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Cities monitor land subsidence by combining satellite interferometric synthetic aperture radar (InSAR) with ground measurements such as GPS/GNSS, precise leveling and, where necessary, borehole extensometers. InSAR shows where the surface is moving across a broad area; ground surveys establish reference elevations and check the satellite signal at selected points. Agencies then compare the movement with groundwater levels, pumping, geology, construction and land-use history to investigate causes and decide where action is needed.
What land-subsidence monitoring is designed to find
Land subsidence is downward movement of the ground surface. It can occur across an entire basin, along a corridor or at isolated hotspots, and the rate can change over time. A city does not learn the cause simply by observing that the surface moved. Monitoring first establishes the pattern, amount and timing of displacement; hydrologic, geologic and land-use data are then used to explain it.
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The practical questions are usually:
- Where is the ground moving, and how is the pattern distributed across neighborhoods, industrial areas, airports or wetlands?
- How fast is it moving during a defined observation period?
- Is movement continuing, accelerating, stabilizing or changing location?
- Does the movement correspond with groundwater-level changes, pumping, construction or known geology?
- Which roads, pipelines, buildings, levees, runways or water systems need closer inspection?
A useful monitoring program therefore treats satellite and ground observations as complementary rather than competing technologies.
How satellite InSAR detects a sinking surface
Repeated radar observations
A radar satellite revisits the same area and records the returned radar signal. By comparing observations from different dates, processors estimate changes in the distance between the satellite and the ground along the radar’s line of sight. The U.S. Geological Survey describes the principle as bouncing radar signals off the surface from the same point in space at different times and measuring the resulting range change as land uplifts or subsides.
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The comparison produces interferograms and derived displacement maps. They can reveal coherent spatial patterns over a city or a larger basin, including localized zones that a sparse network of ground monuments might miss. Under suitable conditions, USGS describes measurements at a scale of a few millimeters, but that is not a universal accuracy specification for every site, processing method or observation period.
What an InSAR map actually measures
InSAR measures relative displacement in the satellite’s viewing geometry. A single line-of-sight result is not automatically the complete vertical movement. The viewing angle, observation span, processing choices and ground conditions all affect interpretation. Analysts may need multiple viewing geometries or independent ground measurements to understand the vertical component and to separate genuine motion from processing or environmental effects.
The map is also not a diagnosis. A red or otherwise high-displacement area says that the surface changed relative to the reference used in the analysis; it does not by itself prove that groundwater pumping, oil extraction, landfill settlement or a natural geologic process caused the change.
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Satellite radar can screen a wide area repeatedly, making it useful for finding unexpected deformation and for seeing how a known problem extends beyond a few monitored points. It can also show that an old hotspot has stabilized while a new one is developing elsewhere. This spatial context is the main reason cities use InSAR to guide where to place or repeat ground surveys.
What ground sensors add
GPS and GNSS benchmarks
Survey-grade GPS/GNSS receivers observe positions at stable geodetic monuments. Repeated occupations provide point measurements of elevation and horizontal position. These points supply a ground reference for satellite products and can confirm whether a mapped displacement pattern is occurring at a specific location.
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Precise leveling between benchmarks
Conventional spirit-leveling surveys measure elevation differences along a route connecting benchmarks. Leveling is spatially limited compared with a satellite map, but it provides a direct, high-quality elevation reference along the surveyed network. Cities can repeat the same routes to detect changes in relative elevation.
Borehole extensometers
An extensometer installed in a borehole tracks compaction or expansion at depth. This helps distinguish surface movement associated with subsurface compaction from motion that may originate in other layers. Houston-area monitoring has used borehole extensometers together with GPS, leveling and InSAR.
The roles are different
| Method | Primary strength | What it measures | Main limitation |
|---|---|---|---|
| InSAR | Broad-area spatial coverage | Relative surface displacement in radar line-of-sight geometry between observation dates | Needs interpretation; one viewing geometry is not automatically full vertical motion and can be affected by ground and processing conditions |
| GPS/GNSS | Reference and validation at known monuments | Position and elevation at selected points | Points are spatially sparse and require survey-grade equipment and stable monuments |
| Precise leveling | Accurate elevation differences along benchmark routes | Relative elevation between benchmarks | Provides corridor measurements rather than continuous citywide coverage |
| Borehole extensometer | Information about deformation at depth | Compaction or expansion in a borehole interval | Represents a small number of installed locations |
No cited source establishes a universal winner for accuracy, cost or revisit frequency. The appropriate mix depends on the city’s geology, hazards, satellite geometry, existing monuments and management decisions.
How a city builds an integrated monitoring workflow
- Define the management question. Decide whether the program is screening a whole basin, protecting critical infrastructure, evaluating a new well field or tracking an already known hotspot. The question determines the area, observation period and ground network.
- Assemble baseline context. Map geology, aquifers, groundwater levels, pumping records, oil and gas activity, reclaimed land, landfill, construction history and infrastructure. These layers are needed later because displacement alone does not identify a cause.
- Process repeat radar observations. Analysts compare radar scenes from the same area and produce interferograms and displacement maps. They document the dates, geometry and processing conditions so that apparent differences are not mistaken for a physical trend.
- Locate candidate hotspots. Broad InSAR coverage identifies coherent zones, sharp gradients and areas where movement appears to be changing. Analysts also note places where radar interpretation is uncertain or where residual noise may remain.
- Install or select ground references. Choose stable GPS/GNSS monuments, leveling benchmarks and, where depth information is important, extensometer sites. Place them to sample both moving and apparently stable areas, as well as assets that need protection.
- Measure baseline elevations and positions. Conduct static GPS/GNSS occupations and first-order leveling before relying on trend comparisons. The baseline establishes the reference against which later surveys and satellite products can be checked.
- Repeat measurements on a management schedule. InSAR supplies observations between satellite acquisition dates, while repeat GPS, leveling or extensometer records provide independent checks and, at selected sites, a different temporal record. The sources do not prescribe one universal revisit cadence.
- Compare datasets, not isolated colors on a map. Check whether satellite displacement agrees with benchmark changes and, where available, depth compaction. Investigate disagreements before publishing a rate or declaring a hotspot.
- Test possible causes. Compare movement timing and location with groundwater-level histories, pumping, extraction, geology, fill or landfill and construction. A causal explanation should fit the spatial and temporal evidence, not just the map’s most intense color.
- Turn results into decisions. Use the findings to adjust water-management programs, inspect or redesign infrastructure, refine well locations and determine where additional surveys or geotechnical investigation are warranted.
Case study: Houston-Galveston, Texas
The Houston-Galveston example illustrates why long-term programs combine several instruments. A USGS study analyzed ERS-1 and ERS-2 radar scenes and processed 27 interferograms from four overlapping scenes for the 1993–2000 investigation. GPS data were used to support and verify the InSAR analysis and to add temporal resolution.
The historical record reported up to 3 meters of accumulated subsidence in parts of the region by 1979. That figure is a cumulative historical amount, not a present-day rate. During the study period, the earlier subsidence area around downtown Houston and the Houston Ship Channel had stabilized, while newer subsidence was occurring farther west and north of Galveston Bay. The geographic boundaries and dates matter; the result should not be presented as a current rate for the entire region.
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USGS summarizes the Houston 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 provided surface maps, point checks and subsurface information that no single method could supply.
Case study: designing a network in the San Diego area
In the San Diego area, USGS worked with the City of San Diego and Sweetwater Authority while planning a network around new groundwater-production wells. Earlier InSAR maps helped identify where deformation had occurred and guided the placement and timing of future ground surveys.
The project established baseline elevations with static GPS measurements at 14 benchmarks and first-order leveling at 20 benchmarks. Future InSAR observations were intended to add spatial detail and help determine the logistics of repeat GPS and leveling work. This is a practical design pattern: use satellite evidence to choose representative ground points, then use those points to validate and interpret subsequent maps.
Case study: uneven movement in New York City
A NASA Jet Propulsion Laboratory summary of a study using ESA Sentinel-1 InSAR mapped vertical land motion across the New York metropolitan area from 2016 through 2023. The metropolitan average was about 0.06 inches, or 1.6 millimeters, of sinking per year during that study period.
That average concealed faster local behavior. LaGuardia Airport runway 13/31 was reported at about 0.15 inches, or 3.7 millimeters, per year, while Arthur Ashe Stadium was reported at about 0.18 inches, or 4.6 millimeters, per year. Some locations were rising. These figures apply to the named sites and study period, not to every borough or to future years.
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The study also shows why citywide averages can be a poor basis for asset decisions. A transportation agency, airport operator or building manager needs the local rate and pattern at its asset, not an average that blends sinking zones with stable or rising ground. NASA attributed the observed pattern to a combination of human land modification and natural post-glacial adjustment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Case study: preliminary NISAR observations in Mexico City
On April 29, 2026, NASA and JPL described preliminary NISAR results for Mexico City and its surroundings. For observations from October 25, 2025, through January 17, 2026, parts of the area appeared to be subsiding by more than half an inch, or more than 2 centimeters, per month.
Those results were explicitly preliminary. NASA noted that some yellow and red areas could contain residual noise expected to decrease as additional data were collected. The report placed the long-running subsidence problem in the context of groundwater pumping and urban development over an ancient lakebed.
NISAR’s L-band radar is intended to help track deformation in challenging, densely vegetated regions. NASA deputy project manager Craig Ferguson said the capability could be important for coastal communities facing the combined effects of subsidence and sea-level rise. Mission products and operational availability evolve, so a city should verify current data status before treating an early product as an operational rate record.
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Groundwater withdrawal and aquifer compaction
When pumping lowers groundwater levels, compressible aquifer materials can compact and the surface can fall. Analysts compare groundwater-level histories and pumping records with the timing and location of displacement. USGS investigations in the Houston-Galveston region and Coachella Valley used this kind of comparison.
Oil and gas extraction
Extraction can alter subsurface pressures and contribute to deformation in some regions. Houston-area documentation identifies oil and gas extraction among the possible drivers alongside groundwater withdrawal. The relative contribution must be established for the particular field and period.
Reclaimed land, landfill and construction
New fill, landfill and major construction can settle, especially where underlying materials are compressible. These causes may be local and may create sharp gradients around a project, so infrastructure records and construction history are essential context for interpreting a hotspot.
Natural geologic processes
Not all motion is caused by recent human activity. The New York study identified natural post-glacial adjustment as one factor, and some locations in the metropolitan map were rising. A monitoring program should therefore test both human and natural explanations.
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Accuracy, uncertainty and common interpretation errors
- Treating line-of-sight displacement as vertical subsidence. Radar measures in its viewing direction. Converting that result into vertical motion requires geometry and, often, independent ground constraints.
- Reading a map as a causal diagnosis. Displacement identifies motion, not whether pumping, extraction, fill or geology produced it.
- Using a citywide average for a specific asset. New York’s average and its faster airport and stadium rates demonstrate why local values matter.
- Ignoring the observation window. Houston’s 1993–2000 analysis and New York’s 2016–2023 map describe defined periods. A historical cumulative amount, such as 3 meters by 1979 in parts of Houston, is not a current annual rate.
- Assuming every colored area is reliable. The preliminary Mexico City NISAR account warned that some apparent high-displacement areas included residual noise.
- Relying on consumer positioning devices. Ordinary navigation GPS is not a substitute for survey-grade GNSS monuments, precise leveling or a controlled geodetic reference.
- Placing all instruments only in the apparent hotspot. Stable reference points and measurements outside the moving zone are needed to determine relative change.
What monitoring results are used for
Water agencies can compare deformation with pumping and groundwater levels when evaluating management strategies. Public-works departments can prioritize inspections of roads, pipelines, levees, runways and buildings where movement gradients are concentrated. Planning departments can require additional investigation for development on compressible or reclaimed ground. Emergency and coastal planners can combine subsidence information with sea-level-rise exposure.
The most defensible output is not just a colored map. It is a documented displacement time series with stated dates and geometry, checked against ground references, accompanied by uncertainty and a reasoned assessment of possible causes. Where the evidence is incomplete, the appropriate decision may be another survey, a borehole instrument or improved groundwater records rather than an immediate causal claim.
A practical checklist for a municipal program
- Define the area, assets and decision the monitoring must support.
- Record satellite acquisition dates, viewing geometry and processing assumptions.
- Maintain stable GPS/GNSS and leveling benchmarks with documented baseline elevations.
- Use extensometers where surface data alone cannot show which subsurface layers are compacting.
- Pair displacement with groundwater, pumping, extraction, geology, fill and construction records.
- Report local hotspots separately from regional averages.
- Label historical, study-period and preliminary values so they are not mistaken for current rates.
- Investigate apparent satellite-ground disagreements before taking regulatory or engineering action.
- Reassess the network as new wells, infrastructure, satellite missions or processing products become available.
Why the combined approach is the answer
Satellites answer “where and how the surface pattern changes?” Ground sensors answer “what is happening at these reference points and, with extensometers, at depth?” Hydrologic and geologic records address “why might it be happening?” Cities need all three perspectives because subsidence is spatially uneven, time-dependent and not uniquely tied to one cause. An integrated network turns wide-area detection into measurements that can support water management, infrastructure planning and targeted investigation.
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