1

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-11

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, 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
2

Taylor et al., 2008
i

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, 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-11

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS:

PROCEDURES, DATA SOURCES AND ANALYSIS

Lisa A. Taylor1

Barry W. Eakins2

Kelly S. Carignan2

Robin R. Warnken1

Tatiana Sazonova2

David C. Schoolcraft1

1

NOAA, 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

Mention of a commercial company or product does not constitute an endorsement by the 

NOAA National Environmental Satellite, Data, and Information Service. Use of information 

from this publication concerning proprietary products or the test of such products for publicity

or advertising purposes is not authorized. 

Corresponding author contact:

Lisa A. Taylor

NOAA, National Geophysical Data Center

Marine Geology and Geophysics Division

325 Broadway, E/GC 3

Boulder, Colorado 80305

Phone: 303-497-6767

Fax: 303-497-6513

E-mail: Lisa.A.Taylor@noaa.gov

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

Also available from the National Technical Information Service (NTIS)

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

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

Contents

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

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

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

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

3.1.1 Shoreline................................................................................................................4

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

3.1.3 Topography..........................................................................................................11

3.2 Establishing Common Datums .................................................................................................12

3.2.1 Vertical datum transformations ...........................................................................12

3.2.2 Horizontal datum transformations.......................................................................12

3.3 Digital Elevation Model Development .....................................................................................13

3.3.1 Verifying consistency between datasets..............................................................13

3.3.2 Interpolating between USACE hydrographic profiles.........................................13

3.3.3 Smoothing of bathymetric data ...........................................................................13

3.3.4 Gridding the data with MB-System.....................................................................14

3.4 Quality Assessment of the DEM...............................................................................................15

3.4.1 Horizontal accuracy.............................................................................................15

3.4.2 Vertical accuracy .................................................................................................15

3.4.3 Slope maps and 3-D perspectives........................................................................15

3.4.4 Comparison with source data files.......................................................................16

3.4.5 Comparison with NGS geodetic monuments......................................................17

4. Summary and Conclusions...............................................................................................................18

5. Acknowledgments............................................................................................................................18

6. References........................................................................................................................................18

7. Data Processing Software ................................................................................................................18

List of Figures

Figure 1. Color image of the Corpus Christi, Texas region........................................................................1

Figure 2. Study area graphic.......................................................................................................................2

Figure 3. Source and coverage of datasets used to compile the Corpus Christi DEM...............................3

Figure 4. Digital coastline segments used to create a ‘combined coastline’ for the Corpus Christi 

region ...................................................................................................................................5

Figure 5. Digital NOS hydrographic survey coverage in the Corpus Christi region .................................6

Figure 6. Location of USACE survey data within dredged shipping channels in the Corpus Christi 

region ...................................................................................................................................9

Figure 7. Source and coverage of topographic datasets used in building the Corpus Christi DEM........10

Figure 8. Color image of the NED data used in the Corpus Christi DEM...............................................11

Figure 9. Histogram of the difference between NOS hydrographic survey H10401and the 

1 arc-second pre-surfaced bathymetric grid ......................................................................14

Figure 10. Slope map of the Corpus Christi DEM.....................................................................................15

Figure 11. Perspective view from the south of the Corpus Christi DEM ..................................................16

Figure 12. Histogram of the difference between NED topographic data and the Corpus Christi 

DEM. .................................................................................................................................16

Figure 13. Histogram of the differences between NGS geodetic monument elevations and the Corpus 

Christi DEM.......................................................................................................................17

Figure 14. Location of NGS monuments and NOAA tide stations in the Corpus Christi region ..............17

List of Tables

Table 1. PMEL specifications for the Corpus Christi DEM .....................................................................3

Table 2. Shoreline datasets used in compiling the Corpus Christi DEM..................................................4
iv

Taylor et al., 2008

Table 3. NOAA Electronic Navigational Charts in the Corpus Christi, Texas region..............................4

Table 4. Bathymetric datasets used in compiling the Corpus Christi DEM .............................................6

Table 5. Digital NOS hydrographic surveys used in compiling the Corpus Christi DEM.......................7

Table 6. USACE bathymetric surveys used in compiling the Corpus Christi DEM ..............................10

Table 7. Topographic datasets used in compiling the Corpus Christi DEM...........................................11

Table 8. Relationship between Mean High Water and other vertical datums in the Corpus Christi 

region .................................................................................................................................12

Table 9. Data hierarchy used to assign gridding weight in MB-System.................................................14
1

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

Digital Elevation Model of Corpus Christi, 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 Corpus Christi, 

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

nctr.pmel.noaa.gov/). The 1/3 arc-second1 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 Corpus Christi DEM. 

Figure 1. Color image of the Corpus Christi, Texas region. Coastline in orange.

1. The Corpus Christi 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 Corpus Christi, Texas (27°47′ N, 97°25′

W) 1/3 arc-second of latitude is equivalent to 10.26 meters; 1/3 arc-second of longitude equals 8.92 meters.
2

Taylor et al., 2008

2. Study Area

The Corpus Christi DEM covers the coastal region of the Gulf coast of Texas known as the Coastal Bend, centered 

on the city of Corpus Christi. The DEM includes the communities of Corpus Christi, Port Aransas, Aransas Pass, 

Ingleside, Portland, and Rockport. It also covers Corpus Christi Bay, Aransas Bay, Nueces Bay, Copano Bay and the 

northern portion of Baffin Bay as well as Padre, Mustang, and San Jose Islands (Fig. 1). 

The southern area of the DEM includes the northern tip of the Padre Island National Seashore separated from 

the mainland by Laguna Madre. Encompassing 130,434 acres and offering a wide variety of flora and fauna as well 

as recreation, it is the longest remaining undeveloped stretch of barrier island in the world (http://www.nps.gov/pais/

planyourvisit/maps.htm). 

Figure 2. Map of Texas Gulf Coast inlets (http://goliath.cbi.tamucc.edu/TexasInletsOnline/

TIO%20Main/index.htm) 
3

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

3. Methodology

The Corpus Christi 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 “worst-case scenario” flooding, respectively. Data processing and evaluation, and DEM assembly and assessment 

are described in the following subsections.

 Table 1: PMEL specifications for the Corpus Christi, Texas DEM.

Grid Area Corpus Christi, Texas

Coverage Area 96.65o to 97.7o W; 27.3o to 28.15o 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 ASCII raster grid

3.1 Data Sources and Processing

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

agencies, including: NOAA’s National Ocean Service (NOS); the National Geospatial-Intelligence Agency; and the 

U.S. Geological Survey (USGS). 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 NOAA tide stations at Corpus Christi 

and offset grids (digital surfaces with values representing interpolated differences between various tidal datums and 

MHW) provided by PMEL.

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

Taylor et al., 2008

3.1.1 Shoreline

Two digital coastline datasets of the Corpus Christi region were analyzed for inclusion in the Corpus Christi 

DEM: NOAA Office of Coast Survey Electronic Navigational Charts and the National Geospatial-Intelligence Agency 

High Water Line (Table 2).

Table 2: Shoreline datasets used in compiling the Corpus Christi DEM.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

NGA 2000

satellite 

derived High 

Water Line

1:75,000 or smaller WGS84 ~High Water 

Line See footnote 2

OCS ENCs 1997 to 

2006

Mean High 

Water 1:10,000 to 1:80,000 WGS84 Mean High Water

http://www.

nauticalcharts.

noaa.gov/mcd/

enc/download_

agreement.htm

1) NOAA Office of Coast Survey Electronic Navigational Charts

Six NOAA nautical charts were available in electronic chart format (#11307, #11308, #11309, #11311, 

#11312 and #11313) for the Corpus Christi region (Table 3) and were downloaded from NOAA’s Office of 

Coast Survey (OCS) website (http://www.nauticalcharts.noaa.gov/). All of the nautical charts were available 

in raster nautical chart (RNC) format—georeferenced map imagery, which are frequently updated—and as 

Electronic Navigational Charts (ENCs)—digital GIS chart components (Fig. 4). The ENCs include coastline 

data files (MHW), which were compared with the NGA coastline dataset, USGS topographic data, and NOS 

hydrographic soundings. The ENCs also include soundings (extracted from NOS hydrographic surveys) and 

land elevations.

The NGA coastline dataset and the six ENCs were used to build a ‘combined coastline’ (Fig. 4). The 

NOAA Coastal Services Center’s ‘Electronic Navigational Chart Data Handler for ArcView’ extension 

(http://www.csc.noaa.gov/products/enc/) was used to import the ENCs into ArcGIS. Editing all of the ENC 

coastline data was necessary to provide more detail in areas where recent bathymetric survey data existed. 

RNCs were reviewed in evaluating the combined coastline to make sure that small scale features were 

represented correctly. 

Table 3: NOAA Electronic Navigational Charts used to build the ‘combined coastline’ for the Corpus Christi DEM.

Chart Number Title Edition Date Scale

11307 Aransas Pass to Baffin Bay 4 06/2006 1:80,000

11308 Redfish Bay to Middle Ground Side A 2 06/2006 1:40,000

11309 Corpus Christi Bay 9 01/2007 1:40,000

11311 Corpus Christi Harbor 8 02/2007 1:10,000

11312 Corpus Christi Bay Port Aransas to Port Ingleside 8 02/2007 1:10,000

11313 Matagorda Light to Aransas Pass 3 01/2001 1:80,000
5

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

Figure 4. NOAA Electronic Navigational Charts and NGA coastline data used to build the 

‘combined coastline’ in the Corpus Christi region.

2) National Geospatial-Intelligence Agency High Water Line

The NGA Office of Global Navigation, Maritime Division2 developed the Global Shoreline Data set 

from digitized orthorectified NASA, 2000 era, LANDSAT GeoCover (multi-spectral imagery). This new 

shoreline is an approximation of the High Water Line with a resolution of 1:75,000 or smaller. The NGA 

coastline provided data where the ENCs were not available.

2. The NGA Office of Global Navigation, Maritime Division is in the process of developing a new version of World Vector Shoreline (WVS®) 

and in support of this effort has acquired a prototype Global Shoreline Data set. This new shoreline is an approximation of the High Water Line; it 

is NOT a Mean High Water Line since the source data have not been tide coordinated (http://www.nga.mil/NGAPortal/DNC.portal?_nfpb=true&_

pageLabel=dnc_portal_page_72). The prototype Global Shoreline Data set (satellite derived High Water Line) in work at NGA has been acquired 

from orthorectified NASA, 2000 era, LANDSAT GeoCover (multi-spectral imagery). [Extracted from metadata.]
6

Taylor et al., 2008

3.1.2 Bathymetry

Bathymetric datasets used in the compilation of the Corpus Christi DEM include 57 NOS hydrographic surveys 

and 51 USACE surveys of dredged shipping channels (Table 4). 

Table 4: Bathymetric datasets used in compiling the Corpus Christi DEM

Source Year Data Type Spatial Resolution

Original 

Horizontal Datum/

Coordinate System

Original 

Vertical Datum URL

USACE, 

Galveston 

District

2004 

to 

2007

Hydrographic 

survey 

soundings

Profiles spaced 25 to 

200 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/

 NOS 

1934 

to 

1994

Hydrographic 

survey 

soundings

Ranges from 10 

m to 1 km (varies 

with scale of survey, 

depth, traffic, and 

probability of 

obstructions)

NAD27 or NAD83 MLLW or 

MLW

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

bathymetry/hydro.html

Figure 5. Bathymetric datasets used in compiling the Corpus Christi DEM.
7

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

1) NOS hydrographic survey data

A total of 57 NOS hydrographic surveys conducted between 1934 and 1994 were utilized in developing 

the Corpus Christi DEM (Table 5; 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 

5). 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 Corpus Christi 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 NED topographic data, the combined coastline, RNCs, and Google Earth satellite imagery.

Table 5: Digital NOS hydrographic surveys used in compiling the Corpus Christi DEM.

NOS Survey 

ID

Year of 

Survey

Survey 

Scale Original Vertical Datum Original Horizontal Datum

H05612 1934 10,000 mean low water NAD27

H05613 1934 10,000 mean low water NAD27

H05693 1935 20,000 mean low water NAD27

H05694 1935 20,000 mean low water NAD27

H05875 1935 20,000 mean low water NAD27

H05916 1935 20,000 mean low water NAD27

H05917 1935 20,000 mean low water NAD27

H06393 1938 20,000 mean low water NAD27

H06394 1938 20,000 mean low water NAD27

H06395 1938 20,000 mean low water NAD27

H06396 1938 20,000 mean low water NAD27

H06397 1938 20,000 mean low water NAD27

H06401 1938 40,000 mean low water NAD27

H06402 1938 40,000 mean low water NAD27

H06403 1938 40,000 mean low water NAD27

H06405 1938 80,000 mean low water NAD27

H06498 1939 80,000 mean low water NAD27

H09002 1968 20,000 mean low water NAD27

H09005 1968 20,000 mean low water NAD27

H10205 1985 20,000 mean lower low water NAD27

D00107 1989 40,000 mean lower low water NAD27

H10320 1989/90 10,000 mean lower low water NAD83

H10321 1989/90 10,000 mean lower low water NAD83

H10322 1989/90 10,000 mean lower low water NAD83

H10323 1989/90 10,000 mean lower low water NAD83

H10324 1989/90 10,000 mean lower low water NAD83

H10325 1989/90 10,000 mean lower low water NAD83

H10332 1989/90 10,000 mean lower low water NAD83

H10326 1990 10,000 mean lower low water NAD83

H10327 1990 10,000 mean lower low water NAD83

H10328 1990 10,000 mean lower low water NAD83

H10329 1990 10,000 mean lower low water NAD83
8

Taylor et al., 2008

H10330 1990 10,000 mean lower low water NAD83

H10359 1990/91 10,000 mean lower low water NAD83

H10360 1990/91 10,000 mean lower low water NAD83

H10361 1990/91 10,000 mean lower low water NAD83

H10362 1990/91 10,000 mean lower low water NAD83

H10363 1990/91 10,000 mean lower low water NAD83

H10364 1990/91 5,000 mean lower low water NAD83

H10365 1991 10,000 mean lower low water NAD83

H10366 1991 10,000 mean lower low water NAD83

H10367 1991 10,000 mean lower low water NAD83

H10368 1991 10,000 mean lower low water NAD83

H10369 1991 10,000 mean lower low water NAD83

H10392 1991 10,000 mean lower low water NAD83

H10399 1991 10,000 mean lower low water NAD83

H10402 1991 10,000 mean lower low water NAD83

H10400 1991/92 10,000 mean lower low water NAD83

H10401 1991/92 10,000 mean lower low water NAD83

F00381 1992 10,000 mean lower low water NAD83

F00386 1993 10,000 mean lower low water NAD83

H10508 1993 10,000 mean lower low water NAD83

H10509 1993/94 10,000 mean lower low water NAD83

H10510 1993/94 10,000 mean lower low water NAD83

F00402 1994 10,000 mean lower low water NAD83

F00405 1994 10,000 mean lower low water NAD83

H10562 1994 10,000 mean lower low water NAD83
9

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

Figure 6. Digital NOS hydrographic survey coverage in the Corpus Christi region. DEM boundary in red, combined coastline in gray.

2) U.S. Army Corps of Engineers Survey

The USACE, Galveston District provided NGDC with hydrographic surveys spanning the Texas 

Gulf Coast within dredged shipping channels and the Intracoastal Waterway (Table 6, Fig. 7). Records 

in the individual survey files that contained zero latitude and longitude values were deleted and the 

resulting data were transformed to shapefiles using FME. The original vertical datum of Mean Low Tide 

(MLT) was determined to be equivalent to Mean Low Water. The surveys were reviewed and edited 

individually in ArcMap. Two surveys contained overlapping elevation points with conflicting values and 

two surveys contained incorrect single elevation points. These surveys were corrected using ArcMap 

editor, referencing corresponding nautical chart data. 
10

Taylor et al., 2008

Table 6: USACE hydrographic surveys used in compiling the Corpus Christi DEM.

Region

Original horizontal 

datum

Original vertical 

datum Spatial Resolution

Port Aransas NAD27 State Plane 

Texas South Mean Low Tide Profiles ~125 m long, spaced 25 to 75 m apart, with <1 m point spacing

Rincon Canal NAD27 State Plane 

Texas South Mean Low Tide Profiles ~75 to 125 m long, spaced ~100 m apart, with <1 m point spacing

Jewl Fulton 

Channel

NAD27 State Plane 

Texas South Mean Low Tide Profiles ~75 to 100 m long, spaced 60 to 125 m apart, with <1 m point spacing

La Quinta 

Channel

NAD27 State Plane 

Texas South Mean Low Tide Profiles ~200 to 400 m long, spaced ~100 m apart, with <1 m point spacing

Fulton NAD27 State Plane 

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

Aransas Pass NAD27 State Plane 

Texas South Mean Low Tide Profiles ~75 m long, spaced 50 to 200 m apart, with <1 m point spacing

GIWW Corpus 

Christi

NAD27 State Plane 

Texas South Central Mean Low Tide Profiles ~100 m long, spaced ~75 to 100 m apart, with <1 m point spacing

Corpus Christi 

Shipping 

Channel

NAD27 State Plane 

Texas South Mean Low Tide Profiles ~250 to 500 m long, spaced ~ 30 to 130 m apart, with <1 m point spacing

 

Figure 7. USACE channel surveys within the Corpus Christi DEM boundary.
11

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

3.1.3 Topography

The topographic dataset used in the Corpus Christi region was obtained from the U.S. Geological Survey (Table 

7; Fig. 8). 

Table 7: Topographic dataset used in compiling the Corpus Christi DEM.

Source Year Data Type Spatial 

Resolution

Original Horizontal 

Datum/Coordinate System

Original 

Vertical Datum URL

USGS 2001 NED 1/3 arcsecond ~10 m NAD83 NAVD88

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

1) USGS NED topography

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

complete 1/3 arc-second coverage of the Corpus Christi region3. Data are in NAD83 geographic coordinates 

and NGVD88 vertical datum (meters), and are available for download as raster DEMs. The extracted bareearth 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 data included “zero” elevation values over the open ocean, which were removed from the 

dataset before gridding. Some anomalous values still remained over the open ocean, which were visually 

inspected and compared with NOAA nautical charts, the combined coastline, and Google Earth satellite 

imagery. ESRI Arc Catalog was used to clip the data to the combined coastline.

Figure 8. USGS National Elevation Dataset (NED) data used in the Corpus Christi DEM 

3. 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]
12

Taylor et al., 2008

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

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

of vertical datums including Mean Lower Low Water (MLLW), Mean Low Water (MLW), and North American 

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

inundation modeling.

1) Bathymetric data

NOS and USACE hydrographic surveys were transformed from MLLW, MLW, and MLT to MHW, using 

FME software, by adding an offset grid provided by PMEL (Table 8). 

2) Topographic data

The USGS NED 1/3 arc-second DEM was originally referenced to NAVD88. It was converted to MHW, 

using FME software by adding a tide-station derived constant offset of -0.317 (Table 8). 

Table 8. Relationship between Mean High Water and other vertical datums in the Corpus Christi region.

Vertical datum Difference to MHW

NAVD88* -0.317

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 averaging values from tide stations #8775870, Corpus Christi and #8775792, 

Packery Channel.

+ Assumed to be equivalent to MLW.

3.2.2 Horizontal datum transformations

Datasets used to compile the Corpus Christi DEM were originally referenced to the horizontal datums of State 

Plane Texas South, State Plane Texas South Central, NAD27, NAD83 geographic, and WGS84 geographic. 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.
13

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

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 river banks in coastline datasets, which had to be removed.

•	 Inconsistencies between the coastline datasets 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 DEM required automated clipping to the combined 

coastline.

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

USACE surveys in dredged shipping channels, differed from older, pre-dredging 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 very closely spaced (up to 1 m apart), while the distance between the profiles can be as 

great as several hundreds of meters. Spatially interpolating in areas that contain dredged channels produced a poor 

representation of the channels due to large distances between profiles. To remedy this, we developed custom code that 

takes three points in each profile (middle, first and the last), identifies similar points in the next profile, and performs 

a linear interpolation between the two sets of points. The resulting dataset contains three lines of closely spaced 

points (10 m apart or less) located in the middle and along the borders of the channel, thus providing a more realistic 

representation of the channel.

3.3.3 Smoothing of bathymetric data

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

in deep water, the NOS survey data have point spacings up to 2 km 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 

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 into a single file, 

along with points extracted from the combined 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 arc-second grid 0.05 degrees 

(~5%) larger than the Corpus Christi 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 combined coastline (to eliminate data interpolation into land areas). The resulting surface 

was compared with the original soundings to ensure grid accuracy (e.g., Fig. 9), converted to a shape file, and then 

exported as an xyz file for use in the final gridding process (see Table 9).
14

Taylor et al., 2008

Figure 9. Histogram of the difference between NOS hydrographic survey H10401 and the 1 arc-second pre-surfaced 

bathymetric grid. Discrepancies between survey soundings and the pre-surface grid result from the averaging of several 

closely spaced soundings.

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 Corpus 

Christi 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 9. Greatest 

weight was given to the USGS NED toprographic DEM. 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 Corpus Christi DEM.

Table 9. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

USACE hydrographic surveys 100

USGS NED topographic DEM 1000

NOS hydrographic surveys: bathymetric soundings 100

Pre-surfaced bathymetric grid 1
15

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

3.4 Quality Assessment of the DEM

3.4.1. Horizontal accuracy

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

the datasets used to determine corresponding DEM cell values. Topographic features within the NED topographic 

dataset have an estimated accuracy of 15 meters. Bathymetric features are resolved only to within a few hundred 

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 Corpus Christi DEM is also highly dependent upon the source 

datasets contributing to DEM cell values. Topographic areas have an estimated vertical accuracy up to 7 meters 

for NED topography. Bathymetric areas have an estimated accuracy of between 0.1 meters and 5% of water depth. 

These 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 Corpus Christi DEM to allow for visual inspection and 

identification of artificial slopes along boundaries between datasets (e.g., Fig. 19). The DEM was transformed to UTM 

Zone 15 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 

(e.g., Fig. 10) 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 Corpus Christi 

DEM in its final version.

Figure 10. Slope map of the Corpus Christi DEM. Flat-lying slopes are white; dark shading denotes 

steep slopes; combined coastline in orange.
16

Taylor et al., 2008

Figure 11. Perspective view from the south of the Corpus Christi DEM. Combined 

coastline in red; vertical exaggeration–times 40.

3.4.4 Comparison with source data files

To ensure grid accuracy, the Corpus Christi 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 NED 

topographic dataset and the Corpus Christi DEM is shown in Figure 12.

Figure 12. Histogram of the difference between NED topographic data and the Corpus Christi DEM.
17

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

3.4.5 Comparison with NGS geodetic monuments

The elevations of 255 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 9) for 

comparison with the Corpus Christi DEM (see Fig. 14 for monument locations). Differences between the Corpus 

Christi DEM and the NGS geodetic monument elevations range from -19 to 2.6 meters, with a negative value indicating 

that the monument elevation is less than the DEM (Fig. 13). Examination of the monuments with the largest positive 

and negative offsets from the DEM revealed that the most of them are located on the top of the man made structures 

such as bridges and buildings. 

Figure 13. Histogram of the differences between NGS geodetic monument elevations and the Corpus Christi DEM. 

Figure 14. Location of NGS monuments and NOAA tide stations in the Corpus Christi region. Tide station used to convert 

between vertical datums; NGS monument elevations used to evaluate the DEM.
18

Taylor et al., 2008

4. Summary and Conclusions

A topographic–bathymetric digital elevation model of the Corpus Christi, Texas region, with cell spacing of 1/3 

arc-second, was developed for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami 

Research. The best available digital data from U.S. federal and state agencies 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, and MB-System software. 

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

•	 Incorporate Texas Water Development Board topographic LiDAR when available.

•	 Incorporate topographic LIDAR data collected by the University of Texas at Austin, Bureau of Economic 

Geology in 2000 from Aransas Pass to the Padre Island National Seashore when available. 

5. Acknowledgments

The development of the Corpus Christi DEM was funded by the NOAA, Pacific Marine Environmental Laboratory. 

The authors thank Chris Chamberlin and Vasily Titov (PMEL), Linda Carberry (USACE, Galveston District), John 

Brock and Amar Nayegandhi (USGS, Florida Integrated Science Center, Coastal and Watershed Studies Team), and 

Iris Wilson (National Park Service).

6. References

Nautical Chart #11307, 37th Edition, 2006. Aransas Pass to Baffin Bay. Scale 1:80,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11308, 23rd Edition, 2005. Redfish Bay to Middleground (Side A). Scale 1:40,000. U.S. Department 

of Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11309, 38th Edition, 2006. Corpus Christi Bay. Scale 1:40,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11311, 24th Edition, 2005. Corpus Christi Harbor. Scale 1:10,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11312, 4th Edition, 2006. Corpus Christi Bay - Port Aransas to Port Ingleside. Scale 1:20,000. U.S. 

Department of Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11313, 23rd Edition, 2005. Matagorda Light to Aransas Pass. Scale 1:80,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

7. Data Processing Software

ArcGIS v. 9.2, 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/
19

DIGITAL ELEVATION MODEL OF CORPUS CHRISTI, TEXAS

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/