noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-55

DIGITAL ELEVATION MODELS OF THE U.S. VIRGIN ISLANDS:

PROCEDURES, DATA SOURCES AND ANALYSIS 

P.R. Grothe

L.A. Taylor

B.W. Eakins

K.S. Carignan

R.J. Caldwell 

E. Lim

D.Z. Friday 

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

August 2012

UNITED STATES

DEPARTMENT OF COMMERCE

Rebecca Blank

Acting Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Dr. Jane Lubchenco

Under Secretary for Oceans

and Atmosphere/Administrator

National Environmental Satellite,

Data, and Information Service

Mary E. Kicza

Assistant Administrator

NOAA Technical Memorandum NESDIS NGDC-55

DIGITAL ELEVATION MODELS OF THE U.S. VIRGIN ISLANDS:

PROCEDURES, DATA SOURCES AND ANALYSIS 

Pamela R. Grothe1

Lisa A. Taylor2

Barry W. Eakins1

Kelly S. Carignan1

R. Jason Caldwell1 

Elliot Lim1

Dorothy Z. Friday1

1Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder 

2

NOAA, National Geophysical Data Center, Boulder, Colorado 

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

August 2012
Grothe et al., 2010

ii

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 project 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/coastal/

Also available from the National Technical Information Service (NTIS)

(http://www.ntis.gov)
Digital Elevation Models of the U.S. Virgin Islands

iii

Contents

1..Introduction 1

2..Study Area 4

3..Methodology 5

3.1 Data Sources and Processing 6

3.1.1 Shoreline 7

3.1.2 Bathymetry 9

3.1.3 Topography 22

3.2 Establishing Common Datums 25

3.2.1 Vertical datum transformations 25

3.2.2 Horizontal datum transformations 26

3.3 Digital Elevation Model Development 27

3.3.1 Verifying consistency between datasets 27

3.3.2 Smoothing of bathymetric data 27

3.3.3 Building the DEMs using MB-System 32

3.4 Quality Assessment of the DEMs 33

3.4.1 Horizontal accuracy 33

3.4.2 Vertical accuracy 33

3.4.3 Slope map and 3-D perspectives 33

3.4.4 Comparison with National Geodetic Survey geodetic monuments 38

3.4.5 Virgin Islands DEMs comparison with source data files 39

4..Summary and Conclusions 43

5..Acknowledgments 43

6..References 43

7..Data Processing Software 44

Appendix A. Vertical Datum Conversion Grids 45

List of Figures

Figure 1. Shaded-relief image of St. Thomas and St. John, U.S. Virgin Islands MHW 1/3

arc-second DEM. 1

Figure 2. Shaded-relief image of St. Croix, U.S. Virgin Islands MHW 1/3 arc-second DEM 2

Figure 3. Shaded-relief image of the U.S. Virgin Islands MHW 1 arc-second DEM 3

Figure 4. Map of the U.S. Virgin Islands region 4

Figure 5. Source and coverage of datasets used in compiling the U.S. Virgin Islands DEMs 6

Figure 6. Digital coastline datasets used in a “combined coastline” of the Virgin Islands region 8

Figure 7. Source and coverage of bathymetric datasets used in compiling the U.S. Virgin Islands

DEMs 10

Figure 8. Spatial coverage of the CCMA multibeam surveys 11

Figure 9. Digital NOS hydrographic survey coverage in the Virgin Islands region 14

Figure 10. Multibeam data coverage in the Virgin Islands region 15

Figure 11. Trackline data coverage in the Virgin Islands region 18

Figure 12. Coverage of digitized hydrographic soundings from georeferenced U.S. nautical

charts 19

Figure 13. Coverage of NOAA ENC soundings used in compiling the U.S. Virgin Islands DEMs 20

Figure 14. Coverage of the USGS EAARL submerged topographic lidar 21

Figure 15. Coverage of gridded multibeam provided by UVI. 21

Figure 16. Source and coverage of topographic datasets used in compiling the U.S. Virgin

Islands DEMs. 22

Figure 17. USGS topographic lidar DEM of St. John. 24

Figure 18. Location of NOAA tide station in the Virgin Islands region used for datum

transformations to MHW. 25
Grothe et al., 2010

iv

Figure 19. Histogram of the differences between all NGDC multibeam data and the

3 arc-second pre-surfaced bathymetric grid 28

Figure 20. Histogram of the differences between all trackline surveys and the 3 arc-second

pre-surfaced bathymetric grid 28

Figure 21. Histogram of the differences between all NOS survey data and the 1/3 arc-second

pre-surfaced bathymetric grid of St. Thomas and St. John. 29

Figure 22. Histogram of the difference between all CCMA multibeam data and the 1/3

arc-second pre-surfaced bathymetric grid of St. Thomas and St. John. 29

Figure 23. Histogram of the difference between on the USGS submerged bathymetric lidar data

and the 1/3 arc-second pre-surface bathymetric grid of St. Thomas and St. John. 30

Figure 24. Histogram of the difference between the CCMA multibeam data and the 1/3

arc-second pre-surface bathymetric grid of St. Croix. 30

Figure 25. Histogram of the difference between the NOS survey data and the 1/3 arc-second

pre-surface bathymetric grid of St. Croix. 31

Figure 26. Histogram of the differences between the University of Virgin Islands multibeam data

and the 1/3 arc-second pre-surface bathymetric grid of St. Croix. 31

Figure 27. Slope map of the 1 arc-second U.S. Virgin Islands DEM 34

Figure 28. Slope map of the 1/3 arc-second St. Thomas and St. John, U.S. Virgin Islands DEM. 35

Figure 29. Slope map of the 1/3 arc-second St. Croix, U.S. Virgin Islands DEM 35

Figure 30. Data contribution plot of the U.S. Virgin Islands 1 arc-second DEM 36

Figure 31. Perspective view from the southwest of the 1 arc-second U.S. Virgin Islands DEM 37

Figure 32. Perspective view from the southwest of the 1/3 arc-second St. Thomas and St. John,

U.S. Virgin Islands DEM. 37

Figure 33. Perspective view from the northeast of the 1/3 arc-second St. Croix, U.S. Virgin

Islands DEM 37

Figure 34. Location of NGS geodetic monuments in the Virgin Islands region. 38

Figure 35. Histogram of the differences between NGS geodetic monument elevations and the

U.S. Virgin Islands 1 arc-second DEM. 38

Figure 36. Histogram of the differences between NOS data points and the 1/3 arc-second

St. Thomas and St. John DEM. 39

Figure 37. Histogram of the differences between USGS EAARL topographic lidar data points

and the 1/3 arc-second St. Thomas and St. John DEM 40

Figure 38. Histogram of the differences between CCMA multibeam data points and the 1/3

arc-second St. Thomas and St. John DEM. 40

Figure 39. Histogram of the differences between NOS data points and the 1/3 arc-second

St. Croix DEM 41

Figure 40. Histogram of the differences between CCMA multibeam data points and the

1/3 arc-second St. Croix DEM. 41

Figure 41. Histogram of the difference between SRTM DEM data points and the 1/3 arc-second

St. Croix DEM 42

Figure 42. Histogram of the difference between the NGDC multibeam data points and the

1 arc-second U.S. Virgin Islands DEM. 42

Figure A1. Color image of the conversion grid from MLLW to MHW using offsets from

tide stations 46

Figure A2. Color image of the conversion grid from MLW to MHW using offset from tide

stations 47

Figure A3. Color image of the conversion grid from MSL to MHW using offset from tide

station 48

Figure A4. Color image of the conversion grid used to convert data from the WGS 84 ellipsoid

to the NAD 83 ellipsoid. 49

Figure A5. Color image of the conversion grid used to convert data from the NAD 83 ellipsoid

to the preliminary VIVD 09 Geoid. VIVD 09 is assumed MSL. 50
Digital Elevation Models of the U.S. Virgin Islands

v

List of Tables

Table 1. PMEL specifications for the U.S. Virgin Islands DEMs 5

Table 2. Shoreline datasets used in developing the U.S. Virgin Islands DEMs 7

Table 3. Shoreline datasets used in developing the Virgin Islands DEMs 7

Table 4. Bathymetric datasets used in compiling the U.S. Virgin Islands DEMs. 9

Table 5. CCMA multibeam surveys used in compiling the U.S. Virgin Islands DEMs 11

Table 6. Digital NOS hydrographic surveys used in compiling the U.S. Virgin Island DEMs 12

Table 7. Multibeam swath sonar surveys used in compiling the U.S. Virgin Islands, DEMs 15

Table 8. Trackline surveys available in the Virgin Islands region. 16

Table 9. Nautical charts of the British Virgin Islands region used for digitizing hydrographic

soundings 19

Table 10. Multibeam surveys from UVI used in compiling the U.S. Virgin Islands DEMs 21

Table 11. Topographic datasets used in compiling the U.S. Virgin Islands MHW DEMs 22

Table 12. Tide stations near the Virgin Islands and the relationships between MHW and other vertical

datums 25

Table 13. Data hierarchy used to assign gridding weight in MB-System 32

Table A1. Difference values between the WGS 84 and NAD 83 Ellipsoids. 45
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Digital Elevation Models of the U.S. Virgin Islands:

Procedures, Data Sources and Analysis

1. Introduction

In October 2010, the National Geophysical Data Center (NGDC), an office of the National Oceanic and Atmospheric Administration (NOAA), developed three bathymetric–topographic digital elevation models (DEMs) of the

U.S. Virgin Islands (Figs. 1 - 3) for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami

Research (http://nctr.pmel.noaa.gov/). Two 1/3 arc-second1 DEMs, one of St Thomas and St. John (Fig. 1) and one of

St. Croix (Fig. 2), and a 1 arc-second DEM of the U.S. Virgin Islands region (Fig. 3), referenced to mean high water

(MHW) will be used as input for the Method of Splitting Tsunami (MOST) model developed by PMEL to simulate

tsunami generation, propagation and inundation. The DEMs were generated from diverse digital datasets in the region

(grid boundary and sources shown in Fig. 5). They will be used for tsunami forecasting, as part of the tsunami forecast

system Short-term Inundation Forecasting for Tsunamis (SIFT) 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 Virgin Islands DEMs.

Figure 1. Shaded-relief image of St. Thomas and St. John, U.S. Virgin Islands MHW 1/3 arc-second DEM. Contour interval is 200 meters for 

the bathymetry and 50 meters for the topography. Image is in Mercator projection.

1. The Virgin Islands DEMs are 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 St. Thomas and St. John, (18°19'48"N,

-64°57'0"W) 1/3 arc-second of latitude is equivalent to 10.25 meters; 1/3 arc-second of longitude equals 9.79 meters; 1 arc-second of latitude is

equivalent to 30.75 meters; 1 arc-second of longitude equals 29.36 meters. At the latitude of St. Croix, (17°42’0”N, -64°45’36”W) 1/3 arc-second

of latitude is equivalent to 10.25 meters; 1/3 arc-second of longitude equals 9.8 meters; 1 arc-second of latitude is equivalent to 30.74 meters; 1

arc-second of longitude equals 29.46.
Grothe et al., 2010

2

Figure 2. Shaded-relief image of St. Croix, U.S. Virgin Islands MHW 1/3 arc-second DEM. Contour interval is 500 meters for the bathymetry 

and 50 meters for the topography. Image is in Mercator projection. 
Digital Elevation Models of the U.S. Virgin Islands

3

Figure 3. Shaded-relief image of the U.S. Virgin Islands MHW 1 arc-second DEM. 

Contour interval is 500 meters for the bathymetry and 50 meters for topography. 

Image is in Mercator projection.
Grothe et al., 2010

4

2. Study Area

The Virgin Islands are a group of islands in the Caribbean, located in the Leeward Islands of the Lesser

Antilles (Fig. 4). The Virgin Island archipelago is made up of United States and British territories. The U.S. Virgin

Islands consist of the main islands of St. Thomas, St. John, and St. Croix and many smaller surrounding islands. The

British Virgin Islands consists of Tortola, Virgin Gorda, and Anegada Islands along with many other smaller islands

and cays.

The Virgin Islands are not known for having damaging tsunamis, but they have occurred. The region is at

risk for tsunamis by several different mechanisms: teletsunamis, or tsunamis that originate from earthquakes across

the Atlantic and other far away regions; local earthquakes along the Puerto Rico trench; and local landslides. There

are only two recorded tsunamis that hit this region from far-field sources, one in 1755 and another in 1761, both from

earthquakes near Lisbon, Portugal. There have also been two significant tsunamis from local earthquakes, one in 1867

and the other in 1918. Both caused considerable damage and run-up heights (~7 meters at St. Croix) for the U.S. and

the British Virgin Islands.

Figure 4. Map of the U.S. Virgin Islands region. Red box denotes the 1 arc-second DEM boundary. 

Image in background taken from ESRI’s online World 2D imagery.
Digital Elevation Models of the U.S. Virgin Islands

5

3. Methodology

The U.S. Virgin Islands DEMs were constructed to meet PMEL specifications (Table 1a - 1c), based on input

requirements for the development of Reference Inundation Models (RIMs) and Standby Inundation Models (SIMs)

(V. Titov, pers. comm.) in support of NOAA’s Tsunami Warning Centers use of SIFT to provide real-time tsunami

forecasts in an operational environment. The best available bathymetric and topographic digital data were obtained by

NGDC and shifted to common horizontal and vertical datums: World Geodetic System of 1984 (WGS 84) and MHW

for maximum flooding. Data were gathered in an area slightly larger (~5%) than the DEM extents. This data “buffer”

ensures that gridding occurs across rather than along the DEM boundaries to prevent edge effects. Data processing and

evaluation, and DEM assembly and assessment are described in the following subsections.

Table 1. PMEL specifications for the U.S. Virgin Islands DEMs. 

Grid Area U.S. Virgin Islands St. Thomas and St. John,

U.S. Virgin Islands

St. Croix,

U.S. Virgin Islands

Coverage Area 64o to 65.15o W;

17.00o to 19.00o N

64.64o to 65.15o W;

18.17o to 18.48o N

64.40o to 65.04o W;

17.61o to 17.88o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984

(WGS 84)

Vertical Datum Mean High Water (MHW)

Vertical Units Meters

Cell Size 1 arc-second 1/3 arc-second 1/3 arc-second

Grid Format ESRI Arc ASCII raster grid
Grothe et al., 2010

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3.1 Data Sources and Processing

Shoreline, bathymetric, and topographic digital datasets (Fig. 5) were obtained from several U.S. federal and

local agencies including: NGDC; NOAA’s National Ocean Service (NOS), Office of Coast Survey (OCS), and Center

for Coastal Monitoring and Assessment (CCMA); the U.S. Geological Survey (USGS); the National Aeronautics and

Space Administration (NASA); and the University of Virgin Islands (UVI). Safe Software’s Feature Manipulation 

Engine (FME)2 data translation tool package and ESRI’s ArcGIS were used to shift datasets to WGS 84 geographic

horizontal datum. FME, GDAL3, and OGR4 were used to convert datasets into ESRI ArcGIS shapefiles and xyz format. The shapefiles and xyz files were then displayed with ArcGIS and Applied Imagery’s Quick Terrain Modeler (QT

Modeler) to assess data quality and manually edit datasets. The methodology used for vertical datum transformations

to in described in Section 3.2.1). ESRI’s online World 2D imagery and NOAA’s Raster Nautical Charts (RNCs) were

used to analyze and modify data. QT Modeler software was used to evaluate processing and gridding techniques.

Figure 5. Source and coverage of datasets used in compiling the U.S. Virgin Islands DEMs. 

Black boxes denote the DEM boundaries. 

2. FME uses the North American Datum Conversion Utility (NADCON; http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.shtml) developed by

NOAA’s National Geodetic Survey (NGS) to convert data from NAD 27 to NAD 83. NADCON is the U.S. Federal Standard for NAD 27 to NAD

83 datum transformations.

3. GDAL is a translator library for raster geospatial data formats that is released under an X/MIT style Open Source license by the Open Source

Geospatial Foundation. As a library, it presents a single abstract data model to the calling application for all supported formats. It also comes with

a variety of useful commandline utilities for data translation and processing.

4. The OGR Simple Features Library is a C++ open source library and commandline tools providing read and write access to a variety of vector file

formats, including ESRI shapefiles. OGR is a part of the GDAL library.
Digital Elevation Models of the U.S. Virgin Islands

7

3.1.1 Shoreline

Coastline datasets of the Virgin Islands region were obtained from NOAA’s OCS as Electronic Navigational

Charts (ENCs)5 (Table 2; Fig. 6). Datasets from six ENCs were used to develop a “combined coastline” of the Virgin

Islands region.

Table 2. Shoreline datasets used in developing the U.S. Virgin Islands DEMs.

Source Year Data Type Spatial 

Resolution

Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum URL

OCS 2003-

2008

ENC®

Coastline

1:10,000 -

1:100,000 WGS 84 geographic Mean High Water http://www.nauticalcharts.noaa.

gov/mcd/enc/

Six ENCs are available for the U.S. Virgin Islands region (Table 3; Fig. 6). They were downloaded from

NOAA’s OCS web site in S-57 format and included coastline data files referenced to MHW. The coastline shapefiles

were extract using FME and compared to large-scale georeferenced Raster Nautical Charts (RNCs). The coastlines

from the highest resolution ENCs were merged together into a “combined coastline”. The combined coastline was

edited as needed and modified to include large offshore rocks using ESRI’S online World 2D imagery. Small piers and

docks were removed from the coastline.

Table 3. Shoreline datasets used in developing the Virgin Islands DEMs. 

Chart Title Edition Edition 

date Format Scale

25640 Puerto Rico and Virgin Islands 43 2008 RNC/ENC 1:326,856

25641 Virgin Gorda to St. Thomas and St. Croix

and Krause Lagoon Channel 27 2004 RNC/ENC 1:100,000 and 1:20,000

25644 Frederiksted Road and Frederiksted Pier 13 2003 RNC/ENC 1:20,000 and 1:2,500

25645 Christiansted Harbor 18 2006 RNC/ENC 1:10,000

25647 Pillsbury Sound 11 2006 RNC/ENC 1:15,000

25649 St. Thomas Harbor 19 2003 RNC/ENC 1:10,000

25650 Virgin Passage and Sonda De Vieques 34 2004 RNC 1:100,000

5. The Office of Coast Survey (OCS) produces NOAA Electronic Navigational Charts (NOAA ENC®) to support the marine transportation

infrastructure and coastal management. NOAA ENC®s are in the International Hydrographic Office (IHO) S-57 international exchange format,

comply with the IHO ENC Product Specification and are provided with incremental updates, which supply Notice to Mariners corrections and other

critical changes. NOAA ENC®s are available for free download on the OCS web site. [Extracted from NOAA OCS web site: http://nauticalcharts.

noaa.gov/mcd/enc/]
Grothe et al., 2010

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Figure 6. Digital coastline datasets used in a “combined coastline” of the Virgin Islands region. 
Digital Elevation Models of the U.S. Virgin Islands

9

3.1.2 Bathymetry

Bathymetric datasets available in the Virgin Islands region include 53 NOS hydrographic surveys; 13 multibeam surveys retrieved from the NGDC multibeam database; nine multibeam surveys from NOAA’s CCMA; two

multibeam surveys from UVI; bathymetric lidar from USGS; hydrographic soundings from ENCs; trackline soundings; and NGDC digitized soundings from nautical charts. (Table 4; Fig. 7).

Table 4. Bathymetric datasets used in compiling the U.S. Virgin Islands DEMs.

Source Year Data Type Spatial Resolution

Original Horizontal Datum/

Coordinate 

System

Original 

Vertical 

Datum

URL

CCMA

2004

to

2009

Multibeam

DEM 1 - 10 meters NAD 83UTM Zone 20 N MLLW

http://ccma.nos.noaa.gov/products/biogeography/usvi_nps/

data.html

NGDC

1909

to

2003

NOS

hydrographic

survey

soundings

Ranges from 5 meters to

1.2 kilometers (varies with

scale of survey, depth,

traffic and probability of

obstructions)

Early Puerto

Rico Island Datum, Puerto Rico

Datum, NAD 27,

NAD 83

MLLW and

MLW

(meters)

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

bathymetry/hydro.html

NGDC

1985

to

2005

Multibeam

swath sonar

surveys

Raw sonar files gridded to

3 arc-second

WGS 84

geographic

Assumed

MSL

(meters)

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

bathymetry/multibeam.html

NGDC

1962

to

2000

Trackline

soundings

100’s of meters along

profiles

WGS 84

geographic

Assumed

MSL

(meters)

http://www.ngdc.

noaa.gov/mgg/geodas/

trackline.html

NGDC 2010

Digitized

chart

soundings

1:80,000, 1:30,000 and 

1:12,473

WGS 84

geographic MHW N/A

OCS

2004

and

2008

Hydrographic

soundings

1:326,856

and 1:100,000

WGS 84

geographic MLLW http://www.nauticalcharts.noaa.

gov/mcd/enc/

USGS 2003

EAARL

submerged

topography

lidar

1 meter WGS 84

UTM zone 20 N

WGS 84

Ellipsoid

referenced

to ITRF

http://pubs.usgs.gov/ds/395/

USGS 2009 Multibeam

DEM 5 meters NAD 83UTM Zone 20 N MLLW N/A

UVI 2009 Multibeam

DEM 1 meter NAD 83UTM Zone 20 N MLLW N/A
Grothe et al., 2010

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Figure 7. Source and coverage of bathymetric datasets used in compiling the U.S. Virgin Islands DEMs. 

Coastline in red. Regions of no data shown in white. Black boxes denote the DEM boundaries. 
Digital Elevation Models of the U.S. Virgin Islands

11

1) Center for Coastal Monitoring and Assessment

NGDC downloaded a unified ten meter bathymetric DEM surface from the CCMA web site that contained all their available multibeam data collected in 2009. CCMA also provided NGDC via ftp with three

surveys collected by the USGS and one survey collect by Geophysics GPR International, Inc for the Caribbean Fisheries Management Council (Table 5; Fig. 8). This data ranged from one meter, five meter, 8 meters,

and 50 meter resolution. The data were provided in GeoTIFF format, with a horizontal datum of NAD 83

UTM Zone 20 N, and vertical datum of mean lower low water (MLLW).

Table 5. CCMA multibeam surveys used in compiling the U.S. Virgin Islands DEMs.

Survey Year of Survey Survey Scale Source

buck_island_01m 2009 1 meter CCMA

EviequesTiki09_5m 2009 5 meters USGS

NStxRon Brown06_50m 2006 50 meters USGS

RedHindMCD_01m 2009 1 meter Caribbean Fisheries Management Council

StJohnShelf_01m 2009 1 meter CCMA

StJohnShelf_08m 2009 8 meter CCMA

unified_bathy _10m 2009 10 meters CCMA

StTSailRockTiki09_5m 2009 5 meters USGS

VirginPass_01m 2009 1 meter CCMA

Figure 8. Spatial coverage of the CCMA-provided multibeam surveys. Light blue represents shallow depths; dark pink represents deep depths. 

Nautical chart #25640 in background. 
Grothe et al., 2010

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2) National Ocean Service hydrographic survey data

Fifty-three hydrographic surveys conducted between 1909 and 2003 were available for use in the development of the Virgin Islands DEMs. Surveys were extracted as xyz files using GEODAS from NGDC’s

online NOS Hydrographic database with a buffer 0.05 degrees (~5%) larger than the 1 arc-second Virgin Islands DEM extent to support data interpolation along grid edges. The downloaded hydrographic survey data

were vertically referenced to MLLW or mean low water (MLW) and horizontally reference to Early Puerto

Rico Datum, Puerto Rico Datum, North America Datum of 1927 (NAD 27), and North America Datum of

1983 (NAD 83) (Table 6; Fig 9).

Data point spacing for the NOS surveys varied by scale. In general, small scale surveys had greater point

spacing than larger scale surveys. The data were converted to WGS 84 and transformed to shapefiles using

FME software. They were then displayed in ESRI ArcMap and reviewed for digitizing errors against scanned

original survey smooth sheets and edited as necessary. The surveys were also compared to other bathymetric

datasets, the combined coastline, and NOS RNCs. Older surveys were clipped to remove soundings that have

been superseded by more recent NOS surveys and multibeam data.

Table 6. Digital NOS hydrographic surveys used in compiling the U.S. Virgin Island DEMs.

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

D00002 1977 50,000 Early Puerto Rico Island Datums MLW

F00277 1985 2,500 Puerto Rico Datum MLLW

F00279 1985 5,000 Puerto Rico Datum MLLW

H02491 1909 10,000 Early Puerto Rico Island Datums MLW

H04652B 1924 20,000 Early Puerto Rico Island Datums MLW

H04653D 1924 10,000 Early Puerto Rico Island Datums MLW

H04743B1 1923 20,000 Unknown MLW

H08877 1966 5,000 Early Puerto Rico Island Datums MLW

H09270 1967 40,000 North American Datum 1927 MLW

H09271 1972 10,000 Early Puerto Rico Island Datums MLW

H09272 1972 10,000 Early Puerto Rico Island Datums MLW

H09273 1972 20,000 Early Puerto Rico Island Datums MLW

H09352 1973 20,000 Early Puerto Rico Island Datums MLW

H09353 1973 10,000 Early Puerto Rico Island Datums MLW

H09365 1973 10,000 Early Puerto Rico Island Datums MLW

H09507 1975 10,000 Early Puerto Rico Island Datums MLW

H09514 1975 10,000 Early Puerto Rico Island Datums MLW

H09515 1975 10,000 Early Puerto Rico Island Datums MLW

H09516 1975 20,000 Early Puerto Rico Island Datums MLW

H09517 1975 20,000 Early Puerto Rico Island Datums MLW

H09601 1976 10,000 Early Puerto Rico Island Datums MLW

H09602 1976 10,000 Early Puerto Rico Island Datums MLW
Digital Elevation Models of the U.S. Virgin Islands

13

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

H09603 1976 10,000 Early Puerto Rico Island Datums MLW

H09604 1976 20,000 Early Puerto Rico Island Datums MLW

H09605 1976 20,000 Early Puerto Rico Island Datums MLW

H09616 1976 10,000 Early Puerto Rico Island Datums MLW

H09617 1976 10,000 Early Puerto Rico Island Datums MLW

H09618 1976 20,000 Early Puerto Rico Island Datums MLW

H09929 1981 5,000 Early Puerto Rico Island Datums MLW

H09930 1981 5,000 Early Puerto Rico Island Datums MLW

H09934 1981 2,500 Puerto Rico Datum MLLW

H09935 1981 10,000 Early Puerto Rico Island Datums MLW

H09936 1981 10,000 Puerto Rico Datum MLLW

H09937 1981 10,000 Early Puerto Rico Island Datums MLW

H09938 1981 10,000 Early Puerto Rico Island Datums MLW

H09992 1982 80,000 Early Puerto Rico Island Datums MLW

H09993 1982 80,000 Early Puerto Rico Island Datums MLW

H09997 1982 10,000 Puerto Rico Datum MLLW

H09998 1982 80,000 Early Puerto Rico Island Datums MLW

H09999 1982 5,000 Early Puerto Rico Island Datums MLW

H10002 1981 10,000 Early Puerto Rico Island Datums MLW

H10003 1982 10,000 Early Puerto Rico Island Datums MLW

H10004 1982 80,000 Early Puerto Rico Island Datums MLW

H10006 1982 10,000 Early Puerto Rico Island Datums MLW

H10007 1982 10,000 Puerto Rico Datum MLLW

H10008 1982 10,000 North American Datum 1927 MLLW

H10009 1982 10,000 Puerto Rico Datum MLLW

H10074 1982 80,000 Puerto Rico Datum MLLW

H10505 1993 5,000 North American Datum 1983 MLLW

H10506 1993 5,000 North American Datum 1983 MLLW

H10211 1985 10,000 Puerto Rico Datum MLLW

H11146 2002 5,000 North American Datum 1983 MLLW

W00004 2003 500,000 North American Datum 1983 MLLW
Grothe et al., 2010

14

Figure 9. Digital NOS hydrographic survey coverage in the Virgin Islands region. Coastline in red.
Digital Elevation Models of the U.S. Virgin Islands

15

3) NGDC multibeam swath sonar surveys

Thirteen multibeam swath sonar surveys were available from the NGDC multibeam bathymetry database for use in the development of the U.S. Virgin Islands DEMs (Table 7; Figure 10). The data were horizontally referenced to WGS 84 geographic datum and were assumed to be vertically referenced to mean sea

level (MSL) datum. The data were gridded to extents with a buffer of 0.05 degrees (5%) larger than the DEM

extents using MB-System6 (http://www.ldeo.columbia.edu/res/pi/MB-System/) at 3 arc-seconds and viewed

in QT Modeler for quality analysis. Editing was performed using QT Modeler to eliminate errors and where

survey data overlapped. The grid was then converted to xyz format and the elevations were transformed from

MSL to MHW.

Table 7. Multibeam swath sonar surveys used in compiling the U.S. Virgin Islands, DEMs.

Survey ID Date Institution Ship

AT04L04 2004 Woods Hole Oceanographic Institution (WHOI) Atlantis

EW0309* 2003 Columbia University, Lamont-Doherty Earth Observatory (CU/LDEO) Maurice Ewing

EW0404 2004 CU/LDEO Maurice Ewing

EW9605 1996 CU/LDEO Maurice Ewing

EW9606 1996 CU/LDEO Maurice Ewing

EW9706 1997 CU/LDEO Maurice Ewing

HLY05TH 2005 Rolling Deck to Repository (R2R) Program USCGC Healy

KN151L4 1997 WHOI Knorr

KN173L02* 2003 WHOI Knorr

NF0406* 2004 NOAA Nancy Foster

NF0505* 2005 NOAA Nancy Foster

RB0303 2003 US Geological Survey (USGS) Uri ten Brink

RC2605* 1985 CU/LDEO Robert Conrad

*indicates data were not used due to poor quality or overlapped by more recent survey

Figure 10. Multibeam data coverage in the Virgin Islands region. Vertical exaggeration 5 times. 

6. MB-System is an open source software package for the processing and display of bathymetry and backscatter imagery data derived from

multibeam, interferometry, and sidescan sonars. The source code for MB-System is freely available (for free) by anonymous ftp (including “point

and click” access through these web pages). A complete description is provided in web pages accessed through the web site. MB-System was

originally developed at the Lamont-Doherty Earth Observatory of Columbia University (L-DEO) and is now a collaborative effort between the

Monterey Bay Aquarium Research Institute (MBARI) and L-DEO. The National Science Foundation has provided the primary support for MBSystem development since 1993. The Packard Foundation has provided significant support through MBARI since 1998. Additional support has

derived from SeaBeam Instruments (1994-1997), NOAA (2002-2004), and others. URL: http://www.ldeo.columbia.edu/res/pi/MB-System/ from

MB-System web site.]
Grothe et al., 2010

16

4) NGDC trackline surveys

Sixty-three single-beam trackline surveys were available from the NGDC Marine Geophysical Trackline

Database for use in the development of the U.S. Virgin Islands DEMs (Table 8; Fig 11). This database is comprised of bathymetry, magnetics, gravity, and seismic navigation data collected during marine cruises from

1953 to present. The data were downloaded as xyz files horizontally referenced to the WGS 84 geographic

and vertically referenced to assumed MSL.

Table 8. Trackline surveys available in the Virgin Islands region.

Cruise ID Ship Year Institution

4873 Gibbs 1970 Defense Mapping Agency (DMA) Hydro/Topo Centre

13976 Gibbs 1970 DMA Hydro/Topo Centre

20276 San Pablo 1967 DMA Hydro/Topo Centre

45783 Vessel TRB-4 1972 DMA Hydro/Topo Centre

19930013 HMY Britannia 1985 United Kingdom Hydrographic Office (UKHO)

20000140 HMS Manchester 2000 UKHO

87004711 Jean Charcot 1987 France - IFREMER

87004712 Jean Charcot 1987 France - IFREMER

A2054L03 Atlantis II 1969 USGS Woods Hole (WHOI)

AKU05 Kurchatov 1969 Russia - IFZ AN SSSR

B82161 Hayes 1982 US Naval Research Laboratory (US NRL)

BA67001 Baffin 1967 Canadian Hydrographic Service (CHS)

BA70002 Baffin 1970 CHS

CH034L01 Chain 1962 WHOI

CH075L01 Chain 1967 WHOI

DI109L1 Discovery 1980 Institute of Oceanographic Sciences, UK

DSDP45GC Glomar Challenger 1976 Scripps Institution of Oceanography (SIO)

DSDP46GC Glomar Challenger 1976 SIO

DSDP51GC Glomar Challenger 1977 SIO

DSDP78AC Glomar Challenger 1981 SIO

DSDP78BC Glomar Challenger 1981 SIO

EW9207 Maurice Ewing 1992 CU/LDEO

FARN0682 Farnella 1982 Natural Environment Research

Council (NERC)

FRNL85-4 Farnella 1985 WHOI

H0506A HNLMS Luymes 1972 Netherlands Hydrographic Office

HU70002 Hudson 1970 CHS

IG1505 Ida Green 1975 University of Texas Institute for

Geophysics (UTIG)

INMD12MV Melville 1978 SIO

KA344602 Kane 1976 US Naval Oceanographic Office (US NAVO)

KA68J Kane 1968 US Navy NORDA

KN054L04 Knorr 1976 WHOI

KN151L4 Knorr 1997 WHOI

LY70B Lynch 1970 US Navy NORDA

LY70F Lynch 1970 US Navy NORDA

ODP171BJ JOIDES Resolution 1997 Ocean Drilling Program/

Texas A and M (ODP/TAMU)

P1085VI Powell 1985 USGS Branch of Pacific Marine Geology
Digital Elevation Models of the U.S. Virgin Islands

17

Cruise ID Ship Year Institution

P1185VI Powell 1985 USGS Branch of Pacific Marine Geology

P286CB Powell 1986 USGS Branch of Pacific Marine Geology

P385CB Powell 1985 USGS Branch of Pacific Marine Geology

P386CB Powell 1986 USGS Branch of Pacific Marine Geology

P486CB Powell 1986 USGS Branch of Pacific Marine Geology

RC0707 Robert Conrad 1963 CU/LDEO

RC0708 Robert Conrad 1963 CU/LDEO

RC0801 Robert Conrad 1963 CU/LDEO

RC0809 Robert Conrad 1964 CU/LDEO

RC0812 Robert Conrad 1964 CU/LDEO

RC1612 Robert Conrad 1973 CU/LDEO

RC1613 Robert Conrad 1973 CU/LDEO

RC1904 Robert Conrad 1975 CU/LDEO

RC1907 Robert Conrad 1976 CU/LDEO

RC2116 Robert Conrad 1978 CU/LDEO

RC2212 Robert Conrad 1979 CU/LDEO

RC2215 Robert Conrad 1979 CU/LDEO

RC2605 Robert Conrad 1985 CU/LDEO

RC2907 Robert Conrad 1988 CU/LDEO

S386CB Starella 1986 USGS Branch of Pacific Marine Geology

TYDE88_5 Tydeman 1988 Netherlands Hydrographic Office

U371CB United Geo I 1971 USGS Branch of Pacific Marine Geology

U771PR United Geo I 1971 USGS Branch of Pacific Marine Geology

V1803 Vema 1962 CU/LDEO

V2001 Vema 1964 CU/LDEO

V2002 Vema 1964 CU/LDEO

V2202 Vema 1966 CU/LDEO

V2607 Vema 1969 CU/LDEO

V3003 Vema 1973 CU/LDEO

WI932010 Wilkes 1972 US NAVO
Grothe et al., 2010

18

Figure 11. Trackline data coverage in the Virgin Islands region. Soundings in blue represent the data used in building the DEMs. Soundings in 

green were removed before building the DEM as newer and higher-resolution data were available for those areas.
Digital Elevation Models of the U.S. Virgin Islands

19

5) NGDC digitized hydrographic sounds

Digital data were sparse near the British Virgin Islands. NGDC scanned and georeferenced three paper

U.S. nautical charts from the Defense Mapping Agency in ArcMap and digitized the soundings for use in

building the DEMs (Table 9; Fig 12). Nautical charts were obtained from the University of Colorado’s Map

Library. The charts contained soundings in meters or fathoms and had a vertical datum of MLLW or MLW.

Soundings on the charts were only digitized in areas where no other data were available or only sparse NOAA

ENC data were available.

Table 9. Nautical charts of the British Virgin Islands region used for digitizing hydrographic soundings. 

Chart Title Edition Edition Date Scale Horizontal Datum Vertical Datum

25609 West Indies Virgin Islands St. Thomas

to Anegada 5th 1996 1:80,000 WGS 84geographic

MLW

(meters)

25610 West Indies British Virgin Islands Approaches to Gorda Straight 8th 1996 1:12,473 WGS 84

geographic

MLLW

(fathoms)

25609 West Indies Virgin Islands Approach

to Road Harbour 22nd 1995 1:30,000 WGS 84geographic

MLW

(meters)

Figure 12. Coverage of digitized hydrographic soundings from georeferenced U.S. nautical charts. 

Scanned georeferenced nautical charts in background. 
Grothe et al., 2010

20

6) Office of Coast Survey Electronic Navigation Chart soundings

Soundings from ENCs #25640 and #25641 around the British Virgin Islands were used in building the

U.S. Virgin Islands DEMs (see Table 3; Fig. 13). The extracted soundings were transformed from MLLW to

MHW.

Figure 13. Coverage of NOAA ENC soundings used in compiling the U.S. Virgin Islands DEMs. 

RNC chart #25640 and #25641 in background. Coastline in red. 
Digital Elevation Models of the U.S. Virgin Islands

21

7) USGS EAARL submerged topographic lidar

A submerged topographic DEM was produced for a portion of the U.S. Virgin Islands cooperatively

by USGS, NASA, and the National Park Service (NPS). The DEM was produced from remotely sensed,

geographically referenced elevation measurements7

. The data required several processing steps to convert to

MHW (see Sec. 3.2.1) which potentially added several centimeters of error to the accuracy of the data.

Figure 14. Coverage of the USGS EAARL submerged topographic lidar.

8) UVI gridded multibeam bathymetry

UVI provided NGDC with two multibeam bathymetry grids (Table 10; Fig. 15) that fall in the 1/3 arcsecond DEM of St. Croix. The data were provided as one meter bathymetric grids in NAD 83 UTM Zone 20

N at MLLW.

Table 10. Multibeam surveys from UVI used in compiling the U.S. Virgin Islands DEMs. 

Survey Name Year of Survey Survey Scale Original Horizontal Datum Original Vertical Datum

lang_1m 2009 1 meter NAD 83 UTM Zone 20 N MLLW

mutton_1m 2009 1 meter NAD 83 UTM Zone 20 N MLLW

Figure 15. Coverage of gridded multibeam provided by UVI.

7. The lidar data were collected using the NASA Experiment Advanced Airborne Research lidar (EAARL) and have a vertical accuracy of 15 cm

and horizontal accuracy within one meter. The data were horizontally referenced in WGS 84 UTM Zone 20 N and vertically referenced in the WGS

84 coordinate system, with an ITRF reference [USGS metadata].
Grothe et al., 2010

22

3.1.3 Topography

Three topographic datasets—1 and 3 arc-second Shuttle Radar Topography Mission (SRTM) DEM, USGS

topographic lidar, and NGDC digitized elevations—were used in building the U.S. Virgin Islands DEMs (Table 11;

Fig 16). The 1 arc-second National Elevation Dataset (NED) topographic DEM from USGS was evaluated but not

used in the building of the DEM. In addition, NGDC digitized elevation points of offshore rocks that were not resolved

completely in the other topographic datasets.

Table 11. Topographic datasets used in compiling the U.S. Virgin Islands MHW DEMs.

Source Year Data Type Spatial 

Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Datum URL

NGDC 2002 Digitized elevations 10 meters WGS 84 geographic MHW NA

NASA 2000 Topographic DEM 3 arc-second and

1 arc-second WGS 84 geographic Assumed MSL http://edcsns17.cr.usgs.

gov/EarthExplorer/

USGS 2003 Topographic lidar 1 meter WGS 84 UTM Zone

20 N

WGS 84 Ellipsoid

referenced to ITRF

http://pubs.usgs.gov/

ds/406/index.html

Figure 16. Source and coverage of topographic datasets used in compiling the U.S. Virgin Islands DEMs.
Digital Elevation Models of the U.S. Virgin Islands

23

1) NGDC digitized elevations

Many offshore rocks were not represented in the available digital elevation datasets. NGDC digitized

offshore rocks using elevation values marked on NOAA RNCs. RNC charts reference elevations in MHW,

therefore no vertical datum transformation was needed.

2) NASA SRTM 1 and 3 arc-second topographic DEMs

The NASA Shuttle Radar Topography Mission obtained elevation data on a near-global scale to generate

the most complete high-resolution digital topographic database of Earth8

. The SRTM consisted of a specially

modified radar system that flew on board the Space Shuttle Endeavour during an 11-day mission in February 2000. Data from this mission have been processed into 1 degree x 1 degree tiles that have been edited to

define the coastline, and are available from the Earth Explorer web site as raster DEMs. The data have not

been processed to bare earth, but meet the absolute horizontal and vertical accuracies of 20 and 16 meters,

respectively.

For the outermost British Virgin Islands, the data have 3 arc-seconds spacing while the remainder of the

most populated U.S. and British Virgin Islands have 1 arc-second spacing (see Fig. 16). The data are referenced to the EGM 96 Geoid. The SRTM data contained elevation values over the open ocean, which were

deleted by clipping to the combined coastline.

3) USGS EAARL topographic lidar

A first surface elevation grid of St. John was produced cooperatively by USGS, NASA, and NPS (Fig.

17). Elevation data were remotely collected using the NASA Experimental Advanced Airborne Research

Lidar (EAARL) to measure ground elevation, tree canopy, and coastal topography. The data are horizontally

referenced to WGS 84 UTM Zone 20 N and vertically referenced to the WGS 84 ellipsoid, with reference to

ITRF. They have a vertical accuracy of 15 cm and a horizontal accuracy of 1 meter.

The dataset represented multiple problems in using it for building the U.S. Virgin Islands DEMs. First,

the data are not bare earth and due to the thick tree canopy represented in the unfiltered lidar data, NGDC

was unable to accurately represent the bare-earth surface in the final DEMs. Second, the data are in WGS 84

Ellipsoid, which required several processing steps to transform to MHW (see sect. 3.2.1), which potentially

decreased vertical accuracy.

8. The SRTM data sets result from a collaborative effort by the National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA – previously known as the National Imagery and Mapping Agency, or NIMA), as well as the participation of the

German and Italian space agencies, to generate a near-global digital elevation model (DEM) of the Earth using radar interferometry. The SRTM

instrument consisted of the Spaceborne Imaging Radar-C (SIR-C) hardware set modified with a Space Station-derived mast and additional antennae

to form an interferometer with a 60 meter long baseline. A description of the SRTM mission can be found in Farr and Kobrick (2000). Synthetic

aperture radars are side-looking instruments and acquire data along continuous swaths. The SRTM swaths extended from about 30 degrees off-nadir

to about 58 degrees off-nadir from an altitude of 233 km, and thus were about 225 km wide. During the data flight the instrument was operated at

all times the orbiter was over land and about 1000 individual swaths were acquired over the ten days of mapping operations. Length of the acquired

swaths range from a few hundred to several thousand km. Each individual data acquisition is referred to as a “data take.” SRTM was the primary

(and pretty much only) payload on the STS-99 mission of the Space Shuttle Endeavour, which launched February 11, 2000 and flew for 11 days.

Following several hours for instrument deployment, activation and checkout, systematic interferometric data were collected for 222.4 consecutive

hours. The instrument operated almost flawlessly and imaged 99.96% of the targeted landmass at least one time, 94.59% at least twice and about

50% at least three or more times. The goal was to image each terrain segment at least twice from different angles (on ascending, or north-going, and

descending orbit passes) to fill in areas shadowed from the radar beam by terrain. This “targeted landmass’ consisted of all land between 56 degrees

south and 60 degrees north latitude, which comprises almost exactly 80% of Earth’s total landmass. [Extracted from SRTM online documentation]
Grothe et al., 2010

24

Figure 17. USGS topographic lidar DEM of St. John.
Digital Elevation Models of the U.S. Virgin Islands

25

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation and evaluation of the U.S. Virgin Islands DEMs were originally referenced

to a number of vertical datums including MLLW, MLW, MSL, EGM 96 Geoid, and the WGS 84 Ellipsoid referenced

to the ITRF. All datasets were transformed to MHW for modeling of maximum flooding. Vertical datum transformations to MHW were accomplished using GDAL, based upon data from NOAA tide stations in the region (Table 12;

Fig. 18). See Appendix A for NGDC’s conversion grid methodology.

Table 12. Tide stations near the Virgin Islands and the relationships between MHW and other vertical datums. 

Station Name Station Number Latitude Longitude

MLLW to MHW 

Difference 

(meters)

MLW to MHW 

Difference 

(meters)

MSL TO MHW 

Difference 

(meters)

Charlotte Amalie 9751639 18.335 -64.92 0.227 0.213 0.113

Christiansted

Harbor, St. Croix 9751364 17.75 -64.705 0.215 0.207 0.103

Culebra 9752235 18.30000 -65.30167 0.285 0.238 0.121

Esperanza, Vieques Island 9752695 18.09333 -65.47000 0.216 0.209 0.104

Lameshur Bay,

St. Johns 9751381 18.31667 -64.72333 0.238 0.222 0.12

Lime Tree Bay,

VI 9751401 17.68333 -64.75333 0.214 0.21 0.102

Figure 18. Location of NOAA tide station in the Virgin Islands region used for datum transformations to MHW.
Grothe et al., 2010

26

1) Bathymetric data

The NOS hydrographic surveys, NGDC multibeam swath sonar surveys, trackline surveys, ENCs,

CCMA and UVI multibeam surveys, and digitized chart sounds were transformed from MLLW, MLW,

and MSL to MHW using the conversion grids. The USGS EAARL submerged topographic lidar data

were transformed to MHW by using the WGS 84 to the NAD 83 Ellipsoid conversion grid, the Hybrid model to VIVD 09 (assumed MSL) conversion grid, and then the MSL to MHW conversion grid..

2) Topographic data

The USGS EAARL topographic lidar data were transformed to MHW using the WGS 84 to the NAD 83

Ellipsoid conversion grid, the Hybrid model to VIVD 09 (assumed MSL) conversion grid, and then the MSL

to MHW conversion grid.

The SRTM data were first transformed from the EGM 96 Geoid to the WGS 84 Ellipsoid using a geoid

height transformation grid acquired from the National Geospatial Intelligence Agency (http://earthycinfo.

nga.mil/GradnG/wgs84/gravitymod/egm96.html). The data were then transformed by using the WGS 84 to

the NAD 83 Ellipsoid conversion grid, the Hybrid model to VIVD 09 (assumed MSL) conversion grid, and

then the MSL to MHW conversion grid.

The multiple vertical datum transformations to MHW potentially introduced cm-scale vertical accuracy errors.

The data were carefully evaluated after each step to ensure the conversion grid was applied correctly and that the final

MHW data accurately portrayed the elevation values in the area.

3.2.2 Horizontal datum transformations

Datasets used to compile the U.S. Virgin Islands DEMs were originally referenced to WGS 84 geographic,

NAD 83 geographic, NAD 27 geographic, NAD 83 UTM Zone 20 N, Early Puerto Rico Island Datum, and Puerto

Rico Datum. The relationships and transformational equations between the geographic horizontal datums are well

established. Transformations to WGS 84 geographic were accomplished using FME software and GDAL’s transformation tool.
Digital Elevation Models of the U.S. Virgin Islands

27

3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

After horizontal and vertical transformations were applied, the resulting ESRI shapefiles and xyz files were

checked in ESRI ArcMap and Quick Terrain Modeler for inter-dataset consistency. Problems and errors were identified and resolved before proceeding with subsequent gridding steps. The evaluated and edited ESRI shapefiles were

then converted to xyz files in preparation for gridding. Problems included:

•	 Inconsistent, overlapping datasets. Older and lower resolution datasets were clipped to newer and higher

resolution datasets.

•	 Data values over the ocean in the SRTM DEMs and the USGS topographic lidar datasets. These datasets

required automated clipping to the combined coastline to remove those values over the open ocean.

•	 SRTM and USGS lidar data not processed to bare earth. Data contains elevation values of dense tree canopy

that could not be removed. 

•	 Digital, measured bathymetric values from NOS surveys date back over 100 years. The older NOS survey

data were excised where more recent bathymetric data exist.

•	 Offshore rocks were not included in the topographic data. Elevation values were digitized based on NOAA

RNCs.

•	 Sparse digital data around the British Virgin Islands. Paper nautical charts were scanned, georeferenced, and

digitized.

•	 No VDatum was available. Most datasets were transformed to MHW using a conversion grid from tide

stations, but several data sources required multiple transformations to achieve a vertical datum of MHW.

3.3.2 Smoothing of bathymetric data

Older NOS hydrographic survey data, ENC soundings, and digitized chart soundings are generally sparse

at the resolution of the U.S. Virgin Islands DEMs in both deep water and in some areas close to shore. In order to

reduce the effect of artifacts in the form of lines or “pimples” in the DEM due to these low resolution datasets, and

to provide effective interpolation into the coastal zone, a 3 arc-second-cell size ‘pre-surface’ bathymetric grid of the

Virgin Islands 1 arc-second DEM, and two 1/3 arc-second-cell size ‘pre-surface’ bathymetric grids of St. Thomas and

St. John, and St. Croix, were generated using GMT9, an NSF-funded software application designed to manipulate data

for mapping purposes (http://gmt.soest.hawaii.edu/).

The older NOS hydrographic point data, ENC soundings, and trackline data were manually clipped to remove overlap with the newer NOS surveys, NGDC multibeam data, and the multibeam surveys provided by CCMA

and UVI. All of the bathymetric data were then combined with points extracted from the adjusted MHW coastline—to

provide a buffer along the entire coastline. The coastline elevation values were set to -1 meter to ensure a bathymetric

surface approaching zero relative to MHW in areas where bathymetric data are sparse or non-existent.

The point data were then median-averaged using the GMT tool ‘blockmedian’ to create a 3 arc-second grid

0.05 degrees (~5%) larger than the 1 arc-second DEM gridding region, and a 1/3 arc-second grid 0.05 degrees larger

than the 1/3 arc-second DEM gridding region St. Thomas and St. John, and St. Croix. The higher resolution grids

required the bathymetric surface to be gridded at the same resolution as the DEM to prevent ‘topographic creep’, in

other words, the topography creeping in the open water due to sparse data along the coast. The GMT tool ‘surface’was

then used to apply a tight spline tension to interpolate elevations for 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 original soundings to ensure grid accuracy. Figures 19 and 20 show

histograms of the NGDC multibeam data and trackline data compared to the 3 arc-second pre-surface bathymetric

grid. Figures 21-26 show histograms of the NOS, CCMA multibeam, and UVI multibeam surveys, and ENC and digitized chart soundings compared to both the 1/3 arc-second pre-surface bathymetric grids. Differences cluster around

zero with the multibeam data have the largest differences of -200 to +170 meters when compared to the bathymetric

9. GMT is an open source collection of ~60 tools for manipulating geographic and Cartesian data sets (including filtering, trend fitting, gridding,

projecting, etc.) and producing Encapsulated PostScript File (EPS) illustrations ranging from simple x-y plots via contour maps to artificially illuminated surfaces and 3-D perspective views. GMT supports ~30 map projections and transformations and comes with support data such as GSHHS

coastlines, rivers, and political boundaries. GMT is developed and maintained by Paul Wessel and Walter H. F. Smith with help from a global set

of volunteers, and is supported by the National Science Foundation. It is released under the GNU General Public License. URL: http://gmt.soest.

hawaii.edu/ [Extracted from GMT web site.]
Grothe et al., 2010

28

surface. Points with the largest differences are located along steep gradients of elevation (e.g., submarine canyons)

where the high-resolution surveys may include over 100 points that are averaged to a single cell elevation value. 

Some inconsistencies were identified while merging the bathymetric datasets due to the range in ages and

resolutions of the NOS hydrographic surveys. In areas where more recent data were available, the older surveys were

either edited or not used. The gridded bathymetric surface was then converted to an xyz file for use in building the

MHW DEMs.

Figure 19. Histogram of the differences between all NGDC multibeam data and the 3 arc-second pre-surfaced bathymetric grid. 

Figure 20. Histogram of the differences between all trackline surveys and the 3 arc-second pre-surfaced bathymetric grid.
Digital Elevation Models of the U.S. Virgin Islands

29

Figure 21. Histogram of the differences between all NOS survey data and the 1/3 arc-second pre-surfaced bathymetric grid of St. Thomas and St. 

John.

Figure 22. Histogram of the difference between all CCMA multibeam data and the 1/3 arc-second pre-surfaced bathymetric grid of St. Thomas 

and St. John.
Grothe et al., 2010

30

Figure 23. Histogram of the difference between on the USGS submerged bathymetric lidar data and the 1/3 arc-second pre-surface bathymetric 

grid of St. Thomas and St. John.

Figure 24. Histogram of the difference between the CCMA multibeam data and the 1/3 arc-second pre-surface bathymetric grid of St. Croix.
Digital Elevation Models of the U.S. Virgin Islands

31

Figure 25. Histogram of the difference between the NOS survey data and the 1/3 arc-second pre-surface bathymetric grid of St. Croix.

Figure 26. Histogram of the differences between the University of Virgin Islands multibeam data and the 1/3 arc-second pre-surface bathymetric 

grid of St. Croix.
Grothe et al., 2010

32

3.3.3 Building the DEMs using MB-System

MB-System was used to create the U.S. Virgin Islands DEMs. The MB-System tool ‘mbgrid’ was used to

apply a tight spline tension to the xyz data, and interpolate values for cells without data. The data hierarchy used in

the ‘mbgrid’ gridding algorithm, as relative gridding weights, is listed in Table 13. Greatest weight was given to the

multibeam surveys and the EAARL lidar surveys. Least weight was given to the pre-surfaced bathymetric grid.

Table 13. Data hierarchy used to assign gridding weight in MB-System

Dataset Relative Gridding Weight

CCMA Multibeam 100

NGDC Multibeam 100

UVI Multibeam 100

NOS Surveys 100

USGS EAARL Submerged Lidar 100

USGS EAARL Topographic Lidar 100

NGDC Digitized Features 100

ENC Soundings 10

SRTM Topographic DEM 10

Trackline Soundings 1

NGDC Bathymetric Surface 0.1
Digital Elevation Models of the U.S. Virgin Islands

33

3.4 Quality Assessment of the DEMs

3.4.1 Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the U.S. Virgin Islands DEMs is dependent upon DEM cell size and source datasets. Topographic features have an estimated horizontal accuracy of

10-30 meters for the 1/3 arc-second DEMs and up to 90 meters for the 1 arc-second DEM: gridded USGS EAARL

topographic lidar data have an accuracy of approximately 1 meter and 1 arc-second SRTM DEM data is accurate to

approximately 30 meters and 3 arc-second SRTM data is accurate to approximately 90 meters. Bathymetric features

are resolved only to within a few hundreds of meters in deep-water areas. Shallow, near-coastal regions have an accuracy approaching that of sub-aerial topographic features. Positional accuracy is limited by the sparseness of deepwater soundings and potentially large positional uncertainty of pre-satellite navigated (e.g., GPS) NOS hydrographic

surveys.

3.4.2 Vertical accuracy

Vertical accuracy of elevation values in the U.S. Virgin Islands DEMs is also dependent upon the source

datasets contributing to DEM cell values. Topographic data have an estimated vertical accuracy of 15 centimeter for

the USGS EAARL topographic lidar data and 20 meters for the SRTM DEMs. Bathymetric values have an estimated

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

measurements from the early 20th century to recent, GPS-navigated multibeam swath sonar survey. Gridding interpolation to determine bathymetric values between sparse, poorly located NOS soundings degrades the vertical accuracy

of elevations in deep water.

3.4.3 Slope map and 3-D perspectives

ESRI ArcCatalog was used to generate a slope grid from the Virgin Islands DEMs to allow for visual inspection and identification of artificial slopes along boundaries between datasets (Figs. 27-29). The DEMs were transformed to WGS 84 UTM Zone 20 North coordinates (horizontal units in meters) in ArcCatalog for derivation of the

slope grid; equivalent horizontal and vertical units are required for effective slope analysis. Analysis of preliminary

grids using QT Modeler revealed suspect data points, which were corrected before recompiling the DEM. Figures

31-33 show a perspective rendering of the final DEMs. Figure 30 shows a data contribution plot of the U.S. Virgin

Islands DEMs.
Grothe et al., 2010

34

Figure 27. Slope map of the 1 arc-second U.S. Virgin Islands DEM. Flat-lying slopes are shown in white; dark shading denotes steep slopes; 

combined coastline indicated in red.
Digital Elevation Models of the U.S. Virgin Islands

35

Figure 28. Slope map of the 1/3 arc-second St. Thomas and St. John, U.S. Virgin Islands DEM. Flat-lying slopes are shown in white; dark 

shading denotes steep slopes; combined coastline indicated in red.

Figure 29. Slope map of the 1/3 arc-second St. Croix, U.S. Virgin Islands DEM. Flat-lying slopes are shown in white; dark shading denotes steep 

slopes; combined coastline indicated in red.
Grothe et al., 2010

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Figure 30. Data contribution plot of the U.S. Virgin Islands 1 arc-second DEM. Grey depicts DEM cells constrained by source data; white 

depicts cells with elevation values derived from interpolation. Due to the scale of the image, sparse soundings may not be visible in the graphic. 

Coastline indicated in red.
Digital Elevation Models of the U.S. Virgin Islands

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Figure 31. Perspective view from the southwest of the 1 arc-second U.S. Virgin Islands DEM. Vertical exaggeration - times 2.

Figure 32. Perspective view from the southwest of the 1/3 arc-second St. Thomas and St. John, U.S. Virgin Islands DEM. 

Vertical exaggeration - times 2.

Figure 33. Perspective view from the northeast of the 1/3 arc-second St. Croix, U.S. Virgin Islands DEM. No vertical exaggeration.
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3.4.4 Comparison with National Geodetic Survey geodetic monuments

The elevations of 335 geodetic monuments were extracted from the NOAA NGS web site (http://www.

ngs.noaa.gov/) in shapefile format (Fig. 34). Only 122 monuments with conditions noted as ‘GOOD’ or ‘MONUMENTED’were included in the analysis. Shapefile attributes give positions in NAD 83 geographic (typically sub-mm

accuracy) and elevations in local datum (in meters), which was assumed to be MSL. The elevations were converted

to MHW and compared to the Virgin Islands MHW DEM (Fig. 35). Differences between the DEM and the monument

elevations range from -73.28 to 79.44 meters. The mean distribution was ~ 3 meters. The overall large differences in

elevations are mostly due to the topographic data not representing bare earth or lidar data on St. John averaged from 1

meter resolution to 30 meters resolution of the grid. The largest difference of +/- 70 meters occurred on offshore rocks

where no topographic data existed. Rocks were digitized based on NOAA RNC charts.

Figure 34. Location of NGS geodetic monuments in the Virgin Islands region.

Figure 35. Histogram of the differences between NGS geodetic monument elevations and the U.S. Virgin Islands 1 arc-second DEM.
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3.4.5 Virgin Islands DEMs comparison with source data files

To ensure grid accuracy, the U.S. Virgin Islands DEMs were compared to source data files. Select bathymetric data and topographic data files were compared to the 1 arc-second DEM of the U.S. Virgin Islands, and the 1/3

arc-second DEMs of St. Thomas and St. John, and St. Croix using GDAL.

A histogram of the differences between data points from NOS data, CCMA multibeam data, topographic

lidar and the 1/3 arc-second St. Thomas and St. John DEM are shown in Figures 36 - 38. In Figure 36, the NOS data

points agree well with the grid with a mean around 0. Difference of 20 meters or more occur on the steep shelf where

multibeam data influence the cell value, and also along the edges of the land where topography data influence the cell

value. In Figure 37, the USGS EAARL topographic data points also agree well with the grid with a mean around 0.

Large differences are due to averaging the dense data points into a 10 meter cell that reflect both tree canopy and bare

earth elevations. In Figure 38, CCMA multibeam data points agree well with the grid with a mean around 0. Differences of up to +/- 100 meters occur where dense data at the edge of the steeply dipping shelf are averaged into a 10

meter grid cell.

Figure 36. Histogram of the differences between NOS data points and the 1/3 arc-second St. Thomas and St. John DEM.
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Figure 37. Histogram of the differences between USGS EAARL topographic lidar data points and the 1/3 arc-second St. Thomas and St. John 

DEM. 

Figure 38. Histogram of the differences between CCMA multibeam data points and the 1/3 arc-second St. Thomas and St. John DEM.
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A histogram of the differences between data points from NOS data, CCMAmultibeam data, and SRTM DEM

data and the 1/3 arc-second St. Croix DEM are shown in Figures 39 - 41. In Figure 39, the NOS data points agree

well with the grid with a mean around of 0. Differences up 100 meters occur on the steep shelf where multibeam data

influence the cell value, and also along the edges of the land where topography data influence the cell value. In Figure

40, CCMA multibeam data points also agree well with the grid with a mean around of 0.2. Differences of up to +/- 180

meters occur in the steep drop off from the shelf where multiple data points are averaged together. In figure 41, SRTM

DEM data points agree quite well with the grid with a mean around 0. Minor differences up to 20 meters occur along

the edge of the coast were bathymetry data falls in the same cell as the SRTM data.

Figure 39. Histogram of the differences between NOS data points and the 1/3 arc-second St. Croix DEM.

Figure 40. Histogram of the differences between CCMA multibeam data points and the 1/3 arc-second St. Croix DEM.
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Figure 41. Histogram of the difference between SRTM DEM data points and the 1/3 arc-second St. Croix DEM.

A histogram of the differences between NGDC multibeam data points and the 1 arc-second U.S. Virgin

Islands DEM are shown in Figure 42. The NGDC multibeam data points agree well with the grid with a mean value

around 0. Few differences up 195 meters occur along the edges of the surveys where the data are influenced by surrounding surveys. 

Figure 42. Histogram of the difference between the NGDC multibeam data points and the 1 arc-second U.S. Virgin Islands DEM.
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4. Summary and Conclusions

Three integrated bathymetric–topographic digital elevation models of the U.S. Virgin Islands region, with

cell sizes of 1 and 1/3 arc-second, were developed for the Pacific Marine Environmental Laboratory (PMEL), NOAA

Center for Tsunami Research. The best available digital data from U.S. federal and academic 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, ESRI ArcGIS World Imagery 2D, FME, GMT,

MB-System, QT Modeler, GDAL, and VDatum software.

Recommendations to improve the U.S. Virgin Islands DEM, based on NGDC’s research and analysis, include:

•	 Conduct high-resolution bathymetric multibeam surveys to cover the remainder area of the two 1/3 arcsecond grids.

•	 Conduct bare-earth topographic lidar surveys of St. Thomas, St. John, and St. Croix.

•	 Conduct high-resolution hydrographic surveys near the British Virgin Islands.

•	 Conduct deep-water multibeam surveys south of St. Croix.

5. Acknowledgments

The creation of the Virgin Islands DEMs was funded by the NOAA Pacific Marine Environmental Laboratory. The authors thank Nazila Merati, Marie Eble, and Vasily Titov (PMEL); Bryan Costa and Tim Battista (CCMA);

Uri ten Brink (USGS); Jeremiah Blondeau (UVI); Amar Nayega (USGS); and Dan Roman (NGS).

6. References

Electronic Navigational Chart #25640, 43rd Edition, 2008. Puerto Rico and Virgin Islands. Scale 1:326,856. U.S.

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

Electronic Navigational Chart #25641, 27th Edition, 2004. Virgin Gorda to St. Thomas and St. Croix and Krause

Lagoon Channel. Scale 1:100,000 and 1:20,000. U.S. Department of Commerce, NOAA, National Ocean

Service, Coast Survey.

Electronic Navigational Chart #25644, 13th Edition, 2003. Frederiksted Road and Frederiksted Pier. Scale 1:20,000

and 1:2,500. U.S. Department of Commerce, NOAA, National Ocean Service, Coast Survey.

Electronic Navigational Chart #25645, 18th Edition, 2006. Christiansted Harbor. Scale 1:10,000. U.S. Department of

Commerce, NOAA, National Ocean Service, Coast Survey.

Electronic Navigational Chart #25647, 11th Edition, 2006. Pillsbury Sound. Scale 1:15,000. U.S. Department of

Commerce, NOAA, National Ocean Service, Coast Survey.

Electronic Navigational Chart #25649, 19th Edition, 2003. St. Thomas Harbor. Scale 1:10,000. U.S. Department of

Commerce, NOAA, National Ocean Service, Coast Survey.

Electronic Navigational Chart #25650, 34th Edition, 2004. Virgin Passage and Sonda De Vieques. Scale 1:100,000.

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

Nautical Chart #25609, 5th Edition, 1996. West Indies Virgin Islands St. Thomas to Anegada. Scale 1:80,000. Defense

Mapping Agency.

Nautical Chart #25610, 8th Edition, 1996. West Indies British Virgin Islands Approaches to Gorda Strait. Scale

1:12,473. Defense Mapping Agency. Nautical Chart #25610, 22nd Edition, 1995. West Indies Virgin Islands

Approach to Road Harbour. Scale 1:30,000. Defense Mapping Agency.
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7. Data Processing Software

ArcGIS v. 9.3.1 – developed and licensed by ESRI, Redlands, California, Hhttp://www.esri.com/H

ESRI World Imagery (ESRI_Imagery_World_2D) – ESRI ArcGIS Resource Centers http://resources.esri.com/

arcgisonlineservices/.

FME 2009 GB – Feature Manipulation Engine, developed and licensed by Safe Software, Vancouver, BC, Canada,

Hhttp://www.safe.com/.

GDAL v. 1.7.1 – Geographic Data Abstraction Library is a translator library maintained by Frank Warmerdam, http://

gdal.org/.

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

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

GMT v. 4.3.4 – Generic Mapping Tools, freeware developed and maintained by Paul Wessel and Walter Smith, funded

by the National Science Foundation, Hhttp://gmt.soest.hawaii.edu/H

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

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

Quick Terrain Modeler v. 7.0.0 – LiDAR processing software developed by John Hopkins University’sApplied Physics

Laboratory (APL) and maintained and licensed by Applied Imagery, Hhttp://www.appliedimagery.com/H
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Appendix A. Vertical Datum Conversion Grids

NGDC created three offset grids approximating the relationship between MHW and MLLW, MHW and MLW,

and MHW and MSL for the Virgin Islands region (Figs. A1 - A3). The grids were built in ArcGIS using the ‘Inverse

Distance Weighting’ tool and the differences, in meters, between the vertical datums as measured at 6 NOAA tide

stations (http://tidesandcurrents.noaa.gov/) (see Table 12; see Fig. 18). The grids span from 63o W to 66o W and 16o N

to 20o N with a grid cell size of 3 arc-seconds. All vertical transformations to MHW were performed using these offset

grids developed by NGDC.

Creating the Virgin Islands DEM proved to be challenging for several data sources with vertical datums in

Ellipsoid heights or in a different geoid than Geoid 09. As of September 2009, there is no official vertical datum of

the Virgin Islands, but the National Geodetic Survey provided NGDC with a preliminary hybrid model (Geoid 09)

that relates the NAD 83 Ellipsoid (NSRS 2007) to the Virgin Islands Vertical Datum 2009 (VIVD 09)10. VIVD 09 is

assumed to be equivalent to MSL. The model was provided to NGDC as xyz format and NGDC used the GMT tool

‘surface’ to smoothly grid the data at 1 arc-minute, spanning from 61o W to 70o W and 15o N to 21o N (Fig. A5).

Before the model could be applied to the data, all data needed to be the NAD 83 Ellipsoid (NSRS 2007). Several

data sources have a vertical datum in the WGS 84 Ellipsoid. A conversion grid was built in ArcGIS using the ‘Inverse

Distance Weighting’ tool using the differences between the WGS 84 and NAD 83 Ellipsoid heights of 14 selected

points in the 1 arc-second DEM (Table A4). The difference values were found using the NGS Horizontal Time

Dependent Positioning web site (http://www.ngs.noaa.gov/cgi-bin/HTDP/htdp.prl?f1=4&f2=1).

Table A1. Difference values between the WGS 84 and NAD 83 Ellipsoids.

Latitude Longitude Difference

-64 19 1.863

-65 19 1.862

-64 17 1.904

-65 17 1.902

-64.25 18.25 1.879

-64.75 18.25 1.878

-64.25 17.75 1.889

-64.75 17.75 1.888

-64.5 19 1.863

-64.5 17 1.903

-64.5 18 1.883

-65.15 19 1.862

-65.15 17 1.902

-65.15 18 1.882

-64 18 1.884

10. This is a Hybrid model for Puerto Rico and Virgin Islands that relates NAD 83 (NSRS 2007) to the Puerto Rico Vertical Datum 2002 (PRVD

02) surface in Puerto Rico and VIVD 09 datum in the U.S. Virgin Islands. VIVD 09 was not officially released at the time this DEM was created,

but any changes in the model are only expected to be on the mm-scale, which is miniscule compared to the overall vertical accuracy of the DEM.
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Figure A1. Color image of the conversion grid from MLLW to MHW using offsets from tide stations. 
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Figure A2. Color image of the conversion grid from MLW to MHW using offset from tide stations.
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Figure A3. Color image of the conversion grid from MSL to MHW using offset from tide station
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Figure A4. Color image of the conversion grid used to convert data from the WGS 84 ellipsoid to the NAD 83 ellipsoid.
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Figure A5. Color image of the conversion grid used to convert data from the NAD 83 ellipsoid to the preliminary VIVD 09 Geoid. VIVD 09 is 

assumed MSL.
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