U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5211

Prepared in cooperation with the Harris-Galveston Subsidence District

Investigation of Land Subsidence in the Houston-Galveston 

Region of Texas By Using the Global Positioning System and 

Interferometric Synthetic Aperture Radar, 1993–2000

Galveston 

Bay

BRAZORIA 

COUNTY

LIBERTY 

COUNTY

CHAMBERS 

 COUNTY

GALVESTON COUNTY

HARRIS COUNTY

FORT BEND COUNTY

WALLER 

COUNTY

MONTGOMERY COUNTY

Investigation of Land Subsidence in the 

Houston-Galveston Region of Texas By 

Using the Global Positioning System and 

Interferometric Synthetic Aperture Radar, 

1993–2000

By Gerald W. Bawden, Michaela R. Johnson, Mark C. Kasmarek, Justin Brandt, 

and Clifton S. Middleton

Prepared in cooperation with the Harris-Galveston Subsidence District

Scientific Investigations Report 2012–5211

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

This and other USGS information products are available at http://store.usgs.gov/

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Box 25286, Denver Federal Center

Denver, CO 80225

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Bawden, G.W., Johnson, M.R., Kasmarek, M.C., Brandt, Justin, and Middleton, C.S., 2012, Investigation of land 

subsidence in the Houston-Galveston region of Texas by using the Global Positioning System and interferometric 

synthetic aperture radar, 1993–2000: U.S. Geological Survey Scientific Investigations Report 2012–5211, 88 p.
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope .............................................................................................................................3

Description of Study Area ..................................................................................................................3

Hydrogeologic Framework .................................................................................................................3

Mechanism and History of Subsidence in the Houston-Galveston Region ...............................5

Previous Investigations .......................................................................................................................6

Methodology ..................................................................................................................................................6

Global Positioning System Processing ............................................................................................6

Baseline-Pair Method ................................................................................................................6

Selection of a Stable Reference Frame ................................................................................12

Interferometric Synthetic Aperture Radar Processing ...............................................................12

Interferogram Overview ..........................................................................................................12

Interferometric Synthetic Aperture Radar Error Considerations .....................................16

Subsidence in the Houston-Galveston Region, 1993–2000 ..................................................................16

Global Positioning System Analysis ...............................................................................................16

Global Positioning System Baseline-Pair Analyses ............................................................16

Time-Series Patterns ................................................................................................................18

Global Positioning System Vertical and Horizontal Velocities ..........................................18

InSAR Magnitude and Extent ...........................................................................................................18

Limitations ....................................................................................................................................................29

Summary .......................................................................................................................................................32

References Cited .........................................................................................................................................32

Appendixes

1. Interferometic Synthetic Aperture Radar Processing and Error Analysis ......................37

2. Global Positioning System Time-series for All GPS Stations with Least-Squares 

Linear-Regression Lines Depicting the Directional Velocity ..............................................45

Figures

1. Map showing the Houston-Galveston region, Texas, with interferometric 

synthetic aperture radar (InSAR) scenes, including radar track and frame 

numbers, and management areas for the Harris-Galveston Subsidence District 

(2010) and Fort Bend Subsidence District (2009) ....................................................................2

2. Hydrogeologic section of the Gulf Coast aquifer system in Harris and adjacent 

counties, Texas .............................................................................................................................4

3. Diagram depicting the mechanism of subsidence in an aquifer composed of 

vertically discontinuous fine to coarse-grained sands and gravels with 

interbedded silts and clays ........................................................................................................5

4. Map showing locations of National Geodetic Survey Continuously Operating 

Reference Station sites, Harris-Galveston Subsidence District Port-A-Measure 

sites, and extensometer sites in the Houston-Galveston region, Texas ............................7
iv

5. Map showing the National Geodetic Survey (NGS) Continuously Operating 

Reference Stations (CORS) and Harris-Galveston Subsidence District Port-AMeasure (PAM) network of Global Positioning System (GPS) stations used 

(pink triangle) and not used (white circle) to evaluate subsidence amounts, 

Houston-Galveston region, Texas .............................................................................................8

6. Global Positioning System (GPS) time series of relative movement with 

least-squares linear-regression lines depicting the directional movement in 

millimeters per year (mm/yr) between A, Continuously Operating Reference 

Stations (CORS) Lake Houston (LKHU) and Northeast 2250 CORS ARP (NETP) 

and B, Addicks 1795 CORS ARP (ADKS) and Lake Houston (LKHU) ...................................11

7. Conceptual depiction of horizontal and vertical movement associated with aquifer 

system compaction ...................................................................................................................13

8. Global Positioning System (GPS) time series of relative movement with leastsquares linear-regression lines depicting the directional movement in millimeters 

per year (mm/yr) between GPS station ANG1 and Continuously Operating 

Reference Station Northeast 2250 CORS ARP (NETP) ........................................................14

9. Simplified interferogram ...........................................................................................................15

10. Examples of interferograms .....................................................................................................17

11. Global Positioning System (GPS) time series for Continuously Operating 

Reference Stations (CORS) Addicks 1795 CORS ARP (ADKS) and Northeast 

2250 CORS ARP (NETP) with least-squares linear-regression lines depicting 

directional velocity ....................................................................................................................19

12. Map showing the Global Positioning System- (GPS-) derived horizontal and 

vertical velocities for Continuously Operating Reference Stations (CORS) and 

Harris-Galveston Subsidence District Port-A-Measure (PAM) sites, with 

Northeast 2250 CORS ARP (NETP) (star) as the reference station ...................................20

13. Comparison of an interferogram with water-level contours for the HoustonGalveston region, Texas ............................................................................................................22

14. Interferometric synthetic aperture radar- (InSAR-) measured subsidence 

from January 14, 1996, to December 3, 2000 .........................................................................23

15. Interferogram for January 30, 1996, to December 30, 1997 showing primary 

areas of subsidence ..................................................................................................................24

16. A 0.9-year interferogram (January 30, 1996, to December 30, 1997) of the HoustonGalveston region, Texas, showing 60 millimeters (mm) of subsidence in the 

northwestern part of the interferogram .................................................................................25

17. A 1.3-year interferogram (January 14, 1997, to May 19, 1998) of the HoustonGalveston region, Texas, showing approximately 60 millimeters (mm) of 

subsidence in the northwestern part of the interferogram ................................................26

18. A 2.3-year interferogram (January 29, 1996, to May 19, 1998) of the HoustonGalveston region, Texas, showing a localized area with more than 90 millimeters 

(mm) of subsidence ...................................................................................................................27

19. Comparison of contoured interferograms (January 29, 1996, to May 19, 1998) in 

the Houston-Galveston region, Texas, with velocities (arrows) in millimeters per 

year (mm/yr) and subsidence rates (circles) in mm/yr derived from Global 

Positioning System (GPS) data ................................................................................................28

20. Detailed interferogram showing subsidence during about 1 year (February 18, 

1997, to February 3, 1998) in an area around Seabrook, Texas, with a vector 

showing the displacement rates in millimeters per year (mm/yr) derived from 

Global Positioning System (GPS) data ...................................................................................30

21. Four detailed interferograms of the Seabrook, Texas, area showing different 

subsidence results ....................................................................................................................31
v

Tables

1. Global Positioning System (GPS) stations used to validate subsidence 

magnitudes determined by using interferometric synthetic aperture radar 

(InSAR) with directional velocities in millimeters per year for north, east, and 

up positional changes between a given GPS station and the Northeast 2250 

Continuously Operating Reference Station antenna reference point (Northeast 

2250 CORS ARP) ...........................................................................................................................9

2. Interferograms generated for the Houston-Galveston region from data obtained 

during 1993–2000 from European remote sensing (ERS) satellites ERS–1 and 

ERS–2 ...........................................................................................................................................21

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

square meter (m2) 10.76 square foot (ft2) 

square kilometer (km2) 0.3861 square mile (mi2)

Flow rate

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

cubic meter per day (m3/d) 35.31 cubic foot per day (ft3/d) 

cubic meter per day (m3/d) 264.2 gallon per day (gal/d) 

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Investigation of Land Subsidence in the HoustonGalveston Region of Texas By Using the Global Positioning 

System and Interferometric Synthetic Aperture Radar, 

1993–2000

By Gerald W. Bawden,1 Michaela R. Johnson,1Mark C. Kasmarek,1 Justin Brandt,1 and Clifton S. Middleton2

Abstract

Since the early 1900s, groundwater has been the 

primary source of municipal, industrial, and agricultural 

water supplies for the Houston-Galveston region, Texas. 

The region’s combination of hydrogeology and nearly 

century-long use of groundwater has resulted in one of the 

largest areas of subsidence in the United States; by 1979, 

as much as 3 meters (m) of subsidence had occurred, and 

approximately 8,300 square kilometers of land had subsided 

more than 0.3 m. The U.S. Geological Survey, in cooperation 

with the Harris-Galveston Subsidence District, used 

interferometric synthetic aperture radar (InSAR) data obtained 

for four overlapping scenes from European remote sensing 

satellites ERS-1 and ERS-2 to analyze land subsidence in the 

Houston-Galveston region of Texas. The InSAR data were 

processed into 27 interferograms that delineate and quantify 

land-subsidence patterns and magnitudes. Contemporaneous 

data from the Global Positioning System (GPS) were 

reprocessed by the National Geodetic Survey and analyzed to 

support, verify, and provide temporal resolution to the InSAR 

investigation. 

The interferograms show that the area of historical 

subsidence in downtown Houston along the Houston Ship 

Channel has stabilized and that recent subsidence occurs 

farther west and north of Galveston Bay. Three areas of recent 

subsidence were delineated along a broad arcuate (bowshaped) feature from Spring, Tex., southwest to Cypress, Tex., 

and south to Sugar Land, Tex., with subsidence rates ranging 

from 15 millimeters per year (mm/yr) to greater than 60 mm/

yr. Multiyear interferograms near Seabrook, Tex., within the 

historical subsidence area and nearby Galveston Bay, show 

several fringes of subsidence (approximately 85 millimeters 

from January 1996 to December 1997) in the area; however 

it is difficult to determine the subsidence magnitude near 

Seabrook because many of the InSAR fringes were truncated 

or ill-defined. Horizontal and vertical GPS data throughout 

the area support the InSAR measured subsidence rates and 

extent. The subsidence rates for a few GPS stations northwest 

of Houston began to decrease in 2007, which may indicate that 

subsidence may be decreasing in these areas.

Introduction

The Houston-Galveston region—comprising Harris and 

Galveston Counties and adjacent parts of Brazoria, Chambers, 

Fort Bend, Grimes, Liberty, Montgomery, San Jacinto, Walker, 

and Waller Counties (fig. 1)—is one of the largest areas of 

subsidence in the United States (Galloway and others, 1999). 

Most of the subsidence in the Houston-Galveston region 

(which includes the greater Houston metropolitan area) 

has occurred as a direct result of groundwater withdrawals 

for municipal supply, industrial use, and irrigation that 

depressured and dewatered the major aquifers in the area, 

thereby causing compaction of the clay layers of the aquifer 

sediments (Kasmarek and others, 2010; Johnson and others, 

2011). Groundwater has historically been the principal source 

of water for municipal, industrial, and agricultural uses, 

and groundwater use in the Houston-Galveston region had 

increased rapidly for many decades to meet the water needs 

of the rapidly growing population (Seifert and Drabek, 2006). 

Since the 1990s, surface water has been increasingly used to 

meet these water needs and reduce reliance on groundwater 

resources in the Houston-Galveston region (Kasmarek and 

Robinson, 2004). 

Since 1975, the U.S. Geological Survey (USGS), in 

cooperation with the Harris-Galveston Subsidence District 

(HGSD), has been monitoring changes in groundwater 

elevations and measuring cumulative clay compaction by 

using extensometers at discrete locations in the HoustonGalveston region. Additionally, since the early 1990s, the 

HGSD has been monitoring land-surface elevation with the 

National Geodetic Survey (NGS) by periodically conducting 

1

U.S. Geological Survey.

2

National Geodetic Survey.
2 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

CHAMBERS

COUNTY

GALVESTON

COUNTY

JEFFERSON

 COUNTY

MONTGOMERY

COUNTY

FORT BEND

COUNTY 

BRAZORIA COUNTY

AUSTIN

COUNTY

WASHINGTON

COUNTY

LIBERTY COUNTY

HARDIN

COUNTY

BURLESON

COUNTY

BRAZOS

COUNTY

ROBERTSON

COUNTY

JACKSON

COUNTY

WALKER

COUNTY

GRIMES

COUNTY

SAN JACINTO

COUNTY

POLK

COUNTY

TYLER

COUNTY

HARRIS COUNTY

COLORADO

COUNTY

WALLER COUNTY

WHARTON COUNTY

MATAGORDA COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

30°30'

30°00'

29°30'

29°00'

94°30'

95°00'

95°30'

96°00'

96°30'

Base from U.S. Geological Survey digital data, 2001, 1:2,000,000

Albers equal-area conic projection

North American Datum of 1983

Study area

TEXAS

Harris-Galveston

 Subsidence District

Fort Bend Subsidence

 District

InSAR scene—With

 scene number (table 2)

EXPLANATION

484/f3015

InSAR, Interferometric synthetic aperture radar

Houston

 Ship

 Channel 

Goose Creek

Oil Field

Hardy Toll

Road

249

§ ̈¦45

288

§ ̈¦10

8

§ ̈¦45

£¤59

£¤90

§ ̈¦10

£¤59

£¤290

Galveston

Baytown

Houston

Sugar

Land 

Richmond

Jersey Village

Pasadena

Rosenberg

Seabrook

Taylor

Lake

Village

Atascocita

Bear Creek

Lake

Houston

 Addicks

Reservoir

Barker

Reservoir

GULF OF MEXICO

Galveston Bay

Trinity

Bay

East Bay

West Bay

San Jacinto Riv er

Spring Creek

Cypress Creek

Colorado River

Buffalo Bay o u

Greens Bayou

Trinity River

Brazos River

Whiteoak Bayo u

484/f2997

212/f2997

484/f3015

212/f3015

Figure 1. The Houston-Galveston region, Texas, with interferometric synthetic aperture radar (InSAR) scenes, including radar track 

and frame numbers, and management areas for the Harris-Galveston Subsidence District (2010) and Fort Bend Subsidence District 

(2009).
Introduction 3

first-order leveling surveys and by using land-based Global 

Positioning System (GPS) methods. But because of the 

extensive area affected by subsidence (Kasmarek and 

others, 2010; Johnson and others, 2011), the complexity 

associated with various magnitudes of groundwater use, and 

the hydrogeology of the Gulf Coast aquifer, a more robust 

spatial coverage of deformation was needed to provide 

information between long-term point measurement locations. 

The satellite-based interferometric synthetic aperture radar 

(InSAR) technology is capable of measuring centimeter-level 

deformation over a large area. Thus, the USGS, in cooperation 

with the HGSD, used InSAR data and reprocessed regional 

GPS data to investigate subsidence in the Houston-Galveston 

region in Texas.

Purpose and Scope

The purpose of this report is to document the magnitude 

and areal extent of land subsidence patterns in the HoustonGalveston region that were measured by using InSAR data 

that were obtained from European remote sensing (ERS) 

satellites ERS-1 and ERS-2 operated by the European Space 

Agency from July 25, 1992, to December 19, 2000, and 

reprocessed regional GPS data that were obtained from the 

NGS for the same time period (European Space Agency, 

2012). The historical subsidence in the Houston-Galveston 

region is mentioned as it relates to the varying methods of 

determining subsidence within the region. GPS time-series 

data are discussed to validate the InSAR analysis and provide 

additional information about the subsidence in regions where 

InSAR could not provide adequate measurements. This report 

first discusses the GPS analysis, then the InSAR analysis, 

and lastly the GPS and InSAR analyses in relation to regional 

subsidence rates and patterns. 

Description of Study Area

The study area exists within four overlapping InSAR 

scenes of the Houston-Galveston region obtained from two 

ERS satellites, ERS–1 and ERS–2 (fig. 1). The HoustonGalveston region is approximately 20,000 square kilometers 

(km2). Houston, Sugar Land, and Baytown are among the 

cities in the Houston-Galveston region, which was the sixth 

largest metropolitan area by population in the United States 

in 2008 with an estimated 5.7 million inhabitants (United 

States Census Bureau, 2009b). The population in the greater 

Houston metropolitan area has grown at a rate exceeding 

the overall growth rates of Texas and the Nation during 

2000–8; population increased by 21.5 percent in the greater 

Houston metropolitan area compared to 16.7 percent for 

Texas and 8 percent for the United States during 2000–8 

(U.S. Census Bureau, 2009a, b). The population of the 

greater Houston metropolitan area also increased 19.8 percent 

during 1990–2000 (United States Census Bureau, 2001), 

which includes the years during which the subsidencerelated data examined in this report were collected 

(1992–2000). 

The climate in the Houston-Galveston region is 

subtropical with 30-year normal temperatures ranging from 

5 to 34 degrees Celsius and 30-year normal precipitation 

of 112 centimeters per year (cm/yr) in Galveston and 122 

cm/yr in Houston (National Oceanic and Atmospheric 

Administration, 2008). Land-surface elevation ranges from 

near sea level upward to about 80 meters (m). The flat land 

slopes toward the coast decreasing in elevation by about 0.5 m 

per km and is prone to flooding from heavy rains and tropical 

storms or from riverine and coastal sources. Subsidence can 

exacerbate coastal flooding especially along the coast where 

effects of sea-level rise are compounded by land subsidence 

(Kreps, 1987). Near the coast of the Houston-Galveston 

region, eustatic (long term) sea-level (ESL) rise, with 

estimates of 1 to 1.2 mm/yr, combines with natural (0.05 mm/

yr) and human-induced (as much as 2.2 mm/yr) subsidence, 

thus producing an estimated relative sea-level (RSL) rise in 

some areas exceeding 2 mm/yr (Paine, 1993).

Hydrogeologic Framework

The Gulf Coast aquifer (also known as the coastal 

lowlands aquifer system) consists of Miocene and younger 

unconsolidated sediments in layers of confining and water 

bearing units (Ryder and Ardis, 2002). The hydrogeologic 

units composing the Gulf Coast aquifer are thinner and closer 

to the surface in the northwest, are oriented parallel along 

the coastline, and dip southeast toward the Gulf of Mexico, 

increasing in thickness (fig. 2). The hydrogeologic units 

composing the Gulf Coast aquifer system are the Chicot 

aquifer, the Evangeline aquifer, the Burkeville confining 

unit, the Jasper aquifer, the Catahoula Sandstone (confining 

unit), the Anahuac Formation, and the Frio Formation (Baker, 

1979). Water recharging the system as rainfall—primarily 

in the outcrops in the northwestern part of the system—

enters the saturated zone and either travels a short distance, 

discharging locally to streams and creeks, or moves to deeper 

zones flowing southeastward to the coast, discharging in 

wells or into the Gulf of Mexico. The water-bearing units of 

the Gulf Coast aquifer are composed primarily of laterally 

and vertically discontinuous fine to coarse-grained sands 

and gravels with interbedded silts and clays. The Chicot, 

Evangeline, and Jasper aquifers are the primary sources for 

municipal groundwater supply in the Houston-Galveston 

region. 

Although the lithology of the Gulf Coast aquifer is 

primarily unconsolidated, normal faulting does exist as the 

Gulf of Mexico basin continues to evolve (Chowdury and 

Turco, 2008). Identifying fault locations and predicting 

movement of faults in these unconsolidated sediments can 

be difficult because of the variability and lack of structural 

control in the underlying rock. The Houston-Galveston 

region has more than 150 identified active faults, many with 

rates of movement from 5 to 20 mm/yr (Verbeek and others, 
A A' WALLER COUNTY GRIMES COUNTY GALVESTON COUNTY HARRIS COUNTY MONTGOMERY COUNTY A 

GRIMES COUNTY MONTGOMERY COUNTY HARRIS COUNTY GALVESTON CO

-1,900 -2,100

-2,300

+100

-100-300-500-700-900

-1,100-1,300-1,500

+300

-1,700

NGVD 29

METERS

NORTHWEST 

NGVD 29, National Geodetic Vertical Datum of 1929

0 4 8 12 16 KILOMETERS0 4 8 12 16 MILES

VERTICAL SCALE GREATLY EXAGGERATED

Undifferentiated Pre-Miocene deposits

Chicot aquifer

Evangeline aquifer

Burkeville confining un Ja it sper aquifer

Approximate land surface

Upper part of Catahoula Sandstone "Frio" Formation

Base of Catahoula Sandstone

Catahoula

confining system (restricted)

Anahuac

Formation

(Holocene- and Pleistocene-age sediment

(Pliocene- and Miocene-age sediments)

(Miocene-age 

(Miocene-age s

(Miocene-age sediments)

(Miocene-age sediments)

Figure 2. Hydrogeologic section of the Gulf Coast aquifer system in Harris and adjacent counties, Texas (modified from Baker, 1979, fig. 4).
Introduction 5

1979). Although these natural faults have been active in the 

recent geologic past, accelerated rates of movement were 

documented in the 20th century (Verbeek and others, 1979). 

The increased fault movement may be accelerated by fluid 

withdrawals or be attributable to a period of greater fault 

activity (Verbeek and others, 1979). 

Mechanism and History of Subsidence in the 

Houston-Galveston Region

Subsidence occurs in the Houston-Galveston region 

through clay compaction and reflects the pattern of 

groundwater withdrawals. The weight of the overburden above 

the aquifer is supported by a combination of pore pressure 

exerted by the fluid in the aquifer and structural strength 

provided from the aquifer skeleton, or the granular network 

of particles in the aquifer (fig. 3). Seasonal groundwater 

withdrawals result in a scenario where deformation of the 

aquifer alternates between compression and expansion. 

When the aquifer is pumped seasonally and aquifer hydraulic 

heads (measured as groundwater levels in wells) remain 

above the preconsolidation stress threshold, the aquifer 

compacts and expands with elastic compaction, and there is 

no permanent subsidence. When groundwater levels fall 

below the preconsolidation stress threshold, inelastic 

compaction of principally the fine-grained deposits (silts and 

clays) occurs, causing permanent subsidence (Galloway and 

others, 1999). 

The Houston-Galveston region has experienced land 

subsidence associated with shallow oil and gas extraction and 

municipal and industrial groundwater withdrawal. Subsidence 

in the Houston-Galveston region was first correlated with 

fluid extraction in 1926 at the Goose Creek oil field (Pratt 

and Johnson, 1926). Although oil and gas withdrawals 

characteristically result in localized areas of subsidence 

confined to the field of production, a larger regional area of 

subsidence is typically associated with shallow groundwater 

withdrawals. The area of subsidence in the Houston-Galveston 

region is one of the largest in the United States; by 1979 as 

much as 3 m of subsidence had occurred and approximately 

8,300 km2 of land had subsided more than 0.3 m (Coplin and 

Galloway, 1999, p. 40). 

Methods used to measure subsidence in the HoustonGalveston region include repeated first-order leveling surveys 

of existing benchmarks, mechanical measurements of aquifersystem compaction by using borehole extensometers, GPS 

measurements at NGS Continuously Operating Reference 

Station (CORS) sites and at portable GPS measuring stations 

Permanent land subsidence caused by

irreversible inelastic deformation 

Compaction of the aquifer system is

concentrated in the fine-grained 

clay and silt layers 

When withdrawals

lower groundwater levels

and raise pressure on the

clay and silt , the clay and 

silt can compact and the 

land surface can subside 

permanently.

Depth

to water

Long-term water-level decline

modulated by the seasonal cycles

of groundwater withdrawals

Clay and silt

Sand and gravel

Original land surface

Resulting

land surface

Time Granular clay and silt

skeleton defining fluidfilled interstitial pore 

spaces that store 

groundwater 

Rearranged by compaction 

the skeleton has reduced 

groundwater storage 

capacity

Figure 3. Diagram depicting the mechanism of subsidence in an aquifer composed of vertically discontinuous fine to coarse-grained 

sands and gravels with interbedded silts and clays (modified from Galloway and others, 1999, p. 9).
6 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

called Port-A-Measure (PAM) sites (Harris-Galveston 

Subsidence District, 2012) (fig. 4), and historical digital 

elevation model comparison (Kasmarek and others, 2009a). 

Additionally, groundwater-level change maps have been 

generated annually since 1977 to document the short-term 

and long-term effects of groundwater withdrawal on the 

availability of the groundwater resource. More information on 

these methods and approaches can be found in Gabrysch and 

Neighbors (2005), Kasmarek and Houston (2007), Kasmarek 

and others (2009b), Kasmarek and others (2010), and Johnson 

and others (2011). 

Previous Investigations

InSAR imagery has been used to estimate land-surface 

subsidence and locate faults in alluvial groundwater basins 

in the western United States (Galloway and others, 1998; 

Amelung and others, 1999; Bawden and others, 2001). The 

spatial and temporal motions associated with both natural 

and human-induced recharge and discharge can be imaged 

with well-timed land-surface deformation maps, called 

interferograms (that is, radar imagery that shows whether 

the ground has moved closer to or farther from an orbiting 

satellite). Aquifer structure, extent, and mechanical properties 

can also be seen or determined from well-timed interferograms 

(Galloway and Hoffman, 2007). Time-dependent subsidence 

and uplift from water-level changes have been documented 

by using InSAR techniques in Los Angeles, California, 

(Bawden and others, 2001) and in the Santa Clara Valley, 

Calif. (Ikehara and others, 1998; Galloway and others, 1999; 

Galloway and others, 2000; Schmidt and Bürgmann, 2003). In 

the Los Angeles basin, Bawden and others (2001) found that 

groundwater pumping and artificial recharge produce as much 

as 60 millimeters (mm) of seasonal uplift and subsidence and 

discovered that a subsurface barrier to groundwater flow is 

offset about 1.9 km from the mapped trace of the NewportInglewood Fault that bounds the southwest margin of the 

Santa Anna basin. Other InSAR studies have found previously 

unrecognized faults as lineaments in the InSAR imagery 

(Amelung and others, 1999; Catchings and others, 2000; 

Galloway and others, 2000). 

The use of InSAR has also been investigated in the 

Houston-Galveston region (Stork and Sneed, 2002; Buckley 

and others, 2003). Stork and Sneed (2002) examined two 

satellite scenes to develop interferograms for 1996–99 with 

results matching the patterns of subsidence measured by 

the extensometers. Buckley and others (2003) developed 

interferograms for an area in Texas that included the eastern 

half of Harris County and most of Galveston County and 

the Galveston Bay area for the period 1996–98. Their study 

observed areas of subsidence in the western and northwestern 

parts of Houston as well as near the Seabrook extensometer. 

Methodology

InSAR data were acquired, analyzed, and processed 

to create interferograms following generalized methods 

described by Schmidt and Bürgman (2003). GPS time-series 

data were processed and InSAR interferograms were created 

to assess subsidence patterns in the Houston-Galveston region. 

These GPS time-series data were used to (1) verify and 

validate InSAR-derived subsidence patterns and magnitude; 

(2) compare the GPS-apparent position of benchmark—when 

subsidence occurs, the position of the benchmark is pulled 

down and toward nearby subsidence or pushed up and away 

from uplift; and (3) provide a stable reference frame for 

the InSAR imagery. InSAR data collection and processing 

methods for the purpose of measuring land surface subsidence 

are summarized in appendix 1 (Schmidt and Bürgman, 2003; 

European Space Agency, 2007). 

Global Positioning System Processing

Baseline-Pair Method

In 2010, the NGS uniformly reprocessed all of the 

regional CORS and PAM GPS station data collected between 

1993 and 2009 (Clifton Middleton, National Geodetic Survey, 

written commun., 2009), and those stations with sufficient data 

were used for baseline-pair analyses (fig. 5). The approach 

used to process the GPS data is described by Soler and others 

(2001) and Soler and Marshall (2002). 

The GPS data were provided as baseline pairs for 

three CORS reference sites. The three CORS reference 

sites are listed by their GPS station name and four-character 

GPS site identifier in table 1—Addicks 1795 CORS ARP 

(ADKS), Lake Houston (LKHU), and Northeast 2250 

CORS ARP (NETP). The GPS station names are assigned 

by the NGS or the Harris-Galveston Subsidence District 

(National Geodetic Survey, 2011; Harris-Galveston 

Subsidence District, 2011). The acronym “ARP” is part 

of GPS station names for some reference sites; the ARP, 

or antenna reference point, is the physical bottom of the 

antenna used in the GPS measurement at each site (Landtech 

Consultants, Inc., 2003). The directional velocities depicting 

relative movement in millimeters per year for north, east, 

and up positional changes between a given GPS site and the 

baseline GPS site, NETP, are listed in table 1. Differences 

between the north, east, and up positions at the LKHU 

and NETP reference sites and between those at the ADKS 

and LKHU reference sites are shown in figs. 6A and 6B, 

respectively.
Methodology 7 CHAMBERS

COUNTY

GALVESTON

COUNTY

JEFFERSON

 COUNTY

MONTGOMERY

COUNTY

FORT BEND

COUNTY 

BRAZORIA COUNTY

AUSTIN

COUNTY

WASHINGTON

COUNTY

LIBERTY COUNTY

HARDIN

COUNTY

BURLESON

COUNTY

BRAZOS

COUNTY

ROBERTSON

COUNTY

JACKSON

COUNTY

WALKER

COUNTY

GRIMES

COUNTY

SAN JACINTO

COUNTY

POLK

COUNTY

TYLER

COUNTY

HARRIS COUNTY

COLORADO

COUNTY

WALLER COUNTY

WHARTON COUNTY

MATAGORDA COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

30°30'

30°00'

29°30'

29°00'

94°30'

95°00'

95°30'

96°00'

96°30'

Base from U.S. Geological Survey digital data

Albers equal-area projection

North American Datum of 1983

Study area

TEXAS

Harris-Galveston

 Subsidence District

Fort Bend Subsidence

 District

InSAR scene—With

 scene number (table 2)

GPS Port-A-Measure (PAM)

 and Continuously Operating

 Reference Stations (CORS)

 and GPS site name (table 1)

Continuously Operating

 Reference Stations (CORS),

 used in GPS analysis to 

 help refine the InSAR 

 movement vectors, and 

 GPS site name (table 1)

Extensometers and short name

EXPLANATION

484/f3015

! PA49

ADKS

# ADDICKS

#

GPS, Global Positioning System

InSAR, Interferometric synthetic aperture radar

GULF OF MEXICO

Galveston Bay

Trinity

Bay

East Bay

West Bay

484/f2997

212/f2997

484/f3015

212/f3015

!

!

! !

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

! !

! !

!

!

!

! !

!

!

!

! ! !

! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

! !

!

!

!

!

!

 PA00

PA01

PA02

PA03

PA04

PA05

 PA06

PA07

PA08

PA09

 PA10

PA11

PA12

PA13

PA14

PA16

PA17

 PA18

 PA19

PA21 PA20

 PA22

PA24

 PA26

 PA27

PA28

 PA29

 PA30

PA31

PA32 PA33

 PA35 PA36

PA37 PA38

PA40

PA41

PA42

PA43

 PA44 PA45

PA46

PA47

PA48

 PA49

PA50

PA51

PA52

PA53

PAM 54

PA55

 PA56

TMCC

TXHE

PAM 23

TXCN

COH6 TXLI

TXRO

COH7

COH1

ANG5 TXGV

TXGA

TXLM

TSI1

ROD1

ZHU1

COH3

COH4

COH2

WLA1

!

COH5

!

HOUS

TXHU

!

!

DWI1

!

GAL1

GAL2

ANG1 ANG6

ANG2

!

PA15

!

TXAG

#

#

# #

#

#

#

#

# #

ADDICKS SOUTHWEST PASADENA

BAYTOWN T

BAYTOWN P

CLEAR LAKE T

CLEAR LAKE P

EAST END

TEXAS CITY

SEABROOK

NASA

LAKE HOUSTON

#

#

#

LKHU

ADKS NETP

Figure 4. Locations of National Geodetic Survey Continuously Operating Reference Station sites, Harris-Galveston Subsidence District 

Port-A-Measure sites, and extensometer sites in the Houston-Galveston region, Texas.
8 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

CHAMBERS

COUNTY

GALVESTON

COUNTY

JEFFERSON

 COUNTY

MONTGOMERY

COUNTY

FORT BEND

COUNTY 

BRAZORIA COUNTY

AUSTIN

COUNTY

WASHINGTON

COUNTY

LIBERTY COUNTY

HARDIN

COUNTY

BURLESON

COUNTY

BRAZOS

COUNTY

ROBERTSON

COUNTY

JACKSON

COUNTY

WALKER

COUNTY

GRIMES

COUNTY

SAN JACINTO

COUNTY

POLK

COUNTY

TYLER

COUNTY

HARRIS COUNTY

COLORADO

COUNTY

WALLER COUNTY

WHARTON COUNTY

MATAGORDA COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

30°30'

30°00'

29°30'

29°00'

94°30'

95°00'

95°30'

96°00'

96°30'

Base from U.S. Geological Survey digital data

Albers equal-area projection

North American Datum of 1983

Study area

TEXAS

Harris-Galveston

 Subsidence District

Fort Bend Subsidence

 District

InSAR scene—With

 scene number (table 2)

GPS sites used in the 

 analysis (table 1)

GPS sites that did not have

 sufficient data for the

 analysis (table 1)

EXPLANATION

484/f3015

GPS, Global Positioning System

InSAR, Interferometric synthetic aperture radar

PA33!

# COH5

GULF OF MEXICO

Galveston Bay

Trinity

Bay

East Bay

West Bay

484/f2997

212/f2997

484/f3015

212/f3015

! WLA1 ROD1

TSI1

ANG5 TXGV

!

TXHU

ANG1 !

ANG6

!

DWI1

!

GAL2

!

! !

!

!

!

! !

! !

!

!

!

!

! !

! !

!

!

!

!

!

! !

!

!

!

!

!

PA05

 PA29

 PA30

 PA31

PA32

 PA33

 PA35

PA37 PA38

 PA40

PA41

 PA42

 PA43

 PA44 PA45

PA46 PA48 PA47

 PA49

PA50

PA51

PA52

PA53

PA54

PA55

 PA56

!

PA15

! PA36

#

# #

TXHE

TXCN

ZHU1

COH7

COH4

COH1

COH2 COH3

COH6

TXRO

TXLM

TXGA

#

COH5

HOUS

ANG2

GAL1

#

#

#

#

# #

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

# #

# #

#

#

#

#

#

#

#

# #

#

#

#

 PA00

PA01

PA02

PA03

 PA04

 PA06

PA07

PA08

PA09

 PA10

PA11

PA12

PA13

 PA14

 PA16

PA17

PA18

 PA19

 PA20 PA21

 PA22

PA24

 PA26

 PA27

PA28

TXLI

ADKS NETP 

PA23

LKHU

TMCC

TXAG #

Figure 5. The National Geodetic Survey (NGS) Continuously Operating Reference Stations (CORS) and Harris-Galveston Subsidence 

District Port-A-Measure (PAM) network of Global Positioning System (GPS) stations used (pink triangle) and not used (white circle) to 

evaluate subsidence amounts, Houston-Galveston region, Texas.
Methodology 9

Table 1. Global Positioning System (GPS) stations used to validate subsidence magnitudes determined by using interferometric 

synthetic aperture radar (InSAR) with directional velocities in millimeters per year for north, east, and up positional changes between 

a given GPS station and the Northeast 2250 Continuously Operating Reference Station antenna reference point (Northeast 2250 

CORS ARP).—Continued

[Velocity in millimeters per year (mm/yr); NA, not applicable; ----- indicates no velocity determined because the Northeast 2250 CORS ARP station (NETP) 

was held fixed as the local baseline pair reference frame]

GPS site 

identifier2

GPS station 

name2 Longitude Latitude Velocity north (mm/yr)

Velocity east 

(mm/yr)

Velocity up

(mm/yr)

ANG1 ANGLETON 1 95° 29′ 7.26′′ W 29° 18′ 5.00′′ N -5.4 -1.1 4.1

ANG2 ANGLETON 2 95° 29′ 5.52′′ W 29° 18′ 5.92′′ N -0.1 0.6 -5.3

ANG5 ANGLETON 5 95° 29′ 6.00′′ W 29° 18′ 5.00′′ N 2.2 0.5 0.7

ANG6 ANGLETON 6 95° 29′ 5.52′′ W 29° 18′ 5.92′′ N 1.4 0.5 4.5

COH1 C OF HOUSTON COOP 95° 32′ 33.36′′ W 29° 40′ 13.20′′ N 1.8 1.1 -11.9

COH2 HOUSTON 2 COOP 95° 24′ 41.75′′ W 29° 37′ 42.69′′ N 1.2 -1.2 -4.2

COH3 HOUSTON 3 COOP 95° 15′ 51.62′′ W 29° 38′ 32.60′′ N 1.1 0.2 1.0

COH4 HOUSTON 4 COOP 95° 12′ 54.57′′ W 29° 46′ 59.39′′ N 1.0 0.4 3.8

COH5 HOUSTON 5 COOP 95°16′29.91′′ W 29°50′40.26′′ N 1.9 -0.0 -1.5

COH6 HOUSTON 6 COOP 95° 11′ 5.27′′ W 30° 2′ 23.04′′ N 1.2 0.7 -5.8

COH7 HOUSTON 7 COOP 95° 29′ 47.76′′ W 29° 52′ 38.13′′ N 2.8 -0.1 -11.4

DWI1 CLUTE COOP 95°24′13.18′′ W 29°0′48.98′′ N -21.4 32.2 -62.7

GAL1 GALVESTON 1 94°44′12.01′′ W 29°19′46.99′′ N 0.9 -0.5 -5.8

GAL2 GALVESTON 2 94°44′12.05′′ W 29°19′48.25′′ N 0.1 0.6 -2.4

HOUS HOUSTON 95°25′58′′ W 29°46′45′′ N 1.6 -3.5 -9.8

PA00 PAM 00 95° 9′ 8.02′′ W 29° 32′ 19.01′′ N -0.6 0.2 -3.1

PA01 PAM 01 95° 36′ 59.80′′ W 29° 54′ 42.73′′ N -0.8 -1.8 -42.8

PA02 PAM 02 95° 24′ 57.09′′ W 30° 0′ 2.34′′ N 1.7 -1.5 -34.1

PA03 PAM 03 95° 36′ 48.15′′ W 29° 49′ 14.91′′ N 1.7 -2.0 -38.4

PA04 PAM 04 95° 35′ 48.68′′ W 29° 37′ 49.40′′ N -3.5 -0.8 -19.5

PA05 PAM 05 95° 35′ 9.24′′ W 29° 47′ 28.33′′ N 1.3 0.0 -25.7

PA06 PAM 06 95° 40′ 40.00′′ W 29° 48′ 58.92′′ N 1.2 -1.9 -30.9

PA07 PAM 07 95° 34′ 35.91′′ W 29° 56′ 10.65′′ N 2.7 1.2 -41.6

PA08 PAM 08 95° 28′ 34.54′′ W 29° 58′ 46.82′′ N 3.4 -2.4 -29.1

PA09 PAM 09 95° 4′ 17.24′′ W 30° 2′ 17.23′′ N 2.5 0.1 -3.1

PA10 PAM 10 95° 47′ 57.01′′ W 29° 33′ 58.97′′ N 4.1 2.2 -4.9

PA11 PAM 11 95° 51′ 54.78′′ W 30° 1′ 55.77′′ N 1.4 0.1 -5.2

PA12 PAM 12 95° 15′ 47.04′′ W 30° 3′ 34.89′′ N 2.3 1.1 -9.4

PA13 PAM 13 95° 29′ 23.94′′ W 30° 11′ 41.29′′ N 0.1 0.4 -18.3

PA14 PAM 14 95° 38′ 38.75′′ W 29° 28′ 25.14′′ N 4.3 -3.2 -7.1

PA15 PAM 15 95° 43′ 11.11′′ W 29° 40′ 59.66′′ N 1.4 -0.5 -10.7

PA16 PAM 16 95° 31′ 38.05′′ W 29° 32′ 40.05′′ N 2.7 -8.7 -10.3

PA17 PAM 17 95° 36′ 55.03′′ W 30° 5′ 28.17′′ N 0.2 -0.6 -20.2

PA18 PAM 18 95° 40′ 41.59′′ W 29° 57′ 53.75′′ N 1.5 0.1 -24.7

PA19 PAM 19 95° 48′ 19.21′′ W 29° 50′ 28.01′′ N 0.7 0.1 -10.6

PA20 PAM 20 95° 0′ 47.64′′ W 29° 31′ 58.44′′ N -0.1 1.7 -0.3

PA21 PAM 21 95° 18′ 43.44′′ W 29° 32′ 43.66′′ N 2.5 -1.8 -5.5

PA22 PAM 22 95° 1′ 14.53′′ W 29° 20′ 4.26′′ N 2.4 -2.0 -5.1

PA23 PAM 23 94° 55′ 3.97′′ W 29° 20′ 6.27′′ N -2.7 0.1 -1.2

PA24 PAM 24 95° 2′ 26.78′′ W 29° 40′ 7.65′′ N 1.5 0.2 0.7

PA26 PAM 26 94° 56′ 17.94′′ W 29° 12′ 37.12′′ N 1.7 -0.1 -2.2

PA27 PAM 27 95° 0′ 55.95′′ W 29° 34′ 59.28′′ N -0.3 3.1 -6.3

PA28 PAM 28 94° 55′ 3.44′′ W 29° 45′ 4.37′′ N -1.6 1.2 0.5

PA29 PAM 29 95° 49′ 19.84′′ W 29° 46′ 8.45′′ N 2.1 -4.8 -13.4

PA30 PAM 30 95° 54′ 6.87′′ W 29° 41′ 21.29′′ N 3.9 -2.5 -6.8

Table 1. Global Positioning System (GPS) stations used to validate subsidence magnitudes determined by using interferometric 

synthetic aperture radar (InSAR) with directional velocities in millimeters per year for north, east, and up positional changes between a 

given GPS station and the Northeast 2250 Continuously Operating Reference Station antenna reference point (Northeast 2250 

CORS ARP).

[Velocity in millimeters per year (mm/yr); NA, not applicable; ----- indicates no velocity determined because the Northeast 2250 CORS ARP station (NETP) 

was held fixed as the local baseline pair reference frame]
10 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

Table 1. Global Positioning System (GPS) stations used to validate subsidence magnitudes determined by using interferometric 

synthetic aperture radar (InSAR) with directional velocities in millimeters per year for north, east, and up positional changes between 

a given GPS station and the Northeast 2250 Continuously Operating Reference Station antenna reference point (Northeast 2250 

CORS ARP).—Continued

[Velocity in millimeters per year (mm/yr); NA, not applicable; ----- indicates no velocity determined because the Northeast 2250 CORS ARP station (NETP) 

was held fixed as the local baseline pair reference frame]

GPS site 

identifier2

GPS station 

name2 Longitude Latitude Velocity north (mm/yr)

Velocity east 

(mm/yr)

Velocity up

(mm/yr)

PA31 PAM 31 95° 50′ 54.13′′ W 29° 23′ 52.85′′ N 8.8 -23.6 1.1

PA32 PAM 32 95° 42′ 26.27′′ W 29° 32′ 26.15′′ N 0.9 -1.3 6.2

PA33 PAM 33 95° 13′ 24.81′′ W 29° 29′ 23.67′′ N -2.0 2.1 7.6

PA35 PAM 35 95° 4′ 56.75′′ W 29° 28′ 21.40′′ N 1.7 1.4 5.9

PA36 PAM 36 94° 56′ 29.82′′ W 29° 29′ 39.02′′ N 1.2 4.8 -0.4

PA37 PAM 37 95° 6′ 3.61′′ W 29° 37′ 50.52′′ N 3.3 -8.7 -2.2

PA38 PAM 38 95° 13′ 22.60′′ W 29° 38′ 57.37′′ N 34.9 -2.4 0.4

PA40 PAM 40 95° 27′ 44.96′′ W 29° 29′ 35.84′′ N -5.3 0.2 -7.9

PA41 PAM 41 95° 28′ 31.77′′ W 29° 39′ 42.85′′ N 12.0 2.9 -10.2

PA42 PAM 42 95° 38′ 7.22′′ W 29° 43′ 56.94′′ N -4.5 -11.4 -8.5

PA43 PAM 43 95° 6′ 38.11′′ W 29° 5′ 35.67′′ N 0.3 3.5 -0.3

PA44 PAM 44 95° 41′ 12.67′′ W 29° 52′ 48.45′′ N 7.8 -4.5 -12.5

PA45 PAM 45 95° 23′ 7.60′′ W 29° 52′ 33.21′′ N 2.0 -1.7 5.8

PA46 PAM 46 95° 36′ 0.18′′ W 30° 1′ 47.87′′ N 0.5 -0.1 -22.6

PA47 PAM 47 95° 25′ 24.73′′ W 30° 5′ 22.37′′ N 2.8 -2.8 -12.9

PA48 PAM 48 95° 40′ 18.13′′ W 30° 2′ 43.27′′ N -1.6 1.8 -11.5

PA49 PAM 49 94° 42′ 5.47′′ W 29° 25′ 20.79′′ N 2.2 2.6 -1.1

PA50 PAM 50 94° 51′ 21.72′′ W 29° 50′ 54.02′′ N 1.1 -2.1 1.4

PA51 PAM 51 95° 17′ 3.08′′ W 29° 55′ 57.14′′ N 4.1 0.2 14.2

PA52 PAM 52 95° 10′ 36.22′′ W 29° 51′ 7.27′′ N 2.3 -0.2 8.5

PA53 PAM 53 95° 3′ 26.20′′ W 29° 54′ 28.88′′ N 3.9 0.5 -0.2

PA54 PAM 54 95° 2′ 3.77′′ W 29° 48′ 5.27′′ N 2.3 -2.3 2.2

PA55 PAM 55 95° 10′ 37.89′′ W 29° 47′ 39.06′′ N 1.0 3.3 6.5

PA56 PAM 56 95° 49′ 0.34′′ W 29° 54′ 9.40′′ N 0.4 4.5 -7.0

ROD1 SPRING 95° 31′ 36.00′′ W 30° 4′ 19.99′′ N 3.7 -0.1 -18.0

TXAG ANGLETON 95° 25′ 7.97′′ W 29° 9′ 49.97′′ N 3.3 -0.1 -1.8

TMCC ---- 95° 23′ 42.82′′ W 29° 42′ 8.42′′ N 0.9 -0.2 -1.5

TSI1 DEERPARK COOP 95°7′21.25′′ W 29°40′23.48′′ N 0.9 1.6 2.3

TXCN CONROE 95° 26′ 28.00′′ W 30° 20′ 56.00′′ N 1.3 0.7 -11.7

TXGA GALVESTON 94° 46′ 21.00′′ W 29° 19′ 39.97′′ N 0.1 2.2 -5.1

TXGV GALVESTON 877 1510 94° 47′ 21.45′′ W 29° 17′ 6.48′′ N 3.0 -1.9 7.2

TXHE HEMPSTEAD 96° 3′ 47.99′′ W 30° 5′ 56.00′′ N 1.0 2.3 -4.1

TXHU HOUSTON RRP2 95°25′57.90′′ W 29°46′45.19′′ N 0.7 -0.4 -3.4

TXLI LIBERTY 94° 46′ 14.99′′ W 30° 3′ 20.99′′ N 0.7 1.7 -2.9

TXLM LA MARQUE 95° 01′ 25.26′′ W 29° 23′ 32.00′′ N 0.4 0.5 -5.5

TXRO ROSENBERG 95° 48′ 25.99′′ W 29° 31′ 8.00′′ N 0.6 2.8 -6.7

WLA1 TOMBALL COOP 95°37′30.60′′ W 30°5′47.09′′ N -3.2 3.3 -17.8

ZHU1 HOUSTON WAAS 1 95° 19′ 53.10′′ W 29° 57′ 42.81′′ N 2.9 -0.8 -7.9

ADKS Addicks 1795 CORS ARP 95° 35′ 11.04′′ W 29° 47′ 27.47′′ N -1.5 2.5 -0.2

LKHU Lake Houston 95° 8′ 44.69′′ W 29° 54′ 48.44′′ N -2.7 0.1 0.7

NETP Northeast 2250 CORS ARP station 95° 20′ 3.17′′ W 29° 47′ 28.14′′ N ----- ----- -----

1

Italicized font indicates the site was not used for the InSAR analysis.

2

Station names and site identifiers are from National Geodetic Survey CORS site search - http://www.ngs.noaa.gov/CORS/ or Harris-Galveston Subsidence 

District - Subsidence Charts - http://mapper.subsidence.org/Chartindex.htm. 
Methodology 11

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−100

−50

0

50

100

−50

−25

0

25

50

−50

−25

0

25

50

−100

−50

0

50

100

0

25

50

−50

−25

0

25

50

−50

−25

Movement, in millimeters Movement, in millimeters

mm/yr 0.2 

mm/yr −2.0 

mm/yr −1.0

mm/yr −2.7 

mm/yr 0.1 

mm/yr 0.7

B. ADKS–LKHU

A. LKHU–NETP GPS data were excluded from analysis after April 19, 2009

mm/yr, Millimeters per year

Figure 6. Global Positioning System (GPS) time series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, Continuously Operating Reference Stations (CORS) Lake Houston 

(LKHU) and Northeast 2250 CORS ARP (NETP) and B, Addicks 1795 CORS ARP (ADKS) and Lake Houston (LKHU). 
12 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

Each pair of fixed GPS stations forms a baseline and 

was analyzed for relative positional change over time in the 

north, east, and up components measured to subcentimeter 

precision. Baseline pairs measured over time for LKHU–

NETP and ADKS–LKHU CORS sites are represented as a 

GPS time series (figs. 6A and 6B, respectively). When there is 

no change in the relative position between paired stations, the 

apparent velocity is zero. If there was an observed change in 

position associated with subsidence or uplift, however, then 

a GPS station moving northeast or away from the reference 

station because of subsidence, for example, would show an 

increased distance in both the north and east components and 

a decreased distance in the up component as the GPS station 

is pulled in towards a subsidence feature (vertical motion 

associated with aquifer compaction) (fig. 7). 

To minimize the influence of seasonal fluctuations on 

the long-term velocity pattern, GPS-measured velocities were 

determined only for time series with more than 3.5 years of 

data (Blewitt and others, 1995; Bawden and others, 2001). 

Concurrent baseline pairs that differed by more than 50 mm 

from the time series were removed, and a least-squares linearregression equation (Helsel and Hirsch, 2002) was computed 

for the remaining data for each of the directional components 

(north, east, and up) of the data to compute the horizontal 

and vertical velocities at each station. Time series that had 

appreciable steps (an appreciable change (20 mm or larger) 

in velocity from one velocity value to the next) in velocity 

in either the horizontal or vertical component were excluded 

from this analysis. For example, data were excluded when 

it was unclear if the positional change was a result of land 

deformation or error. Examples of GPS error sources include 

multipath noise (GPS signal reflected and diffracted off 

surfaces with multiple signals reaching the antenna), antenna 

calibration errors, atmospheric effects, and GPS hardware 

issues (Byun and others, 2002). An example of the unclear 

source of positional change can be seen in the time series for 

GPS sites ANG1 and NETP (fig. 8); the north component 

shifted about 20 mm to the south during 2004–8; neither 

an east nor up component of movement was evident. The 

northerly velocity component for ANG1 is about -5 mm/yr and 

was affected by this shift, which might have been an artifact 

given the lack of change in either the east or up components. 

If the questionable data were removed from the velocity 

calculation, the velocity would be close to zero. Site ANG1 

was not used in the analysis because the time series length 

did not meet the minimum requirement of 3.5 years, but it 

underscores how numerical artifacts and short time series can 

introduce errors into the site velocities. 

Selection of a Stable Reference Frame

Defining a stable reference frame for the GPS data 

is essential to understand how changes in the GPS station 

positions relate to motion of the crust of the Earth resulting 

from hydrologic and tectonic processes. There are three basic 

categories of reference frames: absolute, relative, and baseline 

pair (Rothacher, 2001).

An absolute reference frame integrates GPS data from 

the nationwide NGS CORS network in order to understand 

how a local geodetic network responds on a continental scale. 

For this report, an absolute reference frame allows the user 

to understand how the Houston-Galveston region is moving 

(subsiding) with respect to a presumed stable North American 

continent. The tradeoff for using an absolute reference frame 

is that there needs to be an established GPS network with 

stations that are within a few hundred kilometers of the 

target networks, such that a similar suite of GPS satellites are 

simultaneously observed by both the reference and study area 

GPS receivers. The number of GPS stations and their distances 

from the target networks factor into the positional error in an 

absolute reference frame (Snay and others, 2002). 

Similarly, a relative reference frame network can be 

established by holding the positions fixed at a number of GPS 

stations outside of the deformation area such that positional 

changes can be measured among the stations within the 

local network. For the data analyzed in this report, a relative 

network of GPS stations outside of the deforming regions 

would have had to have been established in 1993, which was 

not the case. 

The third baseline-pair approach calculates the positional 

changes between a well-established GPS reference station 

(present prior to and throughout the time period examined 

and has a period of record greater than 3.5 years) within the 

network and each subsequent GPS station. A network can have 

more than one GPS reference station, which is the station held 

fixed in the baseline pair analysis. For any baseline pair of 

stations that are collecting data at the same time, it is possible 

to calculate very accurate (subcentimeter) distances between 

the GPS reference station and each of the CORS and PAM 

stations. For this analysis, a baseline-pair reference frame 

enabled the measurement of subcentimeter positional changes 

between the original three GPS stations established in 1993 

(ADKS, LKHU, and NETP) and all subsequent regional GPS 

stations, including the PAM stations. 

Interferometric Synthetic Aperture Radar 

Processing

Interferogram Overview

For this report, InSAR data were acquired, analyzed, 

and processed to create interferograms. The processing of 

satellite synthetic aperture radar (SAR) scenes to generate 

interferograms is a multistep process that transforms the initial 

radar echoes emitted by the satellite into georeferenced radar 

images that can detect subcentimeter changes in the landsurface position. The first step focuses two SAR scenes that 

imaged the same area on the ground from the same viewpoint. 
Methodology 13

5 MILLIMETERS

10 MILLIMETERS

0 10 20 KILOMETERS

uplift

subsidence

0

28.3

56.6

Range change, in millimeters

EXPLANATION

EXPLANATION

A A' Line of section

A A'

Elastic model of aquifer subsidence

 0.5-kilometer-deep aquifer

 10 millimeters of contraction

 Poisson’s ratio 0.25

A A'

A. Plan view B. Cross section

C

D

Figure 7. Conceptual depiction of horizontal and vertical movement associated with aquifer system compaction. A, Map view of a 

simulated aquifer subsidence for 10 millimeters (mm) of downward vertical movement, with arrows depicting horizontal movement in 

and towards the area with the maximum subsidence corresponding to profile A–A’. B, Cross-section showing the lateral and vertical 

displacements for profile A–A’. C, Simulated interferometric synthetic aperture radar (InSAR) image of the modeled aquifer system 

compaction with vertical exaggeration to visualize the number of individual fringes on the margins of the basin. D, Simulated InSAR 

image of the modeled aquifer system compaction where the InSAR fringe gradient is the largest on the margins of the basin. 
14 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

The two SAR scenes for this report were acquired from 

different orbits from satellites ERS-1 and ERS-2. The orbital 

period, or the time it takes for the satellite to return to the same 

point in space, was 35 days. Next, the two SAR scenes are 

precisely aligned to each other and the scenes are differenced, 

or values for the same pixel location are subtracted between 

the scenes, to create the interferogram (European Space 

Agency, 2007). 

An interferogram shows the measured range change 

(distance change) between the satellite antenna and any stable 

point on the ground such as a building or rock outcrop. The 

satellite antenna emits a sinusoidal wave signal that travels 

from the satellite to the ground. The wave signal interacts with 

and bounces off of the land surface and sends a fraction of 

the signal back to the satellite. The satellite records the length 

of time for the signal to travel to the land surface and return 

to the satellite, how much signal returned (amplitude), and 

what fraction of one full sine-wave cycle returned (phase). 

The amplitude is not used in calculating the range change. 

The interferogram maps the change in phase distance for 

each pixel on the ground and is typically visualized through 

repeating color bands, or fringes, which represent changes in 

phase distance (European Space Agency, 2007) (fig. 9). Only 

interferograms with good coherency (indicated by an absence 

of appreciable signal scatter between the original wave signal 

and the signal returned from the land surface to the satellite) 

are useful; for these interferograms, image focusing is able to 

combine the individual radar echoes into one coherent image 

(appendix 1). 

In this report, each color band of the C-band sensor 

represents 28.3 mm of either uplift or subsidence depending 

on the color-band pattern (unless otherwise noted, one 

complete color cycle (fringe) on an interferogram is half the 

wavelength (56.6 mm) of the C-band sensor, or 28.3 mm and 

the resolution of one pixel is 30 m2). Relative displacement of 

land surface between regions on the map can be measured by 

counting the number of full (one complete color cycle) and 

partial color fringes between the two regions of interest on the 

interferogram and then multiplying by the fringe-scale factor 

(28.3 mm). For example, an interferogram depicting three 

repeating fringes of the same subsidence color-band pattern 

would represent a total of about 85 mm of subsidence. Ideally, 

UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

Movement, in millimeters

−100

−50

0

50

100

0

25

50

−50

−25

0

25

50

−50

−25

mm/yr −5.4 

mm/yr −1.1 

mm/yr 4.1 

ANG1–NETP

mm/yr, Millimeters per year

Figure 8. Global Positioning System (GPS) time series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between GPS station ANG1 and Continuously Operating Reference Station 

Northeast 2250 CORS ARP (NETP).
Methodology 15

Uplift

0

-28.3

Range change, in millimeters

EXPLANATION

• Count the number of fringes

 From the outer red area (”START”) to the

 center, three red fringes can be counted.

• Multiply the number of fringes by 28.3 millimeters to estimate deformation

 3 x 28.3 = 84.9 millimeters

• Determine range change direction from the scale bar

 The color sequence from the outer edge inward 

 is red-orange-yellow-green-blue-violet, which 

 corresponds to uplift and a decreased distance 

 between the ground and the satellite (range).

Range, in millimeters

+28.3 0 -28.3

An increase

in range

indicates

subsidence

A decreasein rangeindicatesuplift

1 START

2

3

Fringes of deformation

START

1

2

3

START

1 fringe

2 fringes

3 fringes

A. Simplified interferogram showing uplift

B. Steps to interpret an interferogram

Figure 9. Simplified interferogram. A, Approximately three fringes of deformation equalling 84.9 millimeters (mm) of uplift. B, Steps to 

interpret an interferogram.
16 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

if no land surface deformation occurred between the two 

radar scenes and there was a clear sky, then the interferogram 

will be a uniform color without any partial fringes except 

for the influence of local noise sources, such as vegetation, 

atmospheric moisture, and topography (see appendix 1 

for an in-depth discussion on noise sources). If there were 

surface deformation sometime between when the two radar 

scenes were acquired, however, then the resulting fringes on 

the interferogram map would change between neighboring 

pixels on the ground. Appendix 1, “Interferometric Synthetic 

Aperture Radar Processing and Error Analysis,” provides a 

more comprehensive discussion on the InSAR processing 

approach used in this report.

Interferometric Synthetic Aperture Radar Error 

Considerations

Techniques used to create interferograms from satellite 

SAR data have limitations with regards to measuring surface 

motion in regions that are moderately to densely covered 

in vegetation because the presence of dense vegetation, the 

growth of vegetation, and other changes in vegetation can 

appear to alter the phase distance. Atmospheric moisture 

(clouds), high humidity, and topography can also introduce 

signal noise (Mateus and others, 2011), but filtering algorithms 

can enhance the signal quality used to develop interferograms 

and minimize noise associated with vegetation. Many of 

these typical noise (error) sources are found in the HoustonGalveston region. To mitigate the effects of changes in 

vegetation, interferograms were developed from radar images 

acquired close in time. The Houston-Galveston region SAR 

scenes were selected to minimize the time period between 

each acquisition and were selected to study the land surface 

during the same time of the year, that is, during the fall and 

winter when there are fewer leaves, which allows the radar 

signal to reach the ground. 

 The largest single source of data noise, or error, for 

the Houston-Galveston region was atmospheric moisture 

associated with clouds. In general, atmospheric moisture 

delays the radar signal as it travels from the satellite to the 

ground and back. This delay increases the radar signal travel 

time and the calculated range distance and can produce 

artifacts in the interferograms that equate to as much as 300 

mm of delay for the most extreme conditions. Figure 10 

includes an interferogram with few clouds and minimal noise 

(fig. 10A) and an interferogram with storm clouds over part of 

the Houston-Galveston region (fig. 10B). The error associated 

with atmospheric moisture was mitigated by combining or 

stacking radar scenes to average out any atmospheric noise 

and by avoiding SAR scenes from days with measureable 

precipitation. Interferograms with severe atmospheric noise 

were not used in the analysis. High humidity delays the radar 

signal in a manner similar to atmospheric moisture, but its 

magnitude is smaller. To reduce error from humidity, the same 

techniques used to account for atmospheric moisture were 

applied (European Space Agency, 2007).

Land-surface topography is examined during the 

error-assessment process. To minimize topographic errors, 

combinations of SAR scenes with the least orbital variations 

were used. Because the Houston-Galveston region is flat, the 

topography errors were negligible (Schmidt and Bürgman, 

2003; European Space Agency, 2007).

Subsidence in the Houston-Galveston 

Region, 1993–2000

NGS GPS data were used to verify subsidence during 

1993–2000 in the Houston-Galveston region. The NGS 

reprocessed all available GPS data (CORS and PAM stations) 

from January 1, 1993, through December 31, 2009, by 

using the baseline-pair approach. NGS data from January 1, 

2001, through April 19, 2009, were included because these 

additional years of data collection provided a larger dataset 

for comparison purposes. Land-surface horizontal and 

vertical vectors were generated from the time series velocity 

components from the baseline-pair analyses by examining 

each station with respect to the NETP CORS reference 

station. Patterns in the horizontal and vertical GPS time 

series exhibited general agreement with the InSAR-measured 

subsidence. 

Interferograms were constructed from the SAR 

scenes controlled by scene availability and quality. The 

InSAR-derived magnitudes and extents of subsidence 

were determined over multiple time intervals and verified 

by contemporaneous GPS time-series analyses. InSAR 

interferograms showed that the part of the Houston-Galveston 

region with the maximum amount of historical subsidence 

during 1913–17 to 2001 of approximately 2 m or more 

(central to southeastern Harris County, extending south 

toward Galveston Bay) (Kasmarek and others, 2009a) is now 

relatively stable. The relative GPS vertical velocities in this 

area also show little vertical motion relative to the NETP 

reference station, corroborating the InSAR findings.

Global Positioning System Analysis 

Global Positioning System Baseline-Pair 

Analyses

The NGS reprocessed all of the GPS data for the region, 

including those from the CORS and PAM stations, with 

respect to the original three CORS reference stations: ADKS, 

LKHU, and NETP. Baseline differences in north, east, and 

up between paired stations LKHU and NETP, and stations 

ADKS and LKHU, were produced for every epoch date (time 

associated with the coordinates of the base stations) between 

1993 and 2010. Baseline differences for each epoch were also 

determined as new CORS and PAM stations were added to the 
Subsidence in the Houston-Galveston Region, 1993–2000 17

Galveston Bay

Atmospheric noise due 

to storm clouds

Houston

TEXAS

Area enlarged

 95 ̊38' 95 ̊00'

29 ̊36'

29 ̊22'

 95 ̊38' 95 ̊00'

29 ̊36'

29 ̊22'

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1927

Subsidence Uplift

+28.3 

-28.3 

0

Movement, in

 millimeters

EXPLANATIONRelative movement

0 5 10 MILES

0 5 10 KILOMETERS

A

B

Galveston 

Bay

Galveston 

Bay

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

Figure 10. Examples of interferograms. A, Few clouds were present and there was minimal noise, Scene T212-F3015 for January 14, 

1997, to December 30, 1997. B, Storm was present (box), Scene T212-F3015 for January 14, 1997, to July 13, 1999. The fringes in this 

interferogram are the summation of land surface motion (subsidence), noise related to the storm (box), and possible other clouds in the 

scene.
18 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

network of GPS stations. All GPS data were excluded from the 

analysis after April 19, 2009, because there was a prominent 

anomaly that was seen in many of the time-series pairs that 

could not be easily explained but may have been caused by 

unexplained motion at one or more of the three CORS stations 

(note the motion in figs. 6A and 6B after the dashed line at 

April 19, 2009). Given that April 19, 2009, is well beyond 

the InSAR time intervals, the exclusion of this data does not 

affect the interpretation of the interferograms presented in this 

report.

Relative motion between paired stations ADKS and 

LKHU (fig. 6B) and stations NETP and LKHU (fig. 6A) 

between 1993 and 2009 was determined, but baselines 

between ADKS and NETP (fig. 11) were not processed on a 

regular basis, so it is not possible to fully assess the stability 

of the three stations together. The LKHU–NETP baseline pair 

showed that LKHU moved southward towards NETP at rate of 

2.7 mm/yr, or more than 40 mm from 1993 to 2009; however, 

there was no discernible eastward or vertical seasonal motion 

during this period (fig. 6A). 

The ADKS–LKHU baseline pair showed that ADKS 

moved to the south and away from LKHU at a rate of about 

1 mm/yr and to the west at 2 mm/yr with no substantial 

uplift during 1993–2009 (fig. 6B). This movement equates to 

about 20 mm of southward motion and 40 mm of westward 

movement during the time period. The ADKS–LKHU time 

series showed higher seasonal variability than the LKHU–

NETP time series (figs. 6A and 6B), with some seasonal 

variability in motion approaching 40 mm. Based on the 

time-series stability and InSAR analysis, NETP was selected 

as the reference station in the local baseline-pair reference 

frame, and all regional motion was tracked with respect to this 

station. The GPS time series analyzed for all GPS stations are 

available in appendix 2 of this report, which also contains 83 

time series that show the baseline-pair time series between 

NETP and all of the regional stations. 

Time-Series Patterns

The GPS time series showed seasonal motion, long-term 

patterns, and deviations from the long-term horizontal and 

vertical patterns. The GPS station LKHU exhibited the most 

notable seasonal motion (most distinguishable primarily in 

the up component of movement), which oscillates within the 

1–3 mm range, in the time series (fig. 6A). Because the effect 

of seasonal motion on long-term motion patterns diminishes 

when there are more than 3.5 years of data (Blewitt and others, 

1995; Bawden and others, 2001), the GPS station velocities 

determined for LKHU and several of the other GPS stations 

are considered robust. Many of the GPS stations had long 

and stable time series; most of the PAM stations, from PA01 

to PA28, had assessable horizontal and vertical patterns of 

movement. For example, PA01 had a long-term subsidence 

rate of about 43 mm/yr with only a minor southerly motion 

(table 1). PA04 was subsiding at a rate of about 19 mm/yr with 

a southerly velocity of about 3 mm/yr (table 1). LKHU had 

no discernible vertical and easterly motion but had a steady 

southerly velocity of about 3 mm/yr, or more than 40 mm 

since 1993 (fig. 6A). 

Global Positioning System Vertical and 

Horizontal Velocities

The GPS stations with the greatest subsidence rates from 

this analysis were clustered to the northwest and west of the 

Houston-Galveston region (farthest from downtown Houston, 

which is the part of this region with the most historical 

subsidence). Stations recording the most horizontal motion, 

for example, PA08, PA10, PA14, or PA16, were near the 

margins of the subsidence (fig. 12). Subsidence rates measured 

at GPS stations PA01, PA02, PA03, PA06, and PA07 were in 

excess of 30 mm/yr (fig. 12). These stations delineated the 

area of greatest subsidence, which was between the cities of 

Jersey Village, Tex., and Cypress, Tex., and extended to the 

south toward Sugar Land, Tex. (PA01, 42.8 mm/yr; PA02, 

34.1 mm/yr; PA03, 38.4 mm/yr; PA06, 30.9 mm/yr; and 

PA07, 41.6 mm/yr). The horizontal velocities north of NETP 

showed a prominent northerly velocity where stations were 

pulled towards the regions with greater subsidence. Five GPS 

stations west of Houston (PA10, PA14, PA16, PA04, and 

TXRO) had a radial velocity inward (fig. 12); these stations 

correspond to a subsidence trough. Minimal subsidence and 

horizontal deformation rates were generally measured at sites 

near Houston (within the Interstate 610 or inner loop and east 

toward Galveston Bay) (fig. 12). 

InSAR Magnitude and Extent

InSAR imagery was processed and analyzed to measure 

the surface displacement patterns and magnitudes in the 

Houston-Galveston region. In total, 61 ERS-1 and ERS-2 

SAR scenes on four overlapping scenes (fig. 1) were acquired 

and processed into 214 interferograms for this study with 

27 interferograms showing good coherency from September 

2, 1993, to December 3, 2000 (table 2). The SAR acquisition 

dates ranged from July 25, 1992, to December 19, 2000, and 

the time spans of the interferograms ranged from 1 (tandem 

ERS-1/ERS-2 interferograms) to 3,069 days (approximately 

8 years). The interferograms described in this report were 

constructed from data obtained from precise satellite orbits by 

using the two-pass method (Borik and Novotny, 2011) with 

a USGS 30-m digital elevation model and GAMMA InSAR 

software (GAMMA Remote Sensing and Consulting, 2012). 

The InSAR time period examined overlaps the more recent 

end of the time periods of previous studies examining surface 

displacement, or subsidence, in the Houston-Galveston region. 
Subsidence in the Houston-Galveston Region, 1993–2000 19

The recent subsidence patterns interpreted for this report 

are different compared to historical (1906–2000; 1913–17 to 

2001) patterns of subsidence published previously (Gabrysch 

and Neighbors, 2005; Kasmarek and others, 2009a). The 

historical maximum area of subsidence exists near downtown 

Houston east along the Houston Ship Channel in southeastern 

Harris County near Galveston Bay, whereas the InSAR 

analysis shows that most of the recent subsidence in the area 

is farther north (along the Harris–Montgomery County line), 

in western Harris County, and southwest along the Harris–Fort 

Bend County line (Kasmarek and others, 2009a). Water-level 

change contours for the Evangeline aquifer between 1990 and 

2003 showed a rise near downtown Houston and a decline to 

the northwest and southwest of Houston by as much as 30 m 

in a pattern that is similar to the InSAR imagery (fig. 13) but 

not identical, based on differences between the time periods 

analyzed by the datasets (Kasmarek and Lanning-Rush, 2004). 

From January 14, 1996, to December 3, 2000, there was 

about 180 mm of subsidence over an approximately 80-km 

long arcuate (bow shaped) feature on the northwestern side 

of Houston, from Sugar Land in southwest Fort Bend County, 

though Jersey Village in Harris County, to Spring, Tex., in 

north-central Harris County along the Harris–Montgomery 

County border (fig. 14). Visible on the interferogram are 

highly compressed color bands (fringe gradient), which 

represent a linear feature (lineament). The primary area of 

subsidence (referred to as the “northwest subsidence feature” 

or “NWSF”) is centered between the cities of Jersey Village 

and Cypress (fig. 15). Two subsidence features of lesser 

magnitude exist to the north and south of the NWSF. The 

Spring subsidence feature (SPSF) extends to the north and 

east from the NWSF through Spring (fig. 15). The Sugar Land 

subsidence feature (SLSF) extends from the NWSF through 

Sugar Land to the south (fig. 15). Additionally, although 

historical subsidence areas generally are stable or slightly 

rebounding, measureable subsidence was identified in a 4-km2

localized area near Seabrook. 

The NWSF encompasses an approximately 875 km2 area. 

A 0.9-year interferogram from January 30, 1996, to December 

30, 1997, showed localized subsidence of greater than 60 mm 

(> 66 mm/yr) and broad regional (15 to 30 mm/yr) subsidence 

across the entire NWSF (fig. 16). The subsidence continued in 

the NWSF between January 14, 1997, and May 19, 1998, with 

localized subsidence greater than 60 mm (46 mm/yr) (fig. 17), 

UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

Movement, in millimeters

−100

−50

0

50

100

0

25

50

−50

−25

0

25

50

−50

−25

mm/yr −1.5 

mm/yr 2.5 

mm/yr -0.1 

ADKS–NETP

mm/yr, Millimeters per year

Figure 11. Global Positioning System (GPS) time series for Continuously Operating Reference Stations (CORS) Addicks 1795 CORS ARP 

(ADKS) and Northeast 2250 CORS ARP (NETP) with least-squares linear-regression lines depicting directional velocity.
20 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

96°15' 96°00' 95°45' 95°30' 95°15' 95°00' 94°45'

30°15'

30°00'

29°45'

29°30'

29°15'

GULF OF MEXICO

Trinity 

Bay

East Bay

West Bay

Galveston 

Bay

InSAR scenes

Area enlarged

TEXAS

Humble

Spring

Houston

Richmond

Greatwood

Rosenberg

Atascocita

The Woodlands

Sugar Land

Seabrook

Cypress

Jersey Village

59

90

90

290

 6

36

99

288

36

105

249

59

146

321 770

45

10

610

45

10

0 10 20 MILES

0 10 20 KILOMETERS

Rates of horizontal movement,

 in millimeters per year

 10

 5.0

 2.5

-5.0

+5.0

-10

-20

-30

-40

-50

Rates of uplift or subsidence, in

 millimeters per year

EXPLANATION

GPS, Global Positioning System

InSAR, Interferometric synthetic aperture radar

GPS Port-A-Measure (PAM)

 and Continuously Operating

 Reference Stations (CORS)

 and GPS site name (table 1)

PA15

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1983

ANG2

COH1

COH2 COH3

COH4

COH5

COH6

COH7

GAL1

HOUS

LKHU

PA00

PA01

PA02

PA03

PA04

PA05

PA06

PA07

PA08

PA09

PA10

PA11

PA12

PA13

PA14

PA15

PA16

PA17

PA18

PA19

PA20 PA21

PA22

PA23

PA24

PA26

PA27

PA28

TMCC

TXCN

TXGA

TXLI

TXLM

TXRO

ZHU1

TXHE

NETP

TXAG

Figure 12. The Global Positioning System- (GPS-) derived horizontal and vertical velocities for Continuously Operating Reference 

Stations (CORS) and Harris-Galveston Subsidence District Port-A-Measure (PAM) sites, with Northeast 2250 CORS ARP (NETP) (star) 

as the reference station. The size of the circle is scaled to subsidence magnitude in millimeters per year (mm/yr). The arrows show 

horizontal motion in millimeters per year of the GPS stations and are oriented towards subsidence or away from uplift. 
Subsidence in the Houston-Galveston Region, 1993–2000 21

and one localized area subsided more than 90 mm between 

January 29, 1996, and May 19, 1998 (fig. 18). The lineament 

on the southern boundary of the NWSF may be associated 

with differential subsidence across the structure or with 

facies change within the subsurface materials (fig. 18); this 

lineament is visible in most of the interferograms, including 

the January 14, 1996, to December 3, 2000, interferogram of 

the region (fig. 14).

The SPSF encompasses an area approximately 300 km2

north and east of the NWSF (fig. 15). Ongoing subsidence was 

observed in the SPSF, with localized subsidence of about 45 

mm/yr. The magnitude of subsidence for much of the SPSF 

is lower that what is observed in the NWSF (fig. 16). Unlike 

the NWSF, the magnitude and extent of the subsidence in the 

SPSF did not noticeably change with four additional months 

included in the interferogram (January 1996 to May 1998) 

(figs. 16 and 18), which shows that most of the subsidence 

is apparently west of Humble, Tex., and south of the Harris–

Montgomery County line for the time period examined, 

January 1996 to December 1997. The large InSAR-imaged 

subsidence signal in the NWSF is also seen in both the 

horizontal and vertical GPS time-series data (figs. 6 and 19). 

The GPS-measured subsidence rates are similar in magnitude 

and match the arcuate pattern seen in the InSAR imagery. 

Stations that are on the southern side of the area of subsidence 

are moving northward, into the area with the maximum 

subsidence. The GPS and InSAR imagery also corresponds 

in regards to the SPSF at the northeastern end of the NWSF, 

although subsidence rates are lower and confined to a smaller 

region.

The SLSF is an open trough encompassing an area 

approximately 375 km2 (15 km by 25 km) aligned in a 

northwest-southeast direction with localized subsidence 

rates near the center of the trough of approximately 

30–40 mm/yr (fig. 15). The northern extent of the SLSF 

is open-ended and transitions into the much larger NWSF 

(fig. 15). Between January 1996 and December 2000, there 

were more than 180 mm of subsidence in the central region 

of the SLSF, though this amount might not represent the 

full magnitude of subsidence during this period (fig. 14). In 

the SLSF area, localized subsidence over the 4-year period 

resulted in a tight InSAR fringe pattern on the northern side 

of the trough that exceeded the magnitude of subsidence 

detectible with InSAR. The full extent of the subsidence on the 

western margin of the SLSF was difficult to detect with InSAR 

because the SAR scenes had poor coherency in the vegetated 

areas west of Richmond, Tex., and Rosenberg, Tex., associated 

with agriculture production. To minimize the scattering effect 

from the vegetation cover, temporal decorrelation was applied 

(a process used to improve coherency, or correlation between 

the short-term SAR scenes used to create the interferogram). 

There was also similar agriculture-related decorrelation south 

of Greatwood, Tex. 

Table 2. Interferograms generated for the Houston-Galveston 

region from data obtained during 1993–2000 from European remote 

sensing (ERS) satellites ERS–1 and ERS–2.

[ID is a unique identifier for an interferogram created from synthetic aperture 

radar (SAR) master and slave scenes, where the master scene is the beginning 

time scene that is the reference scene with the year month date format, 

YYYYMMDD, and the slave scene is the later time SAR scene that is aligned 

to the master scene with the same year month date format, YYYYMMDD. 

Track is the orbital track of the satellite. Frame is a latitude and longitude 

centered area. Scene is the synthetic aperture radar scene area defined by the 

combination of track and frame]

ID Master 

scene

Slave 

scene

Track Frame Scene

1 19960129 19970114 212 2997 T212_F2997

2 19960129 19970325 212 2997 T212_F2997

3 19960129 19971230 212 2997 T212_F2997

4 19960129 19980519 212 2997 T212_F2997

5 19960130 19971230 212 2997 T212_F2997

6 19951226 19960129 212 3015 T212_F3015

7 19951226 19971230 212 3015 T212_F3015

8 19951226 19980901 212 3015 T212_F3015

9 19951226 19981110 212 3015 T212_F3015

10 19960129 19970325 212 3015 T212_F3015

11 19960129 19971230 212 3015 T212_F3015

12 19960129 19980519 212 3015 T212_F3015

13 19960130 19971230 212 3015 T212_F3015

14 19960130 19981110 212 3015 T212_F3015

15 19970114 19980519 212 3015 T212_F3015

16 19970114 19990713 212 3015 T212_F3015

17 19970218 19980203 212 3015 T212_F3015

18 19930902 19960113 484 2997 T484_F2997

19 19960113 19970518 484 2997 T484_F2997

20 19960113 20001203 484 2997 T484_F2997

21 19960114 20001203 484 2997 T484_F2997

22 19930902 19960113 484 3015 T484_F3015

23 19960113 19970518 484 3015 T484_F3015

24 19960113 20000107 484 3015 T484_F3015

25 19960113 20001203 484 3015 T484_F3015

26 19960114 20001203 484 3015 T484_F3015

27 19961229 20001203 484 3015 T484_F3015
22 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

Figure 13. Comparison of an interferogram with water-level contours for the Houston-Galveston region, Texas. A, Interferometric 

synthetic aperture radar- (InSAR-) measured land subsidence between January 30, 1996, and December 30, 1997. B, Evangeline aquifer 

water-level change contours between 1990 and 2003 (modified from Kasmarek and Lanning-Rush, 2004).

Houston

FM 521

90

45

249

59

10

99

90

548

321

FM 1960

225

State Loop 336

90 59

290

45

610

Galveston Rd

Grand Pky

36

N Sam Houston Pky W

S Sam Houston Pky E

E North Belt

Addicks Satsuma Rd

S Belt

146

90

10

610

6

59

1488

40

-20

-40

-60

60

20

-80

-100 20

40

0

80

60

60

20

-80 -100

100

40

80

-60

-60

100

-40

60

60

60

-80

20

80

40

80

60

20

40

100

0

-80

80

-20

0 20

-20

20 0 -20

-40

20 20

40

80

40

40

60

20 120

20

40 40

20

40

80

-20

-20

80

100

140 100

40

-20

-20

-40

-60

0

0

20

-20

-40

-60

-80

40

40 20

40

60 80

60

40

20 20

80

100 60

80

40

20

-100

60

100

0

40

60

0

120

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1927

FM 521

90

45

249

59

10

99

90

548

321

FM 1960

FM 1764

225

State Loop 336

36

90 59

290

45

610

Galveston Rd

Grand Pky

36

N Sam Houston Pky W

S Sam Houston Pky E

E North Belt

Addicks Satsuma Rd

S Belt

146

288

90

10

610

6

59

146

1488

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1927

Houston

GALVESTON

COUNTY

BRAZORIA

COUNTY

FORT BEND

COUNTY

WALLER

COUNTY

HARRIS

COUNTY

MONTGOMERY COUNTY

GALVESTON

COUNTY

BRAZORIA

COUNTY

FORT BEND

COUNTY

WALLER

COUNTY

LIBERTY COUNTY

CHAMBERS COUNTY

LIBERTY COUNTY

CHAMBERS COUNTY

WALKER

COUNTY

HARRIS

COUNTY

MONTGOMERY COUNTY

Sheldon

Reservoir

Trinity Bay

Lake

Houston

Galveston Bay

Sheldon

Reservoir

Trinity Bay

Lake

Houston

Galveston Bay

95°45' 95°30' 95°15' 95°00’

30°00’

30°15'

29°30'

29°45'

30°00’

30°15'

29°30'

29°45'

95°45' 95°30' 95°15' 95°00’

0 5 10 MILES

0 5 10 KILOMETERS

0 5 10 MILES

0 5 10 KILOMETERS

Area enlarged

TEXAS

Line of equal

 water-level

 rise—Intervals

 20 and 40 feet

Line of equal

 water-level

 decline—

 Intervals 20 

 and 40 feet

Line of zero

 water-level

 change

EXPLANATION

 20

 0

-20subsidence uplift

+28.3 

-28.3 

0

Movement, in

 millimeters

EXPLANATION

Relative movement

A

B
Subsidence in the Houston-Galveston Region, 1993–2000 23

FORT BEND COUNTY

WALLER 

COUNTY

BRAZORIA 

COUNTY

LIBERTY 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

HARRIS COUNTY

FORT BEND COUNTY

WALLER 

COUNTY

MONTGOMERY COUNTY

249

610

45

10

45

90

290

59

59

90

288

Sam Houston Tollway

Galveston 

Bay

Humble

Seabrook

Atascocita

Spring

Sugar Land

The Woodlands

Cypress

Jersey Village

Rosenberg

Richmond

Greatwood

Houston

InSAR scenes

Area enlarged

TEXAS

LINEAMENT

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1983

0 5 10 MILES

0 5 10 KILOMETERS

Interferometric synthetic aperture

 radar (InSAR) fringe contour

County boundary Subsidence Uplift

+28.3 

-28.3 

0

Movement, in millimeters

EXPLANATIONRelative movement

95°50' 95°40' 95°30' 95°20' 95°10' 95°00'

30°10'

30°00'

29°50'

29°40'

29°30'

Figure 14. Interferometric synthetic aperture radar- (InSAR-) measured subsidence from January 14, 1996, to December 3, 2000. 

Contours delineate the interferogram fringes and show greater than 180 millimeters (mm) of subsidence to the northwest over this 

4-year period. The arrow points to highly compressed color bands (fringe gradient), which represent a linear feature (lineament).
24 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

Chambers

County

Galveston Bay

Galveston Bay

NWSF

SLSF

SPSF

Houston

TEXAS

Area enlarged

0 5 10 MILES

0 5 10 KILOMETERS

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Land-surface

 deformation class

 Stable to possible

 uplift

 Stable to 30 mm

 subsidence

 30 to 60 mm subsidence

 60 to 90 mm subsidence

 Greater than 90 mm

 subsidence

InSAR fringe contour

Undefined boundary 

 between InSAR

 fringe contour

MONTGOMERY

COUNTY

CHAMBERS

 COUNTY

HARRIS

COUNTY

FORT BEND

COUNTY

BRAZORIA

COUNTY

GALVESTON COUNTY

Subsidence Uplift

+28.3 

-28.3 

0

Movement, in

 millimeters (mm)

EXPLANATIONRelative movement

InSAR fringe

 contours

Undefined

 boundary 

 between

 InSAR

 fringe

 contours

InSAR, Interferometric synthetic aperture radar

A

B

 95 ̊38' 95 ̊00'

29 ̊36'

30 ̊02'

 95 ̊38' 95 ̊00'

29 ̊36'

30 ̊02'

Spring

Seabrook

Sugar

 Land

Jersey 

Village

Cypress

Spring

Seabrook

Sugar

 Land

Jersey 

Village

Cypress

Sugar Land 

subsidence feature (SLSF)

Spring subsidence feature (SPSF)

Northwest 

subsidence feature (NWSF)

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1927

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1927

Figure 15. Interferogram for January 30, 1996, to December 30, 1997 showing primary areas of subsidence. A, The named subsidence 

features to the northwest. B, Interpolated land-surface deformation contours. The blue- and red-shaded areas represent regions with 

subsidence or uplift.
Subsidence in the Houston-Galveston Region, 1993–2000 25

Figure 16. A 0.9-year interferogram (January 30, 1996, to December 30, 1997) of the Houston-Galveston region, Texas, showing 

60 millimeters (mm) of subsidence in the northwestern part of the interferogram.

BRAZORIA 

COUNTY

LIBERTY 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

HARRIS COUNTY

FORT BEND

COUNTY

WALLER 

COUNTY

MONTGOMERY COUNTY

249

610

45

10

45

90

290

59

59

90

288

Sam Houston Tollway

Galveston 

Bay

Humble

Seabrook

Atascocita

Spring

Sugar Land Sugar Land

The Woodlands

Cypress

Jersey Village

Rosenberg

Richmond

Greatwood

Houston

InSAR scenes

Area enlarged

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1983

0 5 10 MILES

0 5 10 KILOMETERS

County boundary Subsidence Uplift

+28.3 

-28.3 

0

Movement, in millimeters

EXPLANATIONRelative movement

InSAR, Interferometric synthetic aperture radar

95°50' 95°40' 95°30' 95°20' 95°10' 95°00'

30°10'

30°00'

29°50'

29°40'

29°30'

Northwest Spring subsidence feature (SPSF)

subsidence feature (NWSF)

Sugar Land 

subsidence feature (SLSF)

LINEAMENT
26 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

HARRIS COUNTY

BRAZORIA 

COUNTY

LIBERTY 

COUNTY

FORT BEND COUNTY

CHAMBERS 

COUNTY

WALLER 

COUNTY

GALVESTON 

COUNTY

Houston

Richmond

Seabrook

Greatwood Rosenberg

Sugar Land

Jersey Village

610

10

45

10

45

 6

Sam Houston Tollway

59

90

59

288

290

249

90

Galveston 

Bay

InSAR scenes

Area enlarged

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1983

0 5 10 MILES

0 5 10 KILOMETERS

County boundary Subsidence Uplift

+28.3 

-28.3 

0

Movement, in millimeters

EXPLANATIONRelative movement

InSAR, Interferometric synthetic aperture radar

95°50' 95°40' 95°30' 95°20' 95°10' 95°00'

29°50'

29°40'

29°30'

29°20'

Northwest 

subsidence feature (NWSF)

Sugar Land 

subsidence feature (SLSF)

Figure 17. A 1.3-year interferogram (January 14, 1997, to May 19, 1998) of the Houston-Galveston region, Texas, showing approximately 

60 millimeters (mm) of subsidence in the northwestern part of the interferogram.
Subsidence in the Houston-Galveston Region, 1993–2000 27

Figure 18. A 2.3-year interferogram (January 29, 1996, to May 19, 1998) of the Houston-Galveston region, Texas, showing a localized 

area with more than 90 millimeters (mm) of subsidence.

Galveston 

Bay

Localized area

of subsidence

95°50' 95°40' 95°30' 95°20' 95°10' 95°00'

30°10'

30°00'

29°50'

29°40'

29°30'

InSAR scenes

Area enlarged

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1983

0 5 10 MILES

0 5 10 KILOMETERS

Interferometric synthetic aperture

 radar (InSAR) fringe contour

County boundary Subsidence Uplift

+28.3 

-28.3 

0

Movement, in millimeters

EXPLANATIONRelative movement

BRAZORIA 

COUNTY

LIBERTY 

COUNTY

CHAMBERS 

 COUNTY

GALVESTON COUNTY

HARRIS COUNTY

FORT BEND COUNTY

WALLER 

COUNTY

MONTGOMERY COUNTY

249

610

45

10

45

 6

90

290

59

59

90

Sam Houston Tollway

288

Humble

Seabrook

Atascocita

Spring

Sugar Land Sugar Land

The

Woodlands

Cypress

Jersey Village Jersey Village

Rosenberg

Richmond

Greatwood

Houston

Spring

subsidence feature (SPSF)

Northwest 

subsidence feature (NWSF)

Sugar Land 

subsidence feature (SLSF)

LINEAMENT
28 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

Humble

Spring

Houston

Richmond

Greatwood

Rosenberg

Atascocita

The Woodlands

Sugar Land

Seabrook

Cypress

Jersey Village

59

90

90

290

 6

36

99

288

36

105

249

59

146

321 770

45

10

610

45

10

96°15' 96°00' 95°45' 95°30' 95°15' 95°00' 94°45'

30°15'

30°00'

29°45'

29°30'

29°15'

GULF OF MEXICO

Trinity 

Bay

East Bay

West Bay

Galveston 

Bay

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1983

InSAR scenes

Area enlarged

TEXAS

ANG2

COH1

COH2 COH3

COH4

COH5

COH6

COH7

GAL1

HOUS

LKHU

PA00

PA01

PA02

PA03

PA04

PA05

PA06

PA07

PA08

PA09

PA10

PA11

PA12

PA13

PA14

PA15

PA16

PA17

PA18

PA19

PA20

PA21

PA22

PA23

PA24

PA26

PA27

PA28

TMCC

TXCN

TXGA

TXLI

TXLM

TXRO

ZHU1

TXHE

TXAG

0 10 20 MILES

0 10 20 KILOMETERS

Rates of horizontal movement,

 in millimeters per year

 10

 5.0

 2.5

-5.0

+5.0

-10

-20

-30

-40

-50

Rates of uplift or subsidence, in

 millimeters per year

GPS, Global Positioning System

GPS Port-A-Measure (PAM)

 and Continuously Operating

 Reference Stations (CORS)

 and GPS site name (table 1)

PA15

Interferometric synthetic aperture

 radar (InSAR) fringe contour

County boundary

Subsidence Uplift

+28.3 

-28.3 

0

Movement, in

 millimeters

EXPLANATION

Relative movement

Figure 19. Comparison of contoured interferograms (January 29, 1996, to May 19, 1998) in the Houston-Galveston region, Texas, with 

velocities (arrows) in millimeters per year (mm/yr) and subsidence rates (circles) in mm/yr derived from Global Positioning System (GPS) 

data.
Limitations 29

The horizontal and vertical GPS deformation field 

supports the InSAR-measured subsidence in the SLSF. All of 

the GPS stations in the area surrounding the SLSF are moving 

inward towards the subsidence trough, mostly perpendicular 

to the InSAR fringe gradient. The PAM station, PA04, is 

north and east of a localized area of subsidence observed from 

InSAR data (fig. 19), where the interferogram shows about 

40 mm of subsidence (19 mm/yr) between January 29, 1996, 

and May 19, 1998. This amount of subsidence corresponds 

with the GPS-measured subsidence rate of 19.5 mm/yr, in 

which the station is moving to the south at a rate of 3.5 mm/

yr and has a slightly westward velocity at less than 1 mm/yr 

directly towards the area with the greatest subsidence. This 

amount equates to about 290 mm of subsidence between 1994 

and 2009 with greater than 50 mm of southward movement 

of the station. The other stations—PA10, TXRO, PA14, and 

PA16—also have velocities inward towards the subsidence 

trough (fig. 19). 

Although it is within the recently stable area of 

historical subsidence, a localized area of subsidence 

exists near Seabrook. The localized subsidence feature 

encompasses an area about 8 km2 where from February 

18, 1997, to February 3, 1998, about 28 mm of subsidence 

occurred (fig. 20). Multiyear interferograms show appreciable 

(more than 85 mm from January 1996 to December 1997) 

subsidence in the area with several fringes visible; however, 

because of decorrelation and the high subsidence magnitudes, 

it is difficult to determine the maximum subsidence 

magnitude near Seabrook because many of the fringes are 

truncated or ill-defined (fig. 21). The subsidence gradient 

begins northwest of Seabrook with little to no discernible 

subsidence and extends to the southeast beginning near Taylor 

Lake Village, Tex., and extending to the area of maximum 

subsidence near the coast. The PAM GPS station PA27 near 

Seabrook has an eastward velocity of 3 mm/yr (greater than 

20 mm of absolute motion during 2003–9) and is subsiding 

at 6 mm/yr, or nearly double the eastward motion, matching 

the InSAR subsidence signal seen in multiple interferograms 

near Seabrook (fig. 21). The horizontal velocity vectors for 

PA27 points east towards the area with the maximum localized 

subsidence (fig. 20).

Limitations

Two primary limitations exist when using the baselinepair approach for the GPS analyses: (1) all of the motions 

are relative to the one station and are independent of the 

regional deformation field, and (2) all local and regional 

motion measured at the baseline-pair reference station will be 

mapped into the motion at every other GPS station. The first 

limitation is not relevant for the scope of this report because 

it is not necessary to relate the Houston regional subsidence 

to a presumed stable North American continent or other areas 

with known subsidence along the Gulf of Mexico Coast. 

Furthermore, based on the InSAR pixel size (30 m on a side 

or 900 m2), there is no advantage to processing the GPS 

data in a global reference frame to compare with the InSAR 

imagery. To address the second limitation, an assessment of 

which CORS station to hold fixed in the baseline-pair analysis 

is needed to fully characterize the vertical and horizontal 

motions in the GPS data. The objective is to establish whether 

or not the reference station is stable. If the station is not stable, 

then any reference station that is truly subsiding at 10 mm/

yr, paired with any other station subsiding at the same rate, 

would show zero net subsidence. All of the motion is relative 

to the selected reference station, and if both are subsiding at 

the same rate, then there is no net difference between the two 

stations. Continuing with this example, any station that shows 

uplift of 10 mm/yr, paired with any other station showing 

uplift at the same rate, would actually have zero net uplift.

InSAR analysis in this study can be limited by a number 

of factors, some of which are generally described in the 

“Methodology” section of this report and some of which are 

described in further detail in appendix 1. Regional agriculture 

production and dense vegetation, combined with an overall 

lack of urban reflective surfaces in the western and northern 

parts of the study area, limited the resolution of the InSAR 

analysis and any corresponding characterization. Determining 

the maximum surface motion with InSAR requires an 

uninterrupted InSAR fringe pattern where the full number 

of fringes can be uniquely counted, which was not always 

possible. Incoherency to the northern and eastern parts of the 

study area decreased the resolution of the InSAR analysis, 

thereby limiting the subsidence characterization. 
Clear Lake Taylor Lake Village Seabrook Galveston Bay 0 1 2 MIL

0 1 2 KILOMETERS 95°04' 95°00' 29°38' 29°36'

PAM

PAM27

Horizontal rate: 3 mm/yr

Vertical rate: -6 mm/yr

Base modified from U.S. Geological Survey digital data, 1:100,000Geographic Coordinate System, World Geodetic System 1984

Figure 20. Detailed interferogram showing subsidence during about 1 year (February 18, 1997, to February 3, 1998) in an area around Seabrook, Texadisplacement rates in millimeters per year (mm/yr) derived from Global Positioning System (GPS) data.
El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

El Lago

Seabrook

Shoreacres

Bayside Terrace

Taylor Lake

Village

0 0.5 1 0 0.5 1 KILO

0 0.5 1 MILE

0 0.5 1 KILOMETER

0 0.5 1 

0 0.5 1 KILO

0 0.5 1 MILE

0 0.5 1 KILOMETER

Little Cedar Bayo

Taylor Bayou

Pine Gull yPine Gull y

H orsepenH orsepen

Bayou

Big Islan d SloughBig Islan d Slough

Spring G ully Spring G ully

Armand Bayou Armand Bayou

Clear Lake

Galve

Ba

Little Cedar Bayou

Taylor Bayou

Pine Gull yPine Gull y

H orsepenH orsepen

Bayou

Big Islan d SloughBig Islan d Slough

Spring G ully Spring G ully

Armand Bayou Armand Bayou

Clear Lake

Galveston 

Bay

Little Cedar Bayo

Taylor Bayou

Pine Gull yPine Gull y

H orsepenH orsepen

Bayou

Big Islan d SloughBig Islan d Slough

Spring G ully Spring G ully

Armand Bayou Armand Bayou

Clear Lake

Galve

Ba

Little Cedar Bayou

Taylor Bayou

Pine Gull yPine Gull y

H orsepenH orsepen

Bayou

Big Islan d SloughBig Islan d Slough

Spring G ully Spring G ully

Armand Bayou Armand Bayou

Clear Lake

Galveston 

Bay

HARRIS COUNTY

CHAMBERS COUNTY

HARRIS COUNT

CHAMBERS CO

HARRIS COUNTY

CHAMBERS COUNTY

HARRIS COUNT

CHAMBERS CO

~50 mm

~85 mm

~85 

Unable to define 

subsidence estimate

95°06' 95°04' 95°02' 95°00'

29°38'29°36'29°34'

95°06' 95°04' 95°02' 95°00'

29°38'29°36'29°34'

A

B

C

D 30 Jan. 1996 to 30 Dec. 1997 29 Jan. 1996 to 19 Ma

26 Dec. 1995 to 10 Nov. 1998 30 Jan. 1996 to 10 No

Base modified from U.S. Geological Survey digital data, 1:100,000Geographic Coordinate System, World Geodetic System 1984

Figure 21. Four detailed interferograms of the Seabrook, Texas, area showing different subsidence results. 

A, Unable to define subsidence estimate

millimeters (mm) of subsidence. 

C, Approximately 85 mm of subsidence. 

D, Approximately 85 mm of subsidence.
32 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

Summary

Since the early 1900s, groundwater has been the 

primary source of municipal, industrial, and agricultural 

water supplies for the Houston-Galveston region, Texas. The 

region’s combination of hydrogeology and nearly centurylong use of groundwater has resulted in one of the largest 

areas of subsidence in the United States; by 1979 as much as 

3 meters (m) of subsidence had occurred and approximately 

8,300 square kilometers of land had subsided more than 

0.3 m. The U.S. Geological Survey, in cooperation with the 

Harris-Galveston Subsidence District, used interferometric 

synthetic aperture radar (InSAR) data from two European 

remote sensing satellites (ERS-1 and ERS-2) to analyze 

land subsidence in the Houston-Galveston region of Texas. 

The InSAR data were processed into 27 interferograms 

that delineate and quantify land subsidence patterns and 

magnitudes. Contemporaneous data from the Global 

Positioning System (GPS) data were reprocessed by the 

National Geodetic Survey and analyzed to support, verify, and 

provide temporal resolution to the InSAR investigation. 

In 2010, the National Geodetic Survey reprocessed all 

of the regional Continuously Operating Reference Stations 

(CORS) and Port-A-Measure (PAM) GPS data for the 

Houston-Galveston region from 1993 through 2009 with 

respect to the original three CORS stations: Addicks 1795 

CORS ARP (antenna reference point) (ADKS), Lake Houston 

(LKHU), and Northeast 2250 CORS ARP (NETP). A local 

reference frame was used by holding the CORS station, NETP, 

fixed; NETP was selected as the reference station in the local 

baseline-pair reference frame based on the lack of InSAR 

observed motion at the station and stability in the time-series 

when compared with LKHU and ADKS. In total, 61 ERS-1 

and ERS-2 synthetic aperture radar scenes were acquired, 

processed, and used to generate 214 interferograms. From 

the total interferograms produced, only 27 interferograms 

showed reasonable to good coherency, but many contained 

atmospheric noise that could not be used to interpret the 

regional subsidence patterns. 

The interferograms show that the area with the maximum 

historical subsidence has now stabilized and may have a minor 

uplift component with respect to the surrounding region. 

Although the historical maximum area of subsidence exists 

near downtown Houston east along the Houston Ship Channel 

in southeastern Harris County near Galveston Bay, the InSAR 

analysis shows that most of the recent subsidence in the area is 

much farther west of and north of downtown Houston. 

The primary area of subsidence determined by InSAR 

analysis, referred to as the “northwest subsidence feature” 

(NWSF), is centered between Jersey Village, Tex., and 

Cypress, Tex. Two subsidence features of lesser magnitude 

exist to the north and south of the primary area of subsidence 

near the cities of Spring, Tex. (Spring subsidence feature 

(SPSF)) and Sugar Land, Tex. (Sugar Land subsidence feature 

(SLSF)), respectively. Subsidence rates were calculated based 

on the interval of time between the SAR images. Estimated 

subsidence in the NWSF is generally 15 to 30 millimeters 

per year (mm/yr) with localized areas greater than 60 mm/yr, 

whereas estimated subsidence in localized areas in the SPSF 

and SLSF is about 45 mm/yr to 30–40 mm/yr, respectively. 

Multiyear interferograms, near Seabrook, Tex., within the 

historical subsidence area and nearby Galveston Bay, show 

several fringes of subsidence (approximately 85 millimeters 

from January 1996 to December 1997) in the area; however, it 

is difficult to determine the maximum subsidence magnitude 

near Seabrook because many of the fringes are truncated or illdefined. The PAM GPS station, PA27, near Seabrook has an 

eastward velocity of 3 mm/yr (greater than 20 mm of absolute 

motion during 2003–9) and is subsiding at 6 mm/yr, or nearly 

double the eastward motion, matching the InSAR subsidence 

signal. 

Horizontal and vertical GPS velocities are similar in 

magnitude and match the arcuate pattern seen in the InSAR 

imagery. Stations that are on the margins of the subsidence 

features have greater horizontal velocities and are moving 

toward the areas with the maximum subsidence, mostly 

perpendicular to the InSAR fringe gradient. Stations on the 

south side of the SPSF have northern horizontal motion into 

the subsidence area, and stations bounding the SLSF are 

pointing radially inward toward the subsidence trough. The 

PAM station, PA04, which is north and east of the SLSF, has 

a subsidence a rate of 19.5 mm/yr, in which the station is 

moving to the south at a rate of 3.5 mm/yr, and has a slightly 

westward velocity at less than 1 mm/yr, which is directly 

towards the area with the greatest subsidence. This equates 

to about 290 mm of subsidence between 1994 and 2009 with 

greater than 50 mm of southward movement of the station; 

similar patterns are observed with the other GPS stations. 

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Appendix 1—Interferometic Synthetic Aperture Radar 

Processing and Error Analysis

Appendix 1

Appendix 1 39

Introduction

The processing of satellite synthetic aperture radar 

(SAR) scenes to generate interferograms is a multistep 

process that transforms the initial radar echoes emitted by 

the satellite into georeferenced radar images (interferograms) 

that can detect subcentimeter changes in the land surface 

position. This appendix outlines the steps used to process 

the interferograms: SAR scene focusing, creating an 

interferogram, georeferencing, removing topographic 

effects, and unwrapping the interferogram to determine the 

absolute magnitude of the land surface motion. Limitations 

and potential error sources associated with the analysis and 

interpretation of interferograms are also discussed (Schmidt 

and Bürgman, 2003; European Space Agency, 2007). 

SAR Scenes Focus and Alignment

The first step to creating an interferometic synthetic 

aperture radar (InSAR) image is to focus and align two SAR 

scenes collected along the same orbit trajectory (same point in 

space) on different orbits; for optimal results, the orbital path 

between each pass should be closer than 200 meters (m). Raw 

SAR data typically consists of 28,000 lines of data, where each 

line is a radar echo that was emitted from the satellite; traveled 

to the ground surface; interacted with the land, vegetation, 

buildings, and so forth; and returned to the satellite. Image 

focusing combines individual radar echoes into one coherent 

image approximately 100- by 100-kilometers (km) in size for 

European remote sensing (ERS) satellites ERS-1 and ERS-2 

that is typically shown as a grey-scale image with increased 

intensity correlating to greater amount of signal reflected 

back to the satellite (fig. 1.1). Once the scenes have been 

focused, the two scenes need to be precisely aligned. This is 

accomplished by employing a pixel-correlated routine that 

matches common pixel patterns between the master scene (the 

scene that is held fixed, typically the scene with the earliest 

date) (fig. 1.1A) and the slave scene (any scene that is aligned 

to the master, typically the scene with the latest data) (fig. 

1.1B). The orbital ephemerides (orbit position information 

for the satellite during the scene acquisition) for each scene 

provide the initial alignment position, and then the pixel 

correlation routine aligns each image so that the location of 

each pixel in the slave scene matches the master (European 

Space Agency, 2007).

Creating an Interferogram

The next stage of the process is to generate an 

interferogram from the aligned SAR scenes obtained from 

ERS-1 and ERS-2. A pulse of energy is emitted from each 

satellite that propagates through the atmosphere, interacts 

with the ground, and bounces back to the satellite where three 

pieces of information are recorded: (1) the two-way travel 

time from the satellite to the ground and back; (2) the amount 

of signal bounced back, known as magnitude or intensity; and 

(3) the phase of the return signal, which is described below. 

These data are used to generate the interferogram. The travel 

time is the time it takes the pulse to travel from the satellite to 

a point on the ground and then back to the satellite and is used 

to calculate the distance between the satellite and the ground 

(Schmidt and Bürgman, 2003; European Space Agency, 2007; 

Sneed and Brandt, 2007). Since the signal travels the distance 

twice, once from the satellite to the ground and once from the 

ground back to the satellite, the true distance is half, or divided 

by two. Distance is calculated by:

D = (t * c) / 2 (1.1)

where

D is the distance from the satellite to the ground 

surface, in meters

t is the travel time of the pulse from the satellite 

to the ground and back to the satellite, in 

seconds, and

c is the speed of light, in meters per second. 

The magnitude of return signal is a measure of the 

amplitude, intensity, or radar albedo returned for that point 

and is often visualized as a grey-scale photo-like image of the 

land surface. The phase is the fraction of a wavelength that 

is returned to the satellite. ERS-1 and ERS-2 are C-band (56 

millimeters (mm)) radar satellites. The distance between the 

ground and the satellite can be quantified with a very large 

number of full sinusoidal wavelengths (satellite altitude * 56 

mm), plus the fractional wavelength distance, to fully reach 

the ground from the satellite. If the ground surface was stable 

between the two satellite acquisitions, then the phase would 

be the same for both scenes. If subsidence occurred between 

the two satellite acquisition dates, however, then there would 

be a phase shift, in this case a phase lag (a positive change in 

phase between the master and slave scenes) corresponding to 

an increase in the distance between the ground and satellite. 

Alternatively, if there were uplift, then the phase shift would 

be negative (a phase advance) and correspond to a decrease in 

the distance between the ground and satellite (European Space 

Agency, 2007). 

The interferogram is produced by comparing the 

change in phase for each corresponding pixel in the aligned 

SAR scenes. At this stage in the process, the interferogram 

is a non-georeferenced phase-change image that identifies 

changes in land surface elevation, positive or negative, by 

repeating a pattern of colors (fig. 1.2). Each unique pattern, 

or fringe, represents one sinusoidal wavelength (0 to 2π: 

0–28.3 millimeters (mm) for ERS-1 and ERS-2 C-band radar 

satellites) (fig. 1.2) (European Space Agency, 2007). 

Georeferencing the Interferogram

The interferogram is georeferenced by aligning the 

master SAR scene to a digital elevation model (DEM) using 

the same pixel correlation routine described above. A synthetic 
40 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

A B

C D

Figure 1.1. Example synthetic aperture radar (SAR) scenes for the Houston-Galveston region where increased pixel intensity 

correlates to greater radar signal returned to the satellite: A, SAR scene collected on December 30, 1997; B, SAR scene collected 

on July 28, 1998; C, Zoom-in region in the box on A; D, SAR scene from a digital elevation model (DEM) using the orbital position from 

scene A.
Appendix 1 41

Uplift

0

-28.3

Range change, in millimeters

EXPLANATION

• Count the number of fringes

 From the outer red area (”START”) to the

 center, three red fringes can be counted.

• Multiply the number of fringes by 28.3 millimeters to estimate deformation

 3 x 28.3 = 84.9 millimeters

• Determine range change direction from the scale bar

 The color sequence from the outer edge inward 

 is red-orange-yellow-green-blue-violet, which 

 corresponds to uplift and a decreased distance 

 between the ground and the satellite (range).

Range, in millimeters

+28.3 0 -28.3

An increase

in range

indicates

subsidence

A decreasein rangeindicatesuplift

1 START

2

3

Fringes of deformation

START

1

2

3

START

1 fringe

2 fringes

3 fringes

A. Simplified interferogram showing uplift

B. Steps to interpret an interferogram

Figure 1.2. Simplified interferogram. A, Approximately three fringes of deformation equaling 84.9 millimeters (mm) of uplift. B, Steps to 

interpret an interferogram.
42 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

radar scene is generated by mathematically calculating what 

the master SAR would look like based on orbital ephemerides 

and the topographic data in the DEM (fig. 1.1D). The 

topography patterns in the synthetic radar image are pixelcorrelated with patterns on the SAR image such that the SAR 

image is precisely aligned and georeferenced to the DEM. The 

pixel size of the DEM determines the final resolution in the 

interferograms, which typically are composed of square pixels 

that range in edge length: 10, 30, 60, and 90 m. Interferograms 

can be processed at each of these pixel sizes. The smaller the 

pixel size the greater the computer resources required (disk 

space, random access memory (RAM), processing time) and 

the more that small changes to the land surface (tree growth, 

car positional changes, and so forth) influence the overall 

interferogram. A 30-m DEM was used as the base for the 

InSAR processing in this report (European Space Agency, 

2007). 

Removal of Topography Effects

A georeferenced-interferogram is processed to remove 

the effects of topography. Variations in the satellite’s orbital 

position can create a parallax effect that is manifested in the 

interferograms as fringes that conform to the topography. 

Increased separation between the two satellite orbits results in 

increased sensitivity to topography and, therefore, increases 

the number of topographic fringes in the interferogram. 

Topographic fringes are removed by calculating the fringe 

pattern from the DEM, aligning the imagery, and then 

subtracting the topographic fringes from the interferogram. 

The greater the distance between the two satellite orbits 

used to create an interferogram, the more likely errors in the 

DEM will be seen in the imagery. Orbital positions that are 

within 200 m produce usable interferograms. Due to the flat 

topography of the Texas Gulf Coastal Plain, these topographydriven parallax effects are minor and easily identified and 

correlated with the topography of the Houston-Galveston 

region. The effects of topography were automatically removed 

from each interferogram as part of the standard processing 

approach (Schmidt and Bürgman, 2003; European Space 

Agency, 2007). 

Unwrapping the Interferogram

Absolute distance is assigned to the interferogram 

through a process called unwrapping. This process converts 

the interferogram from the phase space (0 to 2π) to absolute 

distance between the ground and satellite based on sufficient 

data density to resolve the fringe gradient across the target 

area. Unwrapping refers to resolving the ambiguous phase 

shifts that result from phase changes that are multiples of a 

full (2 π) cycle. There are assumptions and approaches for 

unwrapping the interferograms that are beyond the scope 

of this report and that largely do not apply to the HoustonGalveston region because of the lack of relief in land surface 

and the signal coherence across the area (European Space 

Agency, 2007).

Limitations and Potential Considerations for 

Error

The techniques used to create interferograms from 

satellite SAR data have limitations measuring surface motion 

in regions with moderate to heavy vegetation, atmospheric 

moisture (clouds), high humidity, and high-relief topography 

(Schmidt and Bürgman, 2003; European Space Agency, 2007). 

The Houston-Galveston region has components of many of the 

typical noise and error sources. Each noise and error source 

and the approach used to mitigate its effects are described 

herein. 

Changes in the vegetation between each satellite image 

are mapped as changes in the distance between ground and 

satellite (range). Radar signals bounce off of dense leaves, 

and when there is a change in the leaf canopy (summer versus 

winter), the range randomly changes across the vegetated land 

and appears in the interferograms as random color patterns, 

or radar incoherence, that are not associated with land-surface 

displacement. To mitigate the effects of changes in vegetation, 

interferograms were developed from radar scenes acquired 

close in time. Vegetation patterns change seasonally, but with 

data collected every 35 days, the impact of vegetation can 

be minimized. The Houston-Galveston region SAR scenes 

were selected to minimize the time period between each 

acquisition and study the land surface during the same time 

of the year, ideally during the fall and winter when there are 

fewer leaves. During this time period (leaf-off conditions), 

the radar signal penetrates the small branches and images the 

land surface and stable tree trunks, thus reducing incoherence. 

Filtering algorithms can enhance the signal quality within 

the interferograms and minimize the noise associated with 

vegetation.

The largest single source of data noise, or error, for 

the Houston-Galveston region was atmospheric moisture 

associated with clouds (fig. 1.3). Atmospheric moisture delays 

the radar signal as it travels from the satellite to the ground 

and back. This delay increases the radar signal travel time and 

the calculated range distance and may produce artifacts in the 

interferograms that equate to as much as 300 mm of delay 

for the most extreme conditions (fig. 1.3). Typically clouds, 

humidity, and fog only produce about 30–50 mm of noise in 

the interferograms. The error associated with atmospheric 

moisture was mitigated by combining or stacking radar 

scenes to average out any atmospheric noise and avoiding 

SAR scenes from days with measureable precipitation. The 

spatial patterns associated with clouds can be identified. For 

example, three radar scenes for the same ground location are 

examined, A, B, and C, with scene B containing clouds. The 

interferogram produced from scenes A and B (A–B) would 

show an increase in satellite range (apparent subsidence) and 

the interferogram produced from scenes B and C (B–C) would 
Appendix 1 43

Galveston Bay

Atmospheric noise due 

to storm clouds

Houston

TEXAS

Area enlarged

 95 ̊38' 95 ̊00'

29 ̊36'

29 ̊22'

 95 ̊38' 95 ̊00'

29 ̊36'

29 ̊22'

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 15

North American Datum of 1927

Subsidence Uplift

+28.3 

-28.3 

0

Movement, in

 millimeters

EXPLANATIONRelative movement

0 5 10 MILES

0 5 10 KILOMETERS

A

B

Galveston 

Bay

Galveston 

Bay

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

BRAZORIA 

COUNTY

CHAMBERS 

COUNTY

GALVESTON COUNTY

FORT BEND COUNTY

HARRIS

COUNTY

Figure 1.3. Examples of interferograms. A, Few clouds were present and there was minimal noise, Scene T212-F3015 for January 

14, 1997, to December 30, 1997. B, Storm was present (box), Scene T212-F3015 for January 14, 1997, to July 13, 1999. The fringes in this 

interferogram are the summation of land surface motion (subsidence), noise related to the storm (box), and possible other clouds in the 

scene.
44 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR

show a decrease in range (apparent uplift) that is opposite in 

magnitude of A–B. Therefore, adding A–B and B–C produces 

an interferogram that is free of the clouds observed in scene B. 

Interferograms with severe atmospheric noise were not used in 

the analysis. Atmospheric humidity delays the radar signal in 

a manner similar to atmospheric moisture, but its magnitude 

is smaller. To reduce error from humidity, the same techniques 

that were applied to atmospheric moisture section above were 

applied (European Space Agency, 2007).

Land surface topography is examined as part of the 

error assessment because errors in the DEM used to remove 

topography in the InSAR processing can result in topographic 

artifact fringes in the interferogram. To avoid topographic 

errors, SAR scene combinations with the least orbital 

variations were used. Because the Houston-Galveston region 

is flat, the topography errors were negligible (Schmidt and 

Bürgman, 2003; European Space Agency, 2007).
Appendix 2—Global Positioning System Time-series for All GPS 

Stations with Least-Squares Linear-Regression Lines Depicting 

the Directional Velocity

Appendix 2

Appendix 2 47

UP NORTH UP EAST EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -5.3 

mm/yr 0.6 

mm/yr −0.1 

mm/yr −5.4

mm/yr -1.1

mm/yr 4.1 

B. ANG2–NETP

A. ANG1–NETP

mm/yr, Millimeters per year

Figure 2.1. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, Angleton 1 (ANG1) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, Angleton 2 (ANG2) and Northeast 2250 CORS ARP (NETP) CORS sites. 
48 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−100

−50

0

50

100

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

−100

−50

0

50

100

0

25

50

−50

−25

0

25

50

−50

−25

mm/yr 4.5 

mm/yr 0.5 

mm/yr 1.4 

mm/yr 2.2

mm/yr 0.5

mm/yr 0.7 

B. ANG6–NETP

A. ANG5–NETP

mm/yr, Millimeters per year

Figure 2.2. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, Angleton 5 (ANG5) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, Angleton 6 (ANG6) and Northeast 2250 CORS ARP (NETP) CORS sites. 
Appendix 2 49

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−100

−50

0

50

100

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

−100

−50

0

50

100

0

25

50

−50

−25

0

25

50

−50

−25

mm/yr -11.9

mm/yr 1.1 

mm/yr 1.8 

mm/yr -17.8

mm/yr 3.3

mm/yr -3.2

B. COH1–NETP

A. WLA1–NETP

mm/yr, Millimeters per year

Figure 2.3. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Tomball COOP (WLA1) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, C OF Houston COOP (COH1) and Northeast 2250 CORS ARP (NETP) CORS 

sites.
50 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 1.0

mm/yr 0.2 

mm/yr 1.1

mm/yr 1.2

mm/yr -1.2

mm/yr -4.2 

B. COH3–NETP

A. COH2–NETP

mm/yr, Millimeters per year

Figure 2.4. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Houston 2 COOP (COH2) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, Houston 3 COOP (COH3) and Northeast 2250 CORS ARP (NETP) CORS 

sites.
Appendix 2 51

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -1.5

mm/yr -0.0 

mm/yr 1.9

mm/yr 1.0

mm/yr 0.4

mm/yr 3.8 

B. COH5–NETP

A. COH4–NETP

mm/yr, Millimeters per year

Figure 2.5. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Houston 4 COOP (COH4) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, Houston 5 COOP (COH5) and Northeast 2250 CORS ARP (NETP) CORS 

sites. 
52 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -11.4

mm/yr -0.1 

mm/yr 2.8

mm/yr 1.2

mm/yr 0.7

mm/yr -5.8 

B. COH7–NETP

A. COH6–NETP

mm/yr, Millimeters per year

Figure 2.6. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Houston 6 COOP (COH6) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, Houston 7 COOP (COH7) and Northeast 2250 CORS ARP (NETP) CORS 

sites. 
Appendix 2 53

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -5.8

mm/yr -0.5 

mm/yr 0.9

mm/yr -21.4

mm/yr 32.2

mm/yr -62.7 

B. GAL1–NETP

A. DWI1–NETP

mm/yr, Millimeters per year

Figure 2.7. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Clute COOP (DWI1) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, Galveston 1 (GAL1) and Northeast 2250 CORS ARP (NETP) CORS sites.
54 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -9.8

mm/yr -3.5 

mm/yr 1.6

mm/yr 0.1

mm/yr 0.6

mm/yr -2.4

B. HOUS–NETP

A. GAL2–NETP

mm/yr, Millimeters per year

Figure 2.8. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Galveston 2 (GAL2) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, Houston (HOUS) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 55

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 0.7

mm/yr 0.1 

mm/yr -2.7

mm/yr -1.5

mm/yr 2.5

mm/yr -0.2

B. LKHU–NETP

A. ADKS–NETP

mm/yr, Millimeters per year

Figure 2.9. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, Addicks 1795 CORS ARP (ADKS) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, Lake Houston (LKHU) and Northeast 2250 CORS ARP (NETP) CORS sites.
56 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

−300

−450

 450

−150

0

150

300

0

25

50

−50

−25

0

25

50

−50

−25

−300

−450

 450

−150

0

150

300 mm/yr -34.1

mm/yr -1.5 

mm/yr 1.7

mm/yr -0.8

mm/yr -1.8

mm/yr -42.8

B. PA02–NETP

A. PA01–NETP

mm/yr, Millimeters per year

Figure 2.10. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 01 (PA01) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 02 (PA02) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 57

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−300

−450

 450

−150

0

150

300

−300

−450

 450

−150

150

300

0

mm/yr -19.5

mm/yr -0.8 

mm/yr -3.5

mm/yr 1.7

mm/yr -2.0

mm/yr -38.4

B. PA04–NETP

A. PA03–NETP

mm/yr, Millimeters per year

Figure 2.11. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 03 (PA03) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 04 (PA04) and Northeast 2250 CORS ARP (NETP) CORS sites.
58 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−300

−450

 450

−150

0

150

300

−300

−450

 450

−150

150

300

0

mm/yr -30.9

mm/yr -1.9 

mm/yr 1.2

mm/yr 1.3

mm/yr 0.0

mm/yr -25.7

B. PA06–NETP

A. PA05–NETP

mm/yr, Millimeters per year

Figure 2.12. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 05 (PA05) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 06 (PA06) and Northeast 2250 CORS ARP (NETP) CORS sites. 
Appendix 2 59

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

−75

0

25

75

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−300

−450

 450

−150

0

150

300

−300

−450

 450

−150

150

300

0

mm/yr -29.1

mm/yr -2.4

mm/yr 3.4

mm/yr 2.7

mm/yr 1.2

mm/yr -41.6

B. PA08–NETP

A. PA07–NETP

mm/yr, Millimeters per year

Figure 2.13. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 07 (PA07) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 08 (PA08) and Northeast 2250 CORS ARP (NETP) CORS sites.
60 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -3.1

mm/yr 0.1

mm/yr 2.5

mm/yr -0.6

mm/yr 0.2

mm/yr -3.1

B. PA09–NETP

A. PA00–NETP

mm/yr, Millimeters per year

Figure 2.14. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 00 (PA00) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 09 (PA09) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 61

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -5.2

mm/yr 0.1

mm/yr 1.4

mm/yr 4.1

mm/yr 2.2

mm/yr -4.9

B. PA11–NETP

A. PA10–NETP

mm/yr, Millimeters per year

Figure 2.15. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 10 (PA10) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 11 (PA11) and Northeast 2250 CORS ARP (NETP) CORS sites.
62 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−300

−450

 450

−150

0

150

300

−300

−450

 450

−150

150

300

0

mm/yr -18.3

mm/yr 0.4

mm/yr 0.1

mm/yr 2.3

mm/yr 1.1

mm/yr -9.4

B. PA13–NETP

A. PA12–NETP

mm/yr, Millimeters per year

Figure 2.16. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 12 (PA12) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 13 (PA13) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 63

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−100

−50

0

50

100

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

−100

−50

0

50

100

0

25

50

−50

−25

0

25

50

−50

−25

mm/yr -10.7

mm/yr -0.5

mm/yr 1.4

mm/yr 4.3

mm/yr -3.2

mm/yr -7.1

B. PA15–NETP

A. PA14–NETP

mm/yr, Millimeters per year

Figure 2.17. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 14 (PA14) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 15 (PA15) and Northeast 2250 CORS ARP (NETP) CORS sites.
64 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−300

−450

 450

−150

0

150

300

−300

−450

 450

−150

150

300

0

mm/yr -24.7

mm/yr 0.1

mm/yr 1.5

mm/yr 0.2

mm/yr -0.6

mm/yr -20.2

B. PA18–NETP

A. PA17–NETP

mm/yr, Millimeters per year

Figure 2.18. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 17 (PA17) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 18 (PA18) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 65

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -10.6

mm/yr 0.1

mm/yr 0.7

mm/yr 2.7

mm/yr -8.7

mm/yr -10.3

B. PA19–NETP

A. PA16–NETP

mm/yr, Millimeters per year

Figure 2.19. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 16 (PA16) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 19 (PA19) and Northeast 2250 CORS ARP (NETP) CORS sites.
66 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -5.5

mm/yr -1.8

mm/yr 2.5

mm/yr -0.1

mm/yr 1.7

mm/yr -0.3

B. PA21–NETP

A. PA20–NETP

mm/yr, Millimeters per year

Figure 2.20. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 20 (PA20) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 21 (PA21) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 67

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -1.2

mm/yr 0.1

mm/yr -2.7

mm/yr 2.4

mm/yr -2.0

mm/yr -5.1

B. PA23–NETP

A. PA22–NETP

mm/yr, Millimeters per year

Figure 2.21. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 22 (PA22) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 23 (PA23) and Northeast 2250 CORS ARP (NETP) CORS sites.
68 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -2.2

mm/yr -0.1

mm/yr 1.7

mm/yr 1.5

mm/yr 0.2

mm/yr 0.7

B. PA26–NETP

A. PA24–NETP

mm/yr, Millimeters per year

Figure 2.22. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 24 (PA24) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 26 (PA26) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 69

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 0.5

mm/yr 1.2

mm/yr -1.6

mm/yr -0.3

mm/yr 3.1

mm/yr -6.3

B. PA28–NETP

A. PA27–NETP

mm/yr, Millimeters per year

Figure 2.23. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 27 (PA27) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 28 (PA28) and Northeast 2250 CORS ARP (NETP) CORS sites.
70 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -6.8

mm/yr -2.5

mm/yr 3.9

mm/yr 2.1

mm/yr -4.8

mm/yr -13.4

B. PA30–NETP

A. PA29–NETP

mm/yr, Millimeters per year

Figure 2.24. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 29 (PA29) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 30 (PA30) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 71

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 6.2

mm/yr -1.3

mm/yr 0.9

mm/yr 8.8

mm/yr -23.6

mm/yr 1.1

B. PA32–NETP

A. PA31–NETP

mm/yr, Millimeters per year

Figure 2.25. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 31 (PA31) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 32 (PA32) and Northeast 2250 CORS ARP (NETP) CORS sites.
72 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 5.9

mm/yr 1.4

mm/yr 1.7

mm/yr -2.0

mm/yr 2.1

mm/yr 7.6

B. PA35–NETP

A. PA33–NETP

mm/yr, Millimeters per year

Figure 2.26. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 33 (PA33) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 35 (PA35) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 73

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -2.2

mm/yr -8.7

mm/yr 3.3

mm/yr 1.2

mm/yr 4.8

mm/yr -0.4

B. PA37–NETP

A. PA36–NETP

mm/yr, Millimeters per year

Figure 2.27. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 36 (PA36) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 37 (PA37) and Northeast 2250 CORS ARP (NETP) CORS sites.
74 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -7.9

mm/yr 0.2

mm/yr -5.3

mm/yr 34.9

mm/yr -2.4

mm/yr 0.4

B. PA40–NETP

A. PA38–NETP

mm/yr, Millimeters per year

Figure 2.28. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 38 (PA38) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 40 (PA40) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 75

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -8.5

mm/yr -11.4

mm/yr -4.5

mm/yr 12.0

mm/yr 2.9

mm/yr -10.2

B. PA42–NETP

A. PA41–NETP

mm/yr, Millimeters per year

Figure 2.29. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 41 (PA41) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 42 (PA42) and Northeast 2250 CORS ARP (NETP) CORS sites.
76 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -12.5

mm/yr -4.5

mm/yr 7.8

mm/yr 0.3

mm/yr 3.5

mm/yr -0.3

B. PA44–NETP

A. PA43–NETP

mm/yr, Millimeters per year

Figure 2.30. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 43 (PA43) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 44 (PA44) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 77

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -22.6

mm/yr -0.1

mm/yr 0.5

mm/yr 2.0

mm/yr -1.7

mm/yr 5.8

B. PA46–NETP

A. PA45–NETP

mm/yr, Millimeters per year

Figure 2.31. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 45 (PA45) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 46 (PA46) and Northeast 2250 CORS ARP (NETP) CORS sites.
78 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -11.5

mm/yr 1.8

mm/yr -1.6

mm/yr 2.8

mm/yr -2.8

mm/yr -12.9

B. PA48–NETP

A. PA47–NETP

mm/yr, Millimeters per year

Figure 2.32. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 47 (PA47) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 48 (PA48) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 79

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 1.4

mm/yr -2.1

mm/yr 1.1

mm/yr 2.2

mm/yr 2.6

mm/yr -1.1

B. PA50–NETP

A. PA49–NETP

mm/yr, Millimeters per year

Figure 2.33. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 49 (PA49) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 50 (PA40) and Northeast 2250 CORS ARP (NETP) CORS sites.
80 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 8.5

mm/yr -0.2

mm/yr 2.3

mm/yr 4.1

mm/yr 0.2

mm/yr 14.2

B. PA52–NETP

A. PA51–NETP

mm/yr, Millimeters per year

Figure 2.34. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 51 (PA51) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 52 (PA52) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 81

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr 2.2

mm/yr -2.3

mm/yr 2.3

mm/yr 3.9

mm/yr 0.5

mm/yr -0.2

B. PA54–NETP

A. PA53–NETP

mm/yr, Millimeters per year

Figure 2.35. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 53 (PA53) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 54 (PA54) and Northeast 2250 CORS ARP (NETP) CORS sites.
82 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -7.0

mm/yr 4.5

mm/yr 0.4

mm/yr 1.0

mm/yr 3.3

mm/yr 6.5

B. PA56–NETP

A. PA55–NETP

mm/yr, Millimeters per year

Figure 2.36. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, PAM 55 (PA55) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, PAM 56 (PA56) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 83

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -1.5

mm/yr -0.2

mm/yr 0.9

mm/yr 3.7

mm/yr -0.1

mm/yr -18.0

B. TMCC–NETP

A. ROD1–NETP

mm/yr, Millimeters per year

Figure 2.37. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Spring (ROD1) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, TMCC and Northeast 2250 CORS ARP (NETP) CORS sites.
84 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -1.8

mm/yr -0.1

mm/yr 3.3

mm/yr 0.9

mm/yr 1.6

mm/yr 2.3

B. TXAG–NETP

A. TSI1–NETP

mm/yr, Millimeters per year

Figure 2.38. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Deerpark COOP (TSI1) and Northeast 2250 CORS ARP (NETP) 

Continuously Operating Reference Station (CORS) sites and B, Angleton (TXAG) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 85

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -11.7

mm/yr 0.7

mm/yr 1.3

mm/yr 0.1

mm/yr 2.2

mm/yr -5.1

B. TXCN–NETP

A. TXGA–NETP

mm/yr, Millimeters per year

Figure 2.39. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, Galveston (TXGA) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, Conroe (TXCN) and Northeast 2250 CORS ARP (NETP) CORS sites.
86 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -3.4

mm/yr -0.4

mm/yr 0.7

mm/yr 1.0

mm/yr 2.3

mm/yr -4.1

B. TXHU–NETP

A. TXHE–NETP

mm/yr, Millimeters per year

Figure 2.40. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between A, Hempstead (TXHE) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, Houston RRP2 (TXHU) and Northeast 2250 CORS ARP (NETP) CORS sites.
Appendix 2 87

UP EAST NORTH UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

−50

−25

0

25

50

−50

−25

0

25

50

Movement, in millimeters Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

−100

−50

0

50

100

mm/yr -5.5

mm/yr 0.5

mm/yr 0.4

mm/yr 0.7

mm/yr 1.7

mm/yr -2.9

B. TXLM–NETP

A. TXLI–NETP

mm/yr, Millimeters per year

Figure 2.41. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting 

the directional movement in millimeters per year (mm/yr) between A, Liberty (TXLI) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites and B, LaMarque (TXLM) and Northeast 2250 CORS ARP (NETP) CORS sites.
88 Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR UP EAST NORTH

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

1993 1994 1996 1998 2000 2002 2004 2006 2008 2010

Movement, in millimeters

0

25

50

−50

−25

0

25

50

−50

−25

−100

−50

0

50

100

mm/yr 0.6

mm/yr 2.8

mm/yr -6.7

TXRO–NETP

mm/yr, Millimeters per year

Figure 2.42. Global Positioning System (GPS) time-series of relative movement with least-squares linear-regression lines depicting the 

directional movement in millimeters per year (mm/yr) between Rosenberg (TXRO) and Northeast 2250 CORS ARP (NETP) Continuously 

Operating Reference Station (CORS) sites.

Publishing support provided by

Lafayette Publishing Service Center

Information regarding water resources in Texas is available at 

http://tx.usgs.gov/

Bawden and others—Investigation of Land Subsidence in the Houston-Galveston Region of Texas By Using GPS and InSAR—SIR 2012–5211