DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-12

DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS:

PROCEDURES, DATA SOURCES AND ANALYSIS 

L.A.Taylor

B.W. Eakins

K.S. Carignan

R.R. Warnken

T. Sazonova

D.C. Schoolcraft

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

January 2008


DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

UNITED STATES

DEPARTMENT OF COMMERCE

Carlos M. Gutierrez

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

VADM Conrad C. Lautenbacher, Jr.

Under Secretary for Oceans

and Atmosphere/Administrator

National Environmental Satellite,

Data, and Information Service

Mary E. Kicza

Assistant Administrator

NOAA Technical Memorandum NESDIS NGDC-12

DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS:

PROCEDURES, DATA SOURCES AND ANALYSIS

Lisa .A. Taylor1

Barry .W. Eakins2

Kelly .S. Carignan2

Robin .R. Warnken1

Tatiana Sazonova2

David C. Schoolcraft1

1NOAA, National Geophysical Data Center, Boulder, Colorado

2

Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder 

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

January 2008
ii

Taylor et al., 2008

NOTICE

Menton of a commercal company or product does not consttute an endorsement by the 

NOAA Natonal Envronmental Satellte, Data, and Informaton Servce. Use of nformaton 

from ths publcaton concernng propretary products or the test of such products for publcty

or advertsng purposes s not authorzed. 

Corresponding author contact:

Lsa A. Taylor

NOAA, Natonal Geophyscal Data Center

Marne Geology and Geophyscs Dvson

35 Broadway, E/GC 3

Boulder, Colorado 80305

Phone: 303-497-6767

Fax: 303-497-653

E-mal: Lsa.A.Taylor@noaa.gov

http://www.ngdc.noaa.gov/mgg/nundaton/

Also avalable from the Natonal Techncal Informaton Servce (NTIS)

(http://www.nts.gov)
iii

DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

Contents

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

2. Study Area..........................................................................................................................................2

3. Methodology ......................................................................................................................................3

3.1 Data Sources and Processing ......................................................................................................3

3.1.1 Shoreline................................................................................................................5

3.1.2 Bathymetry ............................................................................................................6

3.1.3 Topography..........................................................................................................12

3.2 Establishing Common Datums .................................................................................................14

3.2.1 Vertical datum transformations ...........................................................................14

3.2.2 Horizontal datum transformations.......................................................................15

3.3 Digital Elevation Model Development .....................................................................................15

3.3.1 Verifying consistency between datasets..............................................................15

3.3.2	 Interpolating between USACE hydrographic profiles.........................................15

3.3.3 Smoothing of bathymetric data ...........................................................................15

3.3.4 Gridding the data with MB-System.....................................................................16

3.4 Quality Assessment of the DEM...............................................................................................17

3.4.1 Horizontal accuracy.............................................................................................17

3.4.2 Vertical accuracy .................................................................................................17

3.4.3 Slope maps and 3-D perspectives........................................................................17

3.4.4	 Comparison with source data files.......................................................................18

3.4.5 Comparison with NGS geodetic monuments......................................................18

4. Summary and Conclusions...............................................................................................................20

5. Acknowledgments............................................................................................................................20

6. References........................................................................................................................................20

7. Data Processing Software ................................................................................................................20

List of Figures

Figure 1. Color image of the Galveston, Texas region...............................................................................1

Figure 2. Galveston Bay and Galveston Island ..........................................................................................2

Figure 3. Source and coverage of datasets used to compile the Galveston DEM......................................4

Figure 4. TGLO coastline used in building the Galveston DEM...............................................................5

Figure 5. Spatial coverage of bathymetric datasets used to compile the Galveston DEM.........................6

Figure 6. Digital NOS hydrographic survey coverage in the Galveston region.........................................9

Figure 7. Digital USACE hydrographic survey coverage within the Galveston region ..........................11

Figure 8. Source and coverage of topographic datasets used to compile the Galveston DEM................12

Figure 9. Texas Water Development Board topographic LiDAR data sets used to compile

 the Galveston DEM ...........................................................................................................13

Figure 10. Histogram of the difference between NOS hydrographic survey H10584 and the 

 1 arc-second pre-surfaced bathymetric grid ......................................................................16

Figure 11. Slope map of the Galveston DEM ............................................................................................17

Figure 12. Perspective view northeast along Galveston Island, as modeled in the Galveston DEM.........18

Figure 13. Histogram of the difference between one USACE survey (21,728 points) and the 

Galveston DEM. ................................................................................................................18

Figure 14. Histogram of the differences between NGS geodetic monument elevations and the 

Galveston DEM .................................................................................................................19

Figure 15. Location of NGS monuments and NOAA tide stations in the Galveston region......................19

List of Tables

Table 1.	 PMEL specifications for the Galveston, Texas DEM .................................................................3

Table 2. Shoreline datasets used in compiling the Galveston DEM.........................................................5
iv

Taylor et al., 2008

Table 3. Bathymetric datasets used in compiling the Galveston DEM ....................................................6

Table 4. Digital NOS hydrographic surveys used in compiling the Galveston DEM ..............................7

Table 5. USACE hydrographic surveys used in compiling the Galveston DEM ...................................10

Table 6. Topographic datasets used in compiling the Galveston DEM..................................................12

Table 7. Relationship between Mean High Water and other vertical datums in the Galveston

 region ................................................................................................................................15

Table 8. Data hierarchy used to assign gridding weight in MB-System.................................................16


DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

Digital Elevation Model of Galveston, Texas:

Procedures, Data Sources and Analysis

1. Introduction

In May 2007, the National Geophysical Data Center (NGDC), an office of the National Oceanic and Atmospheric

Administration (NOAA), developed a bathymetric–topographic digital elevation model (DEM) of Galveston, Texas 

(Fig. 1) for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami Research (http://nctr.

pmel.noaa.gov/). The 1/3 arc-second coastal DEM will be used as input for the Method of Splitting Tsunami (MOST) 

model developed by PMEL to simulate tsunami generation, propagation and inundation. The DEM was generated 

from diverse digital datasets in the region (grid boundary and sources shown in Fig. 3) and will be used for tsunami

inundation modeling, as part of the tsunami forecast system SIFT (Short-term Inundation Forecasting for Tsunamis) 

currently being developed by PMEL for the NOAA Tsunami Warning Centers. This report provides a summary of the 

data sources and methodology used in developing the Galveston DEM. 

. The Galveston DEM is built upon a grid of cells that are square in geographic coordinates (latitude and longitude), however, the cells are not 

square when converted to projected coordinate systems, such as UTM zones (in meters). At the latitude of Galveston, Texas (29°18′ N, 94°48′ W) 

1/3 arc-second of latitude is equivalent to 10.26 meters; 1/3 arc-second of longitude equals 9.01 meters.

Figure 1. Color image of the Galveston, Texas region. Coastline in black.


Taylor et al., 2008

2. Study Area

The Galveston DEM covers the coastal region centered on the city of Galveston, Texas, and includes the 

communities of Galveston, Texas City, Dickinson, La Porte, Baytown and Gilchrist (Fig. 1). The Galveston Bay 

estuarine system and its tributaries (Fig. 2) consist of six sub-bays: Christmas Bay, West Bay, Lower Galveston Bay, 

Upper Galveston Bay, East Bay, and Trinity Bay (Fig. 1). Galveston Bay covers approximately 600 square miles 

(1,500 km2), and is 30 miles (50 km) long and 17 miles (27 km) wide. The bay is on average 7-9 feet (3 m) deep, 

and supports a wide variety of uses, including industrial processing (such as oil and gas extraction and petrochemical 

operations), shipping, fisheries, recreation, and tourism. These activities have a direct affect on the shorelines of the

bay and its tributaries. Development along the shoreline often creates problems through disturbance or destruction of 

habitats, modification of flood plains, worsening pollution, increasing erosion, and introduction of litter. The Houston 

Ship Channel, connecting the Port of Houston to the Gulf, passes through Galveston Bay, and is regularly dredged by 

the U.S. Army Corps of Engineers. The bay provides nursery and spawning grounds for large amounts of marine life, 

and is important for both commercial and recreational fishing. (http://gbic.tamug.edu/gbeppubs/baybriefings06/GI348_Shoreline_Mgmt.pdf)

The smoothly curved sides of Trinity and Galveston Bays have been sculpted by the scouring action of successive 

hurricane storm surges and runoff events. In contrast, the leading edge of the barrier islands is comparatively straight. 

The Bolivar Peninsula to the east of the inlet shows a tendency to curve where the silt and sand have been trapped by 

a restraining jetty. Galveston Island exhibits a pencil-straight coastal margin—its Gulf coast reinforced with a 17-foot 

high seawall constructed after the devastating hurricane of 1900. Most of the beach along the Galveston seawall was 

lost by wave attack during the storm surge of Hurricane Carla in 1961. (http://www.lpi.usra.edu/publications/slidesets/

oceans/oceanviews/slide_31.html)

Figure 2. Galveston Bay, right (http://gulfsci.usgs.gov/galveston/maps.html) and Galveston Island, 

left, looking southwest (http://www.beg.utexas.edu/news-events/graphics5/coastal0905_pop.jpg).
DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

3. Methodology

The Galveston DEM was developed to meet PMEL specifications (Table 1), based on input requirements for the

MOST inundation model. The best available digital data were obtained by NGDC and shifted to common horizontal 

and vertical datums: World Geodetic System 1984 (WGS84) and Mean High Water (MHW), for modeling of “worstcase scenario” flooding, respectively. Data processing and evaluation, and DEM assembly and assessment are described

in the following subsections.

Table 1: PMEL specifications for the Galveston, Texas DEM.

Grid Area Galveston, Texas

Coverage Area 94.3o to 95.25o W; 28.85o to 29.8o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System 1984 (WGS84)

Vertical Datum Mean High Water (MHW)

Vertical Units Meters

Grid Spacing 1/3 arc-second

Grid Format ESRI Arc ASCII grid

3.1 Data Sources and Processing

Shoreline, bathymetric, and topographic digital datasets (Fig. 3) were obtained from several U.S. federal, state 

and local agencies, including: NOAA’s National Ocean Service (NOS); the U.S. Geological Survey (USGS); the 

Texas Water Development Board; Harris County, Texas; and the Texas General Land Office (TGLO). Safe Software’s

(http://www.safe.com/) FME data translation tool package was used to shift datasets to WGS84 horizontal datum and 

to convert into ESRI (http://www.esri.com/) ArcGIS shape files. The shape files were then displayed with ArcGIS to

assess data quality and manually edit datasets; NGDC’s GEODAS software (http://www.ngdc.noaa.gov/mgg/geodas/) 

was used to manually edit large xyz datasets. Vertical datum transformations to MHW were accomplished using FME, 

based upon data from the NOAA Galveston Pier tide station, and offset grids (digital surfaces with values representing 

interpolated differences between various tidal datums and MHW) provided by PMEL. VDatum model software (http://

nauticalcharts.noaa.gov/csdl/vdatum.htm) was not available for this area. Applied Imagery’s Quick Terrain Modeler 

software (http://www.appliedimagery.com/) was used to edit and assess the quality of the LiDAR data.
Taylor et al., 2008

Figure 3. Source and coverage of datasets used to compile the Galveston DEM.
DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

3.1.1 Shoreline

A high-resolution digital coastline of the Galveston region was obtained from the Texas General Land Office

(TGLO; Table 2). Other digital coastlines form the National Geospatial Intelligence Agency (NGA) and NOAA’s 

available Electronic Nautical Charts were not used as they were of lower resolution than the TGLO coastline. 

Table 2: Shoreline datasets used in compiling the Galveston DEM.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

TGLO 1995 digital 1:24,000 or smaller NAD27 unknown

http://www.glo.

state.tx.us/gisdata/

gisdata.html

1) Texas General Land Office shoreline

The Texas General Land Office shoreline is a compilation of digital coastline segments from U.S.

Geological Survey and U.S. Fish and Wildlife Service National Wetlands Inventory digital line graph files

and from digitized USGS maps. The shoreline contains hydrographic features of the coastal counties of 

Texas, including streams, bayous, canals, ditches, lakes, reservoirs, marshes, tidal flats, bays, and estuaries.

The shoreline was created in 1995 with a horizontal datum of NAD27. The data were extracted and digitized 

by personnel from the Texas General Land Office, Jefferson County Appraisal District and other entities.

NGDC modified the TGLO coastline to remove manmade structures, such as piers, and small inland

streams, canals, and water bodies (Fig. 4). The coastline was also adjusted to be consistent with recent NOS 

and USACE bathymetric surveys. The TGLO coastline was used only for pre-smoothing of bathymetric data 

(see Section 3.3.3), and not as a dataset used for creating the final Galveston DEM.

Figure 4. TGLO coastline (red) used in building the Galveston DEM. Small streams, canals, and 

water bodies (blue lines) in the original dataset were deleted. DEM area in purple.
Taylor et al., 2008

3.1.2 Bathymetry

Bathymetric datasets used in the compilation of the Galveston DEM (Fig. 5) include 76 NOS hydrographic 

surveys and 24 USACE surveys of dredged shipping channels (Table 3). 

Table 3: Bathymetric datasets used in compiling the Galveston DEM.

Source Year Data Type Spatial Resolution

Original 

Horizontal Datum/

Coordinate System

Original 

Vertical Datum URL

USACE, 

Galveston 

District

1996 

to 

2006

Hydrographic 

survey 

soundings

Profiles spaced 10 m 

to 300 m apart.

Point spacing along 

profiles <1 m.

NAD27 State 

Plane Texas South 

or South Central

Mean Low 

Tide

http://www.swg.usace.army.mil/ and 

http://beams.swg.usace.army.mil/

 NOS 

1897 

to 

2002

Hydrographic 

survey 

soundings

Ranges from 10 

m to 1 km (varies 

with scale of survey, 

depth, traffic, and

probability of 

obstructions)

NAD27 or NAD83

Mean Low 

Water or Mean 

Lower Low 

Water

http://www.ngdc.noaa.gov/mgg/

bathymetry/hydro.html

Figure 5. Spatial coverage of bathymetric datasets used to compile the Galveston DEM.
DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

1) NOS hydrographic survey data

A total of 76 NOS hydrographic surveys conducted between 1931 and 2002 were utilized in developing 

the Galveston DEM (Table 4; Fig. 6). The hydrographic survey data were originally vertically referenced to 

Mean Lower Low Water (MLLW) or Mean Low Water (MLW) and horizontally referenced to either NAD27 

or NAD83 datums.

Data point spacing for the NOS surveys varied by collection date. In general, earlier surveys had greater 

point spacing than more recent surveys. All surveys were extracted from NGDC’s online NOS hydrographic 

database (http://www.ngdc.noaa.gov/mgg/bathymetry/hydro.html) in their original, digitized datums (Table 

4). The data were then converted to WGS84 and MHW using FME software, an integrated collection of 

spatial extract, transform, and load tools for data transformation (http://www.safe.com). The surveys were 

subsequently clipped to a polygon 0.05 degrees (~5%) larger than the Galveston DEM area to support data 

interpolation along grid edges. 

After converting all NOS survey data to MHW (see Section 3.2.1), the data were displayed in ESRI 

ArcMap and reviewed for digitizing errors against scanned original survey smooth sheets and compared to 

the topographic LiDAR and NED data, the TGLO coastline, RNCs, and Google Earth satellite imagery. The 

surveys were also clipped to remove soundings that overlap the more recent USACE surveys of dredged 

shipping channels, and where soundings from older surveys have been superceded by more recent NOS 

surveys.

Table 4: Digital NOS hydrographic surveys used in compiling the Galveston DEM.

NOS Survey ID Year of Survey Survey Scale Original Vertical Datum Original Horizontal Datum

H05121 1931 5,000 mean low water NAD27

H05122 1931 5,000 mean low water NAD27

H05123 1931 5,000 mean low water NAD27

H05124 1931 5,000 mean low water NAD27

H05125 1931 5,000 mean low water NAD27

H05126 1931 5,000 mean low water NAD27

H05127 1931 5,000 mean low water NAD27

H05128 1931 5,000 mean low water NAD27

H05398 1933 10,000 mean low water NAD27

H05399 1933 20,000 mean low water NAD27

H05424 1933/34 10,000 mean low water NAD27

H05488 1933/34 10,000 mean low water NAD27

H05489 1934 20,000 mean low water NAD27

H05511 1933 20,000 mean low water NAD27

H05521 1934 20,000 mean low water NAD27

H06251 1937 40,000 mean low water NAD27

H06252 1937 40,000 mean low water NAD27

H06253 1937 40,000 mean low water NAD27

H06398 1938 40,000/20,000 mean low water NAD27

H08693 1962 10,000 mean low water NAD27

H08694 1962 10,000 mean low water NAD27

H08695 1962 12,500 mean low water NAD27

H08740 1963/65 20,000 mean low water NAD27

H08741 1963/65 10,000 mean low water NAD27

H08742 1962/63 10,000 mean low water NAD27

H08743 1963/65 20,000 mean low water NAD27

H08745 1965 20,000 mean low water NAD27

H08746 1962/65 5,000 mean low water NAD27

H08747 1965 10,000 mean low water NAD27

H08748 1962/65 10,000 mean low water NAD27
Taylor et al., 2008

H08749 1965 5,000 mean low water NAD27

H08750 1966 10,000 mean low water NAD27

H08751 1962/63 20,000 mean low water NAD27

H08752 1963/65 20,000 mean low water NAD27

H08795 1964 40,000 mean low water NAD27

H08837 1965 20,000 mean low water NAD27

H08873 1966 20,000 mean low water NAD27

H08876 1966 20,000 mean low water NAD27

H09765 1978 20,000 mean low water NAD27

H09769 1978 20,000 mean low water NAD27

H09774 1978 20,000 mean low water NAD27

H09775 1978 40,000 mean low water NAD27

H09784 1978 40,000 mean low water NAD27

H09843 1979 20,000 mean low water NAD27

H09851 1979 40,000 mean low water NAD27

H09885 1980 40,000 mean low water NAD27

H10011 1982 20,000 mean low water NAD27

H10014 1982 20,000 mean low water NAD27

H10021 1982 20,000 mean low water NAD27

H10111 1983 20,000 mean lower low water NAD27

H10119 1983/84 10,000 mean lower low water NAD27

F00403 1994 20,000 mean lower low water NAD83

F00418 1995 10,000 mean lower low water NAD83

H10574 1994/95 20,000 mean lower low water NAD83

H10584 1994/95 10,000 mean lower low water NAD83

H10585 1994/95 10,000 mean lower low water NAD83

H10586 1994/96 10,000 mean lower low water NAD83

H10588 1995/96 10,000 mean lower low water NAD83

H10589 1995/96 10,000 mean lower low water NAD83

H10614 1995 10,000 mean lower low water NAD83

H10619 1995 10,000 mean lower low water NAD83

H10638 1995 10,000 mean lower low water NAD83

H10660 1995 10,000 mean lower low water NAD83

H10661 1195/96 10,000 mean lower low water NAD83

H10663 1996 10,000 mean lower low water NAD83

H10664 1996 10,000 mean lower low water NAD83

H10666 1996 10,000 mean lower low water NAD83

H10805 1998 20,000 mean lower low water NAD83

H10835 1998/99 20,000 mean lower low water NAD83

H10850 1999 20,000 mean lower low water NAD83

H10873 1999/2000 20,000 mean lower low water NAD83

H10875 1999 20,000 mean lower low water NAD83

H10876 1999/2000 20,000 mean lower low water NAD83

H10915 1999/2000 20,000 mean lower low water NAD83

H10943 1999/2000 20,000 mean lower low water NAD83

H11061 2001/02 40,000 mean lower low water NAD83
DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

Figure 6. Digital NOS hydrographic survey coverage in the Galveston region. Some older surveys were not utilized as they 

have been entirely superceded by more recent surveys. DEM boundary in red.
10

Taylor et al., 2008

2) U.S. Army Corps of Engineers surveys of dredged channels

The USACE, Galveston District provided NGDC with recent bathymetric surveys spanning the 

Texas Gulf Coast, including the Gulf Intracoastal Waterway and dredged shipping channels (Table 5, 

Fig. 7). The surveys were collected from 1996 to 2006, and were referenced to Mean Low Tide (MLT) 

vertical datum, which was assumed to be equivalent to Mean Low Water (MLW). Some files contained

zero latitude and longitude position for individual soundings, which were deleted during conversion to 

WGS84 using FME. 

Table 5: USACE hydrographic surveys used in compiling the Galveston DEM.

Region

Original horizontal 

datum

Original vertical 

datum Spatial Resolution

Anahuac 

Channel

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~65 m long, spaced 60 m to 300 m apart, with<1 m point spacing

Atkinson Island NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~80 m long, spaced ~20 m to 125 m apart, with

<1 m point spacing

Barbour ship 

Channel

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~125 m to 625 m long, spaced 70 m apart,with <1 m point spacing

Bayport Ship 

Channel

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~160 m to 1300 m long, spaced ~50 m to 140m apart, with <1 m point spacing

Brady Island NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~50 m long, spaced ~20 m to 85 m apart, with<1 m point spacing

Cedar Bayou NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~85 m long, spaced 185 m apart, with <1 mpoint spacing

Chocolate 

Bayou

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~60 m long, spaced ~100 m apart, with <1 mpoint spacing

Clear Creek NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~25 m to 55 m long, spaced ~ 50 m to 125 m

apart, with <1 m point spacing

Coast Guard 

Basins

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~25 m to 100 m long, spaced ~10 m apart, with<1 m point spacing

Dickinson 

Bayou

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~75 m long, spaced ~50 m to 125 m apart, with<1 m point spacing

Double Bayou NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~100 m long, spaced ~75 m to 125 m apart,

with <1 m point spacing

Five Mile Cut NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~60 m long, spaced ~60 m to 120 m apart, with

<1 m point spacing

Freeport Harbor NAD83 State Plane 

Texas South Central Mean Low Tide Profiles ~170 m to 300 m long, spaced ~25 m to 125 m

apart, with <1 m point spacing

Galveston 

Harbor

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~450 m to 950 m long, spaced ~50 m to 125 mapart, with <1 m point spacing

GIWW NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~75 m to 1800 m long, spaced ~50 m to 300 m

apart, with <1 m point spacing

Greens Bayou NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~50 m to 250 m long, spaced ~25 m to 125 m

apart, with <1 m point spacing

Houston Ship 

Channel

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~75 m to 250 m long, spaced ~50 m to 125 mapart, with <1 m point spacing

Liberty NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~75 m long, spaced ~50 m to 125 m apart, with

<1 m point spacing

Nasa NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~50 m long, spaced ~75 m apart, with <1 mpoint spacing

Offatts Bayou NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~75 m to 100 m long, spaced ~50 m to 100 m

apart, with <1 m point spacing

Port Bolivar NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~40 m to 300 m long, spaced ~25 m to 65 m

apart, with <1 m point spacing

Sims Bayou NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~40 m to 90 m long, spaced ~10 m to 30 m

apart, with <1 m point spacing

Texas City 

Harbor

NAD83 State Plane 

Texas South Central Mean Low Tide Profiles ~50 m to 300 m long, spaced ~10 m to 100 m

apart, with <1 m point spacing

Trinity River NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~10 m to 100 m long, spaced ~50 m to 75 m

apart, with <1 m point spacing
11

DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

Figure 7. Digital USACE hydrographic survey coverage within the Galveston region. DEM boundary in red.
12

Taylor et al., 2008

3.1.3 Topography

Topographic datasets in the Galveston region were obtained from the Texas Water Development Board, Harris 

County, Texas, and the U.S. Geological Survey (Table 6; Fig. 8).

Table 6: Topographic datasets used in compiling the Galveston DEM.

Source Year Data Type Spatial 

Resolution

Original Horizontal 

Datum/Coordinate System

Original 

Vertical Datum URL

Texas Water 

Development 

Board

2006 Bare-earth 

LiDAR

average <

5 m

NAD83 UTM zone 14 

North NAVD88 (feet) http://www.twdb.state.

tx.us/home/index.asp

Harris County 

Flood Control 

District 

2002 Bare-earth

DEM 15 ft NAD83 Texas State Plane, South Central NAVD88 (feet) http://www.tsarp.org

USGS 2001 NED DEM 1/3 arcsecond NAD83 geographic NAVD88

(meters) http://ned.usgs.gov/

NGDC 2007 digitized jetty ~10 m WGS84 geographic MHW

Figure 8. Source and coverage of topographic datasets used to compile the Galveston DEM.
13

DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

1) Texas Water Development Board topographic LiDAR

The Texas Water Development Board (TWDB) provided NGDC with topographic LiDAR datasets for 

several coastal counties of the state of Texas, including Galveston, Brazoria, Chambers, and Jefferson (Fig.

9). The LiDAR data had been processed to bare earth and supplied in tiles, with each data tile covering 

approximately 3 km2. Data are in NAD83 UTM Zone 14, and NAVD88 (feet).

The LiDAR data files contained position, elevation and intensity values for both land and water areas.

Examination of the data indicated that values less than 1 foot in elevation and with intensity values less than 

1 represented water-surface returns. FME was used in initial processing to remove all those elevation values 

less than 1 ft and intensity values less than 1. The LiDAR files were then evaluated and edited using QT

Modeler—specifically removing values remaining over water, as well as from piers and occasional points

with anomalously high elevation. A final total of 1,121,029,758 TWDB LiDAR points from the four counties

were used in building the Galveston DEM.

Figure 9. Texas Water Development Board topographic LiDAR data sets used to

compile the Galveston DEM, separated by county. 
14

Taylor et al., 2008

2) Harris County Flood Control District topographic LiDAR DEM

Bare-earth topographic LiDAR data of Harris County, Texas were collected in October 2001 with an 

Airborne LiDAR Topographic Mapping System (ALTMS). The data were obtained in DEM format with a 

grid cell spacing of 15 feet, in NAD83 State Plane Texas South Central and NAVD88. The LiDAR data were 

acquired for use in the Tropical Storm Allison Recovery Project (http://www.tsarp.org/).

3) USGS NED topographic DEM

The U.S. Geological Survey (USGS) National Elevation Dataset (NED; http://ned.usgs.gov/) provided 

complete 1/3 arc-second coverage of the Galveston region. Data are in NAD83 geographic coordinates and 

NGVD88 vertical datum (meters), and are available for download as raster DEMs. The extracted bare-earth 

elevations have a vertical accuracy of +/- 7 to 15 meters depending on source data resolution. See the USGS 

Seamless web site for specific source information (http://seamless.usgs.gov/). The dataset was derived from 

USGS quadrangle maps and aerial photographs based on topographic surveys; it has been revised using data 

collected in 1999 and 2004.

The NED DEM included “zero” elevation values over the open ocean, which were removed from the 

dataset by clipping to the TGLO coastline.

4) NGDC-digitized Galveston south jetty

The jetty at the entrance to Galveston Bay is only partly represented in the available topographic 

datasets. The TWDB LiDAR data for Galveston County includes the northern jetty, but not the southern one. 

The southern jetty is represented in the NASA Space Shuttle Radar Topography Mission, though it is both 

mislocated relative to the most recent NOS hydrographic surveys and has elevations that range from -15 to 

+9 meters; the northern jetty is consistently about 0.5 meters above MHW. NGDC chose to hand digitize this 

feature as a collection of points approximately 10 meters apart, with 0.5 meter elevation above MHW (see 

Fig. 3 for location).

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation and evaluation of the Galveston DEM were originally referenced to a number of 

vertical datums including Mean Lower Low Water (MLLW), Mean Low Water (MLW), Mean Low Tide (MLT), and 

North American Vertical Datum of 1988 (NAVD88). All datasets were transformed to MHW to provide the worst-case 

scenario for inundation modeling. Units were converted from feet to meters as appropriate.

1) Bathymetric data

The NOS hydrographic surveys and the USACE surveys were transformed from MLLW, MLW and MLT 

to MHW, using FME software, by adding an offset grid provided by PMEL. 

2) Topographic data

The USGS NED 1/3 arc-second DEM, the Harris County DEM, and the Texas Water Development 

Board LiDAR data were originally referenced to NAVD88. Conversion to MHW, using FME software, 

was accomplished by adding a constant offset of -0.377 m (Table 7) derived from the Galveston Pier tidestation. 

. The USGS National Elevation Dataset (NED) has been developed by merging the highest-resolution, best quality elevation data available across 

the United States into a seamless raster format. NED is the result of the maturation of the USGS effort to provide 1:24,000-scale Digital Elevation 

Model (DEM) data for the conterminous U.S. and 1:63,360-scale DEM data for Georgia. The dataset provides seamless coverage of the United 

States, HI, AK, and the island territories. NED has a consistent projection (Geographic), resolution (1 arc second), and elevation units (meters). The 

horizontal datum is NAD83, except for AK, which is NAD27. The vertical datum is NAVD88, except for AK, which is NGVD29. NED is a living 

dataset that is updated bimonthly to incorporate the “best available” DEM data. As more 1/3 arc second (10 m) data covers the U.S., then this will 

also be a seamless dataset. [Extracted from USGS NED website]
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DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

Table 7. Relationship between Mean High Water and other vertical datums in the Galveston region.

Vertical datum Difference to MHW

NAVD88 -0.377 meters

MLW Determined by adding PMEL offset grid

Mean Low Tide+ Determined by adding PMEL offset grid

MLLW Determined by adding PMEL offset grid

* Datum relationships determined by values from tide station #8771450 Galveston Pier.

+ Assumed to be equivalent to MLW.

3.2.2 Horizontal datum transformations

Datasets used to compile the Galveston DEM were originally referenced to NAD83 UTM Zone 14, NAD83 Texas 

State Plane – South Central, NAD27 geographic, NAD83 geographic, or WGS84 geographic horizontal datums. The 

relationships and transformational equations between these horizontal datums are well established. All data were 

converted to a horizontal datum of WGS84 using FME software.

3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

After horizontal and vertical transformations were applied, the resulting ESRI shape files were checked in ESRI

ArcMap for inter-dataset consistency. Problems and errors were identified and resolved before proceeding with

subsequent gridding steps. The evaluated and edited ESRI shape files were then converted to xyz files in preparation

for gridding. Problems included:

•	 Presence of man-made structures and extensive small streams, canal and water bodies in the TGLO coastline 

dataset, which had to be removed.

•	 Inconsistencies between the coastline dataset and bathymetric, and topographic datasets. These inconsistencies 

are partly the result of differing resolution between datasets and of morphologic change in the highly dynamic 

coastal zone.

•	 Data values over the open ocean and rivers in the NED, Harris County DEMs and TWDB LiDAR data. Each 

dataset required automated clipping to the TGLO coastline.

•	 Digital, measured bathymetric values from NOS surveys date back over 70 years. More recent data, such as 

USACE surveys in dredged shipping channels and the Gulf Intracoastal Waterway, differed from older, predredging NOS data by as much as 10 meters. The older NOS survey data were excised where more recent 

bathymetric data exists.

3.3.2	 Interpolating between USACE hydrographic profiles

USACE hydrographic surveys were conducted along profiles perpendicular to the axis of each channel. Data

points along the profiles are closely spaced (up to 1 m apart), but the distance between the profiles can be as great as

several hundreds of meters. Initial gridding produced a poor representation of the channels due to the large distances 

between profiles; higher elevations along the flanks of the channelstended to interpolate acrossthe channels, producing

isolated bathymetric “wells” rather than a more accurate linear channel morphology. To remedy this, NGDC developed 

custom code to extract the middle point in each profile and perform a linear interpolation between these middle points.

The resulting dataset contains lines of closely spaced points (10 m apart) located in the middle (deepest part) of each 

channel, thus providing a more realistic representation of the channels in the final Galveston DEM.

3.3.3 Smoothing of bathymetric data

The NOS hydrographic surveys are generally sparse at the resolution of the 1/3 arc-second Galveston DEM: in 

deep water, the NOS survey data have point spacings up to 600 m apart. In order to reduce the effect of artifacts in the 

form of lines of “pimples” in the DEM due to this low resolution dataset, and to provide effective interpolation into 
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Taylor et al., 2008

the coastal zone, a 1 arc-second-spacing ‘pre-surface’ or grid was generated using GMT, an NSF-funded share-ware 

software application designed to manipulate data for mapping purposes (http://gmt.soest.Texas.edu/).

The NOS hydrographic point data, in xyz format, were combined with the USACE soundings and interpolated 

soundings into a single file, along with points extracted from the TGLO coastline—to provide a “zero” buffer along

the entire coastline. These point data were then median-averaged using the GMT tool ‘blockmedian’ to create a 1 arcsecond grid 0.05 degrees (~5%) larger than the Galveston DEM gridding region. The GMT tool ‘surface’ then applied 

a tight spline tension to interpolate cells without data values. The GMT grid created by ‘surface’ was converted into 

an ESRI Arc ASCII grid file, and clipped to the TGLO coastline (to eliminate data interpolation into land areas). The

resulting surface was compared with original soundings to ensure grid accuracy (e.g., Fig. 10), converted to a shape 

file, and then exported as an xyz file for use in the final gridding process (see Table 8).

Figure 10. Histogram of the difference between NOS hydrographic survey H10584 and the 1 arc-second pre-surfaced 

bathymetric grid. Large discrepancies between survey soundings and the pre-surface grid occur where multiple, closely 

spaced points, in regions with significant relief, contribute to one cell value.

3.3.4 Gridding the data with MB-System

MB-System (http://www.ldeo.columbia.edu/res/pi/MB-System/) was used to create the 1/3 arc-second Galveston 

DEM. MB-System is an NSF-funded share-ware software application specifically designed to manipulate submarine

multibeam sonar data, though it can utilize a wide variety of data types, including generic xyz data. The MB-System 

tool ‘mbgrid’ applied a tight spline tension to the xyz data, and interpolated values for cells without data. The data 

hierarchy used in the ‘mbgrid’ gridding algorithm, as relative gridding weights, is listed in Table 8. Greatest weight 

was given to the USACE bathymetric surveys. Least weight was given to the pre-surfaced 1 arc-second bathymetric 

grid. Gridding was performed in quadrants, each with a 5% data overlap buffer. The resulting Arc ASCII grids were 

seamlessly merged in ArcCatalog to create the final 1/3 arc-second Galveston DEM.

Table 8. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

USACE bathymetry 100,000

TWDB topographic LiDAR 1000

Harris County topographic LiDAR DEM 1000

USGS NED topographic DEM 1

NOS hydrographic surveys: bathymetric soundings 100

Pre-surfaced bathymetric grid 1
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DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

3.4 Quality Assessment of the DEM

3.4.1. Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Galveston DEM is dependent upon the 

datasets used to determine corresponding DEM cell values. Topographic features have an estimated accuracy of 10 

to 15 meters: Harris County and TWDB topographic LiDAR data have an accuracy of approximately 1 meter, NED 

topography is accurate to within about 15 meters. Bathymetric features are resolved only to within a few tens of 

meters in deep-water areas. Shallow, near-coastal regions, rivers, and dredged shipping channels have an accuracy 

approaching that of subaerial topographic features. Positional accuracy is limited by: the sparseness of deep-water 

soundings; potentially large positional uncertainty of pre-satellite navigated (e.g., GPS) NOS hydrographic surveys; 

and by the rapid morphologic change that occurs in this dynamic region. 

3.4.2 Vertical accuracy

Vertical accuracy of elevation values for the Galveston DEM is also highly dependent upon the source datasets 

contributing to DEM cell values. Topographic areas have an estimated vertical accuracy between 0.1 to 0.3 meters for 

Harris County and TWDB LiDAR data and up to 7 meters for NED topography. Bathymetric areas have an estimated 

accuracy of between 0.1 meters and 5% of water depth. Those values were derived from the wide range of input data 

sounding measurements from the early 20th century to recent, GPS-navigated sonar surveys. Gridding interpolation to 

determine values between sparse, poorly-located NOS soundings degrades the vertical accuracy of elevations in deep 

water. 

3.4.3 Slope maps and 3-D perspectives

ESRI ArcCatalog was used to generate a slope grid from the Galveston DEM to allow for visual inspection and 

identification of artificial slopes along boundaries between datasets (Fig. 11). The DEM was transformed to UTM

Zone 14 coordinates (horizontal units in meters) in ArcCatalog for derivation of the slope grid; equivalent horizontal 

and vertical units are required for effective slope analysis. Three-dimensional viewing of the UTM-transformed DEM 

(Fig. 12) was accomplished using ESRI ArcScene. Analysis of preliminary grids revealed suspect data points, which 

were corrected before recompiling the DEM. Figure 1 shows a color image of the 1/3 arc-second Galveston DEM in 

its final version.

Figure 11. Slope map of the Galveston DEM. Flat-lying slopes are white; dark shading denotes 

steep slopes; TGLO coastline in red.
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Taylor et al., 2008

Figure 12. Perspective view northeast along Galveston Island, as modeled in the 

Galveston DEM. TGLO coastline in black; vertical exaggeration–times 50.

3.4.4	 Comparison with source data files

To ensure grid accuracy, the Galveston DEM was compared to select source data files. Files were chosen on

the basis of their contribution to the grid-cell values in their coverage areas (i.e., had the greatest weight and did 

not significantly overlap other data files with comparable weight). A histogram of the difference between a TWDB

topographic LiDAR survey file and the Galveston DEM is shown in Figure 13. Differences cluster around zero, with

only a handful of soundings, in regions of steep topography, exceeding 0.5-meter discrepancy from the DEM.

Figure 13. Histogram of the difference between one USACE survey (along the Gulf Intracoastal Waterway) and the 

Galveston DEM.

3.4.5 Comparison with NGS geodetic monuments

The elevations of 1414 NOAA NGS geodetic monuments were extracted from online shape files of monument

datasheets (http://www.ngs.noaa.gov/cgi-bin/datasheet.prl), which give monument positions in NAD83 (sub-mm 

accuracy) and elevations in NAVD88 (in meters). Elevations were shifted to MHW vertical datum (see Table 7) for 

comparison with the Galveston DEM (see Fig. 15 for monument locations). Differences between the Galveston DEM 

and the NGS geodetic monument elevations range from -19 to 21 meters, with a negative value indicating that the 

monument elevation is less than the DEM (Fig. 14). Examination of the monuments with the largest offsets from the 

DEM revealed that they are located on bridges, piers, buoys, or within tunnels.
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DIGITAL ELEVATION MODEL OF GALVESTON, TEXAS

Figure 14. Histogram of the differences between NGS geodetic monument elevations and the Galveston DEM. The largest 

differences derive from monuments located on bridges, piers, buoys or within tunnels. 

Figure 15. Location of NGS monuments and NOAA tide stations in the Galveston region. 

NGS monument elevations were used to evaluate the DEM.
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Taylor et al., 2008

4. Summary and Conclusions

A topographic–bathymetric digital elevation model of the Galveston, Texas region, with cell spacing of 1/3 arcsecond, was developed for the Pacific Marine Environmental Laboratory (PMEL) NOAACenter for Tsunami Research.

The best available digital data from U.S. federal, state and local agencies, and academic institutions were obtained 

by NGDC, shifted to common horizontal and vertical datums, and evaluated and edited before DEM generation. The 

data were quality checked, processed and gridded using ESRI ArcGIS, FME, GMT, MB-System and Quick Terrain 

Modeler software. 

Recommendations to improve the Galveston DEM, based on NGDC’s research and analysis, are listed below:

•	 Obtain LiDAR data processed to bare earth for the southwest corner of the DEM.

5. Acknowledgments

The NOAAPacific Marine Environmental Laboratory funded the development of the Galveston DEM. The authors

thank Chris Chamberlin and Vasily Titov (PMEL), Linda Carberry (USACE, Galveston District), Drew Decker (Texas 

Water Development Board), Tiffany Hepner (University of Texas, Bureau of Economic Geology) and Jacob Spen

(Harris County Flood Control District).

6. References

Coast Pilot 5, 34th Edition, 2006. Atlantic Coast: Gulf of Mexico, Puerto Rico, and Virgin Islands. U.S. Department of 

Commerce, NOAA, National Ocean Service.

Nautical Chart #11360, 7th Edition, 2006. Galveston River Approach. Scale 1:456,394. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

7. Data Processing Software

ArcGIS v. 9.1, developed and licensed by ESRI, Redlands, California, http://www.esri.com/

Electronic Navigational Chart Data Handler for ArcView, developed by NOAA Coastal Services Center, http://www.

csc.noaa.gov/products/enc/

FME 2006 GB – Feature Manipulation Engine, developed and licensed by Safe Software, Vancouver, BC, Canada, 

http://www.safe.com/

GEODAS v. 5 – Geophysical Data System, shareware developed and maintained by Dan Metzger, NOAA National 

Geophysical Data Center, http://www.ngdc.noaa.gov/mgg/geodas/

GMT v. 4.1.4 – Generic Mapping Tools, shareware developed and maintained by Paul Wessel and Walter Smith, 

funded by the National Science Foundation, http://gmt.soest.Texas.edu/

MB-System v. 5.1.0, shareware developed and maintained by David W. Caress and Dale N. Chayes, funded by the 

National Science Foundation, http://www.ldeo.columbia.edu/res/pi/MB-System/

Quick Terrain Modeler v. 6.0.1, LiDAR processing software developed by John Hopkins University’s Applied Physics

Laboratory (APL) and maintained and licensed by Applied Imagery, http://www.appliedimagery.com/