks, and first-order leveling established 20 additional benchmark elevations. Planned repeat InSAR was intended to add spatial detail and help determine where and when ground surveys should be repeated. The project illustrates a deliberate sequence: use satellite evidence to design a manageable ground network, then use both data types together.
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New York City
A NASA Jet Propulsion Laboratory summary mapped vertical land motion across the New York metropolitan area with ESA Sentinel-1 InSAR from 2016 through 2023. The metropolitan average was about 0.06 inches, or 1.6 millimeters, of sinking per year during that period. Local rates were faster at LaGuardia Airport runway 13/31, about 0.15 inches (3.7 millimeters) per year, and at Arthur Ashe Stadium, about 0.18 inches (4.6 millimeters) per year. Some locations were rising. The contrast demonstrates why citywide averages cannot substitute for hotspot maps. NASA attributed the pattern to a combination of human land modification and natural post-glacial adjustment. Lead author Brett Buzzanga said, “We’ve produced such a detailed map of vertical land motion in the New York City area that there are features popping out that haven’t been noticed before.”
Mexico City and preliminary NISAR observations
In an April 29, 2026 report, NASA and the Jet Propulsion Laboratory described preliminary NISAR measurements showing parts of Mexico City and its surroundings subsiding by more than half an inch, or more than 2 centimeters, per month between October 25, 2025 and January 17, 2026. The report linked the long-running problem to groundwater pumping and urban development over an ancient lakebed. It also warned that some yellow and red areas could contain residual noise expected to decrease as more data were collected. These values are preliminary, date-bounded observations, not a universal rate for Mexico City or other cities.
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NASA deputy project manager Craig Ferguson said, “NISAR’s long wavelength L-band radar will make it possible to detect and track land subsidence in more challenging and densely vegetated regions such as coastal communities where they may have the compounding effects of both land subsidence and sea level rise.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How accurate is satellite subsidence monitoring?
Under suitable conditions, InSAR can resolve displacement on the order of a few millimeters, according to USGS descriptions. That is a statement about achievable measurement scale in appropriate applications, not a fixed error bar for every city, date or processing chain. Accuracy and usefulness depend on:
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- Radar wavelength, orbit and viewing geometry.
- Time separation and the stability of the radar-reflecting surface.
- Atmospheric delay and other processing errors.
- The quality and stability of ground control points.
- Whether the observed motion is vertical, horizontal or a combination.
- The length of the time series and the rate being measured.
GPS/GNSS and leveling do not make the map automatically correct: a benchmark can itself move, and sparse points cannot represent every neighborhood. The strongest practice is independent comparison, explicit uncertainty reporting and investigation of disagreements.
Common interpretation mistakes and fixes
Treating a color map as a cause map
Mistake: assuming red or blue directly identifies groundwater pumping, a failing structure or a particular fault.
Fix: compare the displacement pattern with pumping, groundwater, geology, construction and depth records before assigning a cause.
Reading line-of-sight motion as pure vertical settlement
Mistake: converting a single radar measurement directly into vertical millimeters.
Fix: account for viewing geometry and, where possible, combine ascending and descending observations with GNSS or leveling.
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Using a citywide average for a local asset
Mistake: applying a metropolitan mean to a runway, bridge, pipeline or building site.
Fix: inspect the local time series and install or reoccupy nearby benchmarks.
Ignoring the date range
Mistake: presenting a historical cumulative value or a short preliminary interval as a current long-term rate.
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Fix: state the exact observation dates, study area and whether the result is preliminary.
Assuming every bright anomaly is real movement
Mistake: treating noisy or decorrelated pixels as confirmed subsidence.
Fix: review coherence and quality layers, look for persistence through time and seek ground confirmation.
How monitoring supports city decisions
Water managers can compare deformation with groundwater-level histories and pumping changes to evaluate whether restrictions or recharge efforts are affecting compaction. Engineers can screen roads, runways, pipelines, levees and foundations for differential movement rather than relying on a regional average. Emergency and planning teams can use persistent trends to prioritize inspections, while geologists can target boreholes and samples where surface and depth records disagree.
The most defensible program is adaptive. Broad InSAR coverage identifies patterns; benchmarks verify them; extensometers test depth behavior; and hydrogeologic and construction records explain timing. As new satellite acquisitions and ground surveys arrive, the network can shift effort toward accelerating zones, infrastructure corridors or areas where uncertainty remains high.
Quick Recap
Key takeaways
- InSAR provides wide-area maps of relative surface displacement from repeat radar observations.
- GPS/GNSS and leveling supply reference elevations at selected points; extensometers reveal compaction at depth.
- Satellite and ground methods are complementary, not competing substitutes.
- Subsidence can vary sharply across a city and can change direction or rate over time.
- Groundwater pumping, extraction, fill, construction and natural geological adjustment may all contribute, so movement alone does not identify a cause.
- Every result needs its dates, geometry, reference frame, quality limits and independent checks.
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