Digital Elevation Models of Pago Pago, American Samoa:

Procedures, Data Sources and Analysis

Prepared for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami 

Research by the NOAA National Geophysical Data Center (NGDC)

September 30, 2009

Lim, E.1, L.A. Taylor2, B.W. Eakins1, K.S. Carignan1, P.R. Medley1 . R.J. Caldwell1, and D.Z. Friday1

Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder1

NOAA, National Geophysical Data Center, Boulder, Colorado2

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/inundation/

Digital Elevation Models of Pago Pago, American Samoa

iii

Contents

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

2. Study Area..........................................................................................................................................3

3. Methodology ......................................................................................................................................4

3.1 Data Sources and Processing ......................................................................................................5

3.1.1 Shoreline................................................................................................................6

3.1.2 Bathymetry ............................................................................................................8

3.1.3 Topography..........................................................................................................15

3.1.4 Bathymetry-Topography .....................................................................................17

3.2 Establishing Common Datums .................................................................................................18

3.2.1 Vertical datum transformations ...........................................................................18

3.2.2 Horizontal datum transformations.......................................................................18

3.3 Digital Elevation Model Development .....................................................................................19

3.3.1 Verifying consistency between datasets..............................................................19

3.3.2 Smoothing of bathymetric data ...........................................................................19

3.3.3 Gridding the data with MB-System.....................................................................20

3.4 Quality Assessment of the DEMs.............................................................................................20

3.4.1 Horizontal accuracy.............................................................................................20

3.4.2 Vertical accuracy .................................................................................................20

3.4.3 Slope maps and 3-D perspectives........................................................................20

3.4.4 Comparison with source data files.......................................................................23

3.4.5 Comparison with NGS geodetic monuments......................................................24

4. Summary and Conclusions...............................................................................................................25

5. Acknowledgments............................................................................................................................25

6. References........................................................................................................................................25

7. Data Processing Software ................................................................................................................26

List of Figures

Figure 1. Shaded-relief image of the 3 arc-second Pago Pago DEM.........................................................1

Figure 2. Shaded-relief image of the 1/3 arc-second Pago Pago DEM......................................................2

Figure 3. Photograph of Pago Pago Harbor ...............................................................................................3

Figure 4. Source and coverage of datasets available in the American Samoa region ................................5

Figure 5. IKONOS satellite imagery of Tutuila Island ..............................................................................6

Figure 6. NGDC-digitized coastline...........................................................................................................7

Figure 7. Spatial coverage of bathymetric datasets available in the American Samoa region...................8

Figure 8. Coverage of multibeam swath sonar surveys used in compiling the 

 Pago Pago DEMs...........................................................................................................................................10

Figure 9. Spatial coverage of trackline surveys in the Pago Pago region ................................................12

Figure 10. Spatial coverage of estimated depths from Gaia Geo-Analytical draped over IKONOS 

 satellite imagery.................................................................................................................13

Figure 11. NGDC-digitized depths at Pago Pago International Airport.....................................................14

Figure 12. 1 arc-second NED topographic DEM offset 705 meters to the southeast with underlying

 IKONOS satellite imagery.................................................................................................16

Figure 13. NGDC-digitized elevations at Pago Pago International Airport...............................................16

Figure 14. Spatial coverage of the NAVOCEANO bathymetric-topographic lidar...................................17

Figure 15. Histogram of the differences between NAVOCEANO lidar and the 1 arc-second 

 bathymetric grid.................................................................................................................19

Figure 16. Slope map of the 1/3 arc-second Pago Pago DEM...................................................................21

Figure 17. Perspective view from the northeast of the 1/3 arc-second Pago Pago DEM ..........................21

Figure 18. Slope map of the 3 arc-second Pago Pago DEM......................................................................22

Figure 19. Perspective view from the northwest of the 3 arc-second Pago Pago DEM ............................22
Lim et al., 2009

iv

Figure 20. Histogram of the differences between the Kilo Moana multibeam swath sonar survey 

 and the 3 arc-second Pago Pago DEM..............................................................................23

Figure 21. Histogram of the differences between NED data and the 3 arc-second Pago Pago DEM........23

Figure 22. Location of NGS geodetic monuments.....................................................................................24

Figure 23. Histogram of the differences between NGS geodetic monuments and the 3 arc-second4

 Pago Pago DEM................................................................................................................25

List of Tables

Table 1a. PMEL specifications for the 3 arc-second Pago Pago DEM ......................................................4

Table 1b. PMEL specifications for the 1/3 arc-second Pago Pago DEM ...................................................4

Table 2. Shoreline datasets used in compiling the Pago Pago DEMs.......................................................6

Table 3. Bathymetric datasets used in compiling the Pago Pago DEMs..................................................8

Table 4. Multibeam swath sonar surveys used in compiling the Pago Pago DEMs.................................9

Table 5. Trackline surveys used in compiling the 3 arc-second Pago Pago DEM .................................11

Table 6. NOAA nautical chart in the Pago Pago region .........................................................................14

Table 7. Topographic datasets used in compiling the Pago Pago DEMs................................................15

Table 8. Bathymetric-topographic dataset used in compiling the Pago Pago DEMs.............................17

Table 9. Relationships between mean high water and mean sea level in the Pago Pago region ............18

Table 10. Data hierarchy used to assign gridding weight in MB-System.................................................20
Digital Elevation Models of Pago Pago, American Samoa:

Procedures, Data Sources and Analysis

1. Introduction

The National Geophysical Data Center (NGDC), an office of the National Oceanic and Atmospheric Administration 

(NOAA), has developed two integrated bathymetric–topographic digital elevation models (DEMs) centered on Pago 

Pago, American Samoa, for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami 

Research (http://nctr.pmel.noaa.gov/). The coastal DEMs will be used as input for the Method of Splitting Tsunami 

(MOST) model development by PMEL to simulate tsunami generation, propagation, and inundation. A 3 arc-second 

DEM1 (Fig. 1) was generated from diverse digital datasets in the region (grid boundary and sources shown in Fig. 4) 

and will be used for tsunami modeling, as part of the tsunami forecast system SIFT (Short-term Inundation Forecasting 

for Tsunamis) developed by PMEL for the NOAA Tsunami Warning Centers. To increase the forecasting accuracy of 

SIFT, a smaller 1/3 arc-second DEM (Fig. 2) was generated for the immediate area surrounding Pago Pago, where 

high-resolution multibeam bathymetric data were available. This report provides a summary of the data sources and 

methodology used in developing the Pago Pago DEMs.

Figure 1. Shaded-relief image of the 3 arc-second Pago Pago DEM. Contour interval is 1000 meters for bathymetry 

and 250 meters for topography.

1. The Pago Pago 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 Pago Pago (14°17’S, 170°42’W) 1/3 arcsecond of latitude is equivalent to 10.24 meters; 1/3 arc-second longitude equals 9.99 meters. Three arc-seconds of latitude is equivalent to 92.20 

meters; 3 arc-seconds of longitude is equivalent to 89.92 meters.
Lim et al., 2009

2

Figure 2. Shaded-relief image of the 1/3 arc-second Pago Pago DEM. Contour interval is 500 meters for the bathymetry 

and 200 meters for the topography. 
Digital Elevation Models of Pago Pago, American Samoa

3

2. Study Area

The Pago Pago DEMs encompass the American Samoa islands of Tutuila, Ofu, Olosega, and Ta’u (see Figs. 

1, 2, and 4). American Samoa is an unincorporated territory of the United States and is located between 11 and 15 

degrees south latitude and 171 and 169 degrees west longitude in the South Pacific Ocean. Tutuila is the largest island 

and covers an area of 55 square miles. Tutuila is the eroded summit of a large basaltic volcano that formed 1.54 to 

1 million years ago during the Early Pleistocene era. Offshore, the island is characterized by a drowned coastline 

and underdeveloped fringing reefs. Inland areas feature lush, steep, and narrow valleys. Pago Pago, the capital of 

American Samoa, is home to roughly 11,500 people and is located in Pago Pago Harbor on the island of Tutuila (Fig. 

3). Pago Pago Harbor is one of the world’s largest natural harbors and was formed by submergence of the volcano’s 

caldera.

(http://www.soest.hawaii.edu/pibhmc/pibhmc_nwhi.htm)

Figure 3. Photograph of Pago Pago Harbor.

(http://lh5.ggpht.com/_aup3I7mpcB4/RyEl-IcvR4I/AAAAAAAAEmo/veqZZMlMTLc/Pago+Pago+Harbour+(left).jpg)
Lim et al., 2009

4

3. Methodology

The Pago Pago DEMs were constructed to meet PMEL specifications (Tables 1a and 1b), 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 digital data were obtained by NGDC and shifted to common 

horizontal and vertical datums of World Geodetic System 1984 (WGS 84) and mean high water (MHW), respectively, 

for modeling of 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 1a: PMEL specifications for the 3 arc-second Pago Pago DEM.

Grid Area American Samoa

Coverage Area 171.14o to 169.30o W; 14.73o to 13.83o S

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System 1984 (WGS 84)

Vertical Datum Mean high water (MHW)

Vertical Units Meters

Cell Size 3 arc-second

Grid Format ESRI Arc ASCII grid

Table 1b: PMEL specifications for the 1/3 arc-second Pago Pago DEM.

Grid Area Tutuila Island

Coverage Area 170.95o to 170.45o W; 14.40o to 14.18o S

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System 1984 (WGS 84)

Vertical Datum Mean high water (MHW)

Vertical Units Meters

Cell Size 1/3 arc-second

Grid Format ESRI Arc ASCII grid
Digital Elevation Models of Pago Pago, American Samoa

5

3.1 Data Sources and Processing

Shoreline, bathymetric, and topographic digital datasets (Fig. 4) were obtained from several U.S. federal 

agencies, academic institutions, and private companies including: NOAA NGDC and Coastal Services Center (CSC); 

Scripps Institution of Oceanography (SIO); Gaia Geo-Analytical; the U.S. Geological Survey (USGS); and the Naval 

Oceanographic Office (NAVOCEANO). NGDC reviewed, but did not use, data available from Fagetele Bay National 

Marine Sanctuary (FBNMS; http://dusk.geo.orst.edu/djl/samoa/), because the datasets were available from the primary 

sources. Safe Software’s (http://www.safe.com/) FME data translation tool package was used to shift datasets to WGS 

84 horizontal datum and to convert them into ESRI (http://www.esri.com/) ArcGIS shapefiles. The shapefiles were 

then displayed with ArcGIS to assess data quality and manually edit datasets. Vertical datum transformations to MHW 

were accomplished using FME and ArcGIS, based upon data from NOAA tide station #1770000 at Pago Pago. Applied 

Imagery’s Quick Terrain Modeler software (http://www.appliedimagery.com/) was used for evaluating datasets before 

the final gridding process.

Figure 4. Source and coverage of datasets available for the American Samoa region. Areas of no data are white. 
Lim et al., 2009

6

3.1.1 Shoreline

In 2002, NOAA Coastal Services Center (CSC) digitized a coastline of the American Samoa islands of Tutuila, 

Ofu, Olosega, and Ta’u (Table 2). The vectorized shorelines were derived from one meter, panchromatic 2002 IKONOS 

satellite imagery (e.g., Fig. 5). Areas of the coastline that were obscured by cloud cover in the IKONOS imagery were 

derived from USGS digital quadrangles by NOS/CSC. 

NGDC digitized the coastline at Pago Pago International Airport to represent ponds along the runway (Fig. 6). 

The coastline was digitized using 2002 IKONOS satellite imagery, which has a resolution of approximately 1 meter. 

Table 2: Shoreline datasets used in compiling the Pago Pago DEMs. 

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Coordinate System URL

CSC 2002 vector ~ 1 meter WGS 84 geographic MHW http://dusk.geo.orst.

edu/djl/samoa/

NGDC 2009 Digitized 

coastline n/a WGS 84 geographic n/a n/a

Figure 5. IKONOS satellite imagery of Tutuila Island. CSC coastline in red.
Digital Elevation Models of Pago Pago, American Samoa

7

NOS/CSC coastline

Figure 6. NGDC-digitized coastline. Coastline was digitized to represent elevations below zero along the runway at Pago Pago 

International Airport. IKONOS satellite imagery in background.

CSC coastline
Lim et al., 2009

8

3.1.2 Bathymetry

Bathymetric datasets available for the compilation of the Pago Pago DEMs included: fifteen multibeam swath 

sonar surveys from the NGDC Multibeam Bathymetry Database, estimated depths derived from satellite imagery 

from Gaia Geo-Analytical, and 26 trackline geophysics surveys (Table 3; Fig. 7). The NOS hydrographic survey 

(http://www.ngdc.noaa.gov/mgg/bathymetry/hydro.html) available for the region (H09188) was not used in DEM 

development because it was superseded by more recent, higher-resolution data.

Table 3: Bathymetric datasets used in compiling the Pago Pago DEMs.

Source Year Data Type Spatial Resolution

Original 

Horizontal Datum/

Coordinate System

Original 

Vertical 

Datum

URL

NGDC

1996 

to 

2005

Multibeam swath 

sonar

raw MB files gridded 

to 1 and 3 arcseconds

WGS 84 

geographic

assumed 

MSL

http://www.ngdc.

noaa.gov/mgg/

bathymetry/

multibeam.html

Gaia GeoAnalytical 2008

Estimated depths 

from satellite 

imagery

~ 5 meters WGS 84 

geographic

assumed 

MSL

http://dusk2.geo.orst.

edu/djl/samoa/

NGDC

1962

to 

1998

Trackline (single 

beam echo-sounder)

soundings up to 

100’s of meters along 

profiles spaced kms 

apart

WGS 84 

geographic

assumed 

MSL

http://www.ngdc.

noaa.gov/mgg/

geodas/trackline.html

NGDC 2009 Digitized points n/a WGS 84 

geographic n/a n/a

Figure 7. Spatial coverage of bathymetric datasets available in the American Samoa region. Areas of no data are white. Topography is green.
Digital Elevation Models of Pago Pago, American Samoa

9

1) NGDC Multibeam Swath Sonar Surveys

Fifteen multibeam swath sonar surveys were available from the NGDC Multibeam Bathymetry Database 

(http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html) for use in building the Pago Pago DEMs 

(Table 4; Fig. 8). This database is comprised of the original swath sonar data from surveys conducted mostly 

by the U.S. academic fleet. Using MB-System2, the data were gridded by survey to 1 and 6 arc-second cell 

size for the 1/3 and 3 arc-second DEMs, respectively. After assessing individual survey quality, the gridded 

data were transformed to MHW (see section 3.2.1) and converted to shapefiles for editing in ArcMap. Prior 

to gridding of the preliminary bathymetric surfaces and final grids, NGDC edited out noise along the swath 

edges and then converted the files to xyz format using FME.

 Table 4: Multibeam swath sonar surveys used in compiling the Pago Pago DEMs.

Cruise ID Ship Year

Original Vertical 

Datum

Original Horizontal 

Datum Institution

AHI-04-02 Ahi 2002 Assumed MSL WGS 84 geographic NOAA Pacific Islands Fisheries

Science Center

AHI-06-02 Ahi 2002 Assumed MSL WGS 84 geographic NOAA Pacific Islands Fisheries

Science Center

AVON02MV Melville 1999 Assumed MSL WGS 84 geographic SIO

AVON03MV Melville 1999 Assumed MSL WGS 84 geographic SIO

BMRG08MV Melville 1996 Assumed MSL WGS 84 geographic SIO

BMRG09MV Melville 1996 Assumed MSL WGS 84 geographic SIO

COOK15MV Melville 2001 Assumed MSL WGS 84 geographic SIO

DRFT09RR Roger Revelle 2002 Assumed MSL WGS 84 geographic SIO

DRFT10RR Roger Revelle 2002 Assumed MSL WGS 84 geographic SIO

HI-06-02 Hi'ialakai 2002 Assumed MSL WGS 84 geographic NOAA Pacific Islands Fisheries

Science Center

KIWI05RR Roger Revelle 1997 Assumed MSL WGS 84 geographic SIO

KIWI12RR Roger Revelle 1998 Assumed MSL WGS 84 geographic SIO

KMO506 Kilo Moana 2005 Assumed MSL WGS 84 geographic SIO

USF2001samoa Bellows 2001 Assumed MSL WGS 84 geographic University of South Florida

USF2002samoa Bellows 2002 Assumed MSL WGS 84 geographic University of South Florida

2. 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 MB-System 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/ [Extracted from MB-System web site.]
Lim et al., 2009

10

Figure 8. Coverage of multibeam swath sonar surveys used in compiling the Pago Pago DEMs. Topography shown in green.
Digital Elevation Models of Pago Pago, American Samoa

11

2) NGDC Trackline surveys

Twenty-three single-beam echo-sounder surveys were available from the NGDC Marine Geophysical 

Trackline Database (http://www.ngdc.noaa.gov/mgg/geodas/trackline.html) for use in building the Pago 

Pago DEMs (Table 5; Fig. 9). This database is comprised of bathymetry, magnetics, gravity, and seismic 

navigation data collected along ship track during marine cruises from 1953 to present. The bathymetric data 

were downloaded as xyz files in WGS 84 and MSL, and converted to MHW using FME.

The tracklines are spaced tens of kilometers apart and were only used where there were no high-resolution 

multibeam bathymetric data. Trackline data were not used in building the 1/3 arc-second DEM.

Table 5: Trackline surveys used in compiling the 3 arc-second Pago Pago DEM.

Cruise ID Ship Year

Original Vertical 

Datum

Original Horizontal 

Datum Institution

82031602 Kana Keoki 1985 assumed MSL WGS 84 geographic University of Hawaii

AMPHH03AR Argo 1980 assumed MSL WGS 84 geographic SIO

CK80-1 Machias 1983 assumed MSL WGS 84 geographic University of Hawaii

CK80-2 Machias 1983 assumed MSL WGS 84 geographic University of Hawaii

ELT31 Eltanin 1979 assumed MSL WGS 84 geographic Lamont-Doherty Earth Observatory

ERDC02WT T. Washington 1974 assumed MSL WGS 84 geographic SIO

GECS-FMV Melville 1974 assumed MSL WGS 84 geographic SIO

MW8701 Moana Wave 1987 assumed MSL WGS 84 geographic University of Hawaii

MW8702 Moana Wave 1987 assumed MSL WGS 84 geographic University of Hawaii

NBP98-6A Nathaniel Palmer 1998 assumed MSL WGS 84 geographic SIO

NOVA02AR Argo 1967 assumed MSL WGS 84 geographic SIO

NOVA06HO Horizon 1967 assumed MSL WGS 84 geographic SIO

NOVA07AR Argo 1967 assumed MSL WGS 84 geographic SIO

NOVA08AR Argo 1967 assumed MSL WGS 84 geographic SIO

PPTU03WT T. Washington 1985 assumed MSL WGS 84 geographic SIO

SOTW10WT T. Washington 1972 assumed MSL WGS 84 geographic SIO

SOTW11WT T. Washington 1972 assumed MSL WGS 84 geographic SIO

STYX02AZ Agassiz 1968 assumed MSL WGS 84 geographic SIO

STYX05AZ Agassiz 1968 assumed MSL WGS 84 geographic SIO

WS79-1 Machias 1979 assumed MSL WGS 84 geographic University of Hawaii

V1814 Vema 1962 assumed MSL WGS 84 geographic Lamont-Doherty Earth Observatory

V1906 Vema 1963 assumed MSL WGS 84 geographic Lamont-Doherty Earth Observatory

V3610 Vema 1980 assumed MSL WGS 84 geographic Lamont-Doherty Earth Observatory
Lim et al., 2009

12

Figure 9. Spatial coverage of trackline surveys in the Pago Pago region. Trackline data shown in blue were not used in the final DEM 

gridding, as there were high-resolution multibeam bathymetric data available in those areas. Topography shown in green.
Digital Elevation Models of Pago Pago, American Samoa

13

3) Gaia Geo-Analytical Estimated Depths 

Kyle Hogrefe of Gaia Geo-Analytical, in affiliation with Oregon State University (OSU), completed 

a master thesis on a depth analysis of Tutuila using high-resolution satellite imagery (Hogrefe, 2008). Mr. 

Hogrefe graciously provided NGDC with the resulting data (Fig. 10). 

Mr. Hogrefe derived bathymetry from the 2002 multispectral IKONOS satellite imagery provided by 

the National Center for Coastal Monitoring and Assessment (NCCMA). Bathymetry values shallower than 

25 meters were derived by gauging the relative attenuation of blue and green spectral radiance as a function 

of depth. Environment for Visualizing Images 4.5 was used to analyze and process the image. Data editing 

and integration were performed using ArcGIS 9.2. Corrections were made for atmospheric absorption and 

scattering.

Since no vertical datum was documented, NGDC assumed the data were essentially equivalent to MSL. 

The raster files were converted to shapefiles and transformed to MHW using FME for assessing the data. 

Figure 10. Spatial coverage of estimated depths from Gaia Geo-Analytical draped over IKONOS satellite imagery. 

Gaia
Lim et al., 2009

14

4) NGDC-Digitized Depths

 NGDC digitized bathymetric values at Pago Pago International Airport using IKONOS satellite imagery 

to represent bodies of water not present in other datasets. Elevations of -1 meter were assigned to these points 

(Fig. 11). 

Figure 11. NGDC-digitzed depths at Pago Pago International Airport. IKONOS satellite imagery in background.

5) NOAA Nautical Chart

NOAA nautical chart #83484 was available for the Samoa Islands as a georeferenced Raster Nautical 

Chart (RNC; Table 6). The chart was downloaded from NOAA’s Office of Coast Survey website (http://

nauticalcharts.noaa.gov). RNC #83484 was used to assess the quality of bathymetric datasets in the region.

 

Table 6: NOAA nautical chart in the Pago Pago region.

Chart Title Edition Edition Date Format Scale

83484 Samoa Islands 11th 2006 RNC 1:10,000 to 

1:80,000
Digital Elevation Models of Pago Pago, American Samoa

15

3.1.3 Topography

The USGS 1 arc-second National Elevation Dataset (NED) DEM provided full coverage of the islands and 

was used to build the Pago Pago DEMs (Table 7; Fig. 12). The NED data was supplemented with NGDC-digitized 

elevations.

Table 7: Topographic datasets used in compiling the Pago Pago DEMs.

Source Year Data Type Spatial

Resolution

Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum URL

USGS 1999-

2006 NED DEM 1 arcsecond WGS 84 geographica MSLa http://ned.usgs.gov/

NGDC 2009 Digitized 

points n/a WGS 84 geographic n/a n/a

a The metadata for the USGS NED topographic data indicates that the horizontal and vertical datums are NAD 83 geographic 

and NAVD88, respectively. The actual datums as denoted on the USGS topographic quadrangles for the

region indicate that the horizontal datum is WGS 84 geographic and vertical datum is mean sea level.

1) USGS NED topographic DEM

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

complete 1 arc-second coverage of the Samoa Islands3. Data are documented in the metadata as being in 

NAD 83 geographic coordinates and NAVD88 vertical datum (meters), and are available for download as 

raster DEMs. The bare-earth elevations have a vertical accuracy of +/- 7 to 15 meters depending on source 

data resolution. See the USGS Seamless web site for specific source information (http://seamless.usgs.gov/). 

The dataset was derived from USGS quadrangle maps and aerial photographs based on topographic surveys; 

it has been revised using data collected in 1999 and 2006. 

NGDC visually compared georeferenced IKONOS satellite imagery, USGS topographic quadrangles, 

a NOAA RNC, NGS monuments, lidar data, multibeam data, and estimated depths in the Pago Pago region 

with the NED dataset. Although all of these datasets align, the NED topographic DEM was offset to the 

southeast by ~705 meters (Fig. 12). The NED topographic dataset was rectified to align with the IKONOS 

satellite imagery. 

NGDC also visually compared georeferenced images (TIFFs) of USGS topographic quadrangles in the 

Samoa Islands with the NED dataset before and after its conversion to MHW. The 40-foot contours on the 

quadrangles are referenced to a vertical datum of MSL; the coastlines approximate the location of the MHW 

line. NGDC has concluded that the NED DEM in the Samoa Islands are actually in a mixed vertical datum 

(e.g., see NED documentation in Lim et al., 2009), with values above 40 feet relative to MSL, and the coastal 

“zero” value relative to MHW. The original NED DEM also included “zero” elevation values over the open 

ocean, which were removed from the dataset by clipping to the combined coastline. Values between zero 

(MHW) and 40 feet (MSL) are not consistent with either datum. Note that in the Samoa Islands, the MHW 

coastline is at approximately the 1-foot (0.38 meter) MSL contour (see Table 9). 

In an effort to overcome this mixing of vertical datums, the NED DEM was converted from MSL to 

MHW by subtracting a constant value of 0.382 meters. After the conversion, some areas along the coast had 

elevations less than zero. To prevent inappropriate coastal flooding, elevations in the converted data that were 

greater than or equal to 0.5 meters were extracted directly from the grids. Elevations that were less than 0.5 

meters were assigned a value of 0.5 meters above MHW. To smooth the data for use in building the 1/3 arcsecond Pago Pago DEM, the NED DEM was sampled to 1/3 arc-second.

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 U.S. territories. NED has a consistent projection (Geographic), resolution (1 arc second), and elevation units (meters). The horizontal 

datum is NAD 83, except for AK (NAD 27) and the Pacific Islands (WGS 84). The vertical datum is NAVD88, except for AK (NGVD29) and the Pacific Islands (MSL). NED is a living dataset that is updated bimonthly to incorporate the “best available” DEM data. As more 1/3 arc second (10 meter) data covers the U.S., then this will also be a seamless dataset. [Extracted from USGS NED website and adapted based on inspection of NED data]
Lim et al., 2009

16

Figure 12. 1 arc-second NED topographic DEM offset 705 meters to the southeast relative to underlying IKONOS satellite imagery

2) NGDC-Digitized Elevations

NGDC digitized elevation points to supplement the NED at Pago Pago International Airport and where

breakwaters are present within the 1/3 arc-second Pago Pago DEM (e.g., Fig. 13). 

.

Figure 13. NGDC-digitzed elevations at Pago Pago International Airport. IKONOS satellite imagery in background.
Digital Elevation Models of Pago Pago, American Samoa

17

3.1.4 Bathymetry-Topography

Bathymetric-topographic lidar data obtained from the Naval Oceanographic Office (NAVOCEANO) provides 

coverage of the coasts of Pago Pago Harbor (Table 8; Fig. 14). The survey is from May 2006 and has a spatial resolution 

of 5 meters. Its horizontal and vertical datums are WGS 84 geographic and mean sea level (MSL), respectively. The 

lidar data were not processed to bare-earth so, because of the lush vegetation in the area, topographic values were not 

used in DEM development. Bathymetric values were converted from MSL to MHW by subtracting a constant value 

of 0.382 meters (see Table 9). 

Table 8: Bathymetric-topographic dataset used in compiling the Pago Pago DEMs.

Source Year Data Type Spatial

Resolution

Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum URL

NAVOCEANO 2006

Bathymetrictopographic 

lidar

5 meters WGS 84 Geographic MSL https://www.navo.

navy.mil/

Figure 14. Spatial coverage of the NAVOCEANO bathymetric-topographic lidar. IKONOS satellite imagery in background.
Lim et al., 2009

18

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation and evaluation of the Pago Pago DEMs were originally referenced to a vertical 

datum of MSL. All datasets were transformed to MHW.

1) Bathymetric data

The multibeam, trackline surveys, and estimated depths were transformed from MSL to MHW using 

FME software, by subtracting a constant offset of 0.382 meters, as measured at the Pago Pago NOAA tide 

station #1770000 (Table 9; http://tidesandcurrents.noaa.gov/). 

2) Topographic data

The NED topographic DEM was determined to be in MSL vertical datum. Conversion to MHW was 

accomplished by subtracting a constant value of 0.382 meters (Table 9). 

 Table 9: Relationships between mean high water and mean sea level in the Pago Pago region.

Vertical Datums Difference to MHW

MSL 0.382 meters

Note: Datum relationship was determined using NOAA tidal station #1770000 at Pago Pago.

 

3.2.2 Horizontal datum transformations

All datasets used to compile the Pago Pago DEMs were originally referenced to WGS 84 geographic and no 

horizontal datum conversions were necessary. 
Digital Elevation Models of Pago Pago, American Samoa

19

3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

After vertical transformations were applied, the resulting ESRI shapefiles were checked in ArcMap for consistency 

between datasets. 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:

•	 Multibeam swath sonar survey data contained many erroneous values along the edges of swaths.

•	 Outside of Pago Pago Harbor, estimated depths were the only recent data available for the shallow water.

•	 NED topographic DEM was shifted south relative to the georeferenced IKONOS satellite imagery, USGS 

topographic quadrangles, a NOAA RNC, NGS monuments, lidar data, multibeam data, and estimated 

depths.

•	 Incomplete high-resolution multibeam bathymetry in the deep ocean.

•	 Many anomalous estimated depths (20-25 meters) right at the coast.

3.3.2 Smoothing of bathymetric data

Two ‘pre-surface’ bathymetric grids were generated for the Pago Pago DEMs due to the varying resolution of 

data coverage in the deep ocean. The NGDC multibeam swath sonar surveys are high resolution with beam spacing 

approximately 10 meters apart in shallow water, a marked contrast with the trackline surveys spaced 1 to 15 kilometers 

apart. The grids were generated using GMT4. The bathymetric point data were median-averaged using the GMT tool 

‘blockmedian’ to create 1 and 3 arc-second grids 0.05 degrees (~5%) larger than the 1/3 and 3 arc-second Pago Pago 

DEM gridding extents, respectively. 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 final coastline (to eliminate data interpolation into land areas). The resulting surface was 

compared with bathymetric source data to ensure grid accuracy (e.g., Fig. 15) and exported as an xyz file for use in the 

final gridding process (see Table 10). The statistical analysis of the differences between the 1 arc-second bathymetric 

surface and bathymetric values extracted from the NAVOCEANO lidar show that the majority of the lidar depths 

are in agreement with the bathymetric surface (Fig. 15). The largest differences result from averaging of multiple, 

closely-spaced lidar depths in regions of steep bathymetry.

Figure 15. Histogram of the differences between NAVOCEANO lidar and the 1 arc-second bathymetric grid.

4. 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.]
Lim et al., 2009

20

3.3.3 Gridding the data with MB-System

MB-System was used to create the Pago Pago 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 10. Greatest weight was given to the digitized 

features, lidar, and NED topography. Least weight was given to the coastline, trackline surveys, and the pre-surfaced 

bathymetric grid. 

 Table 10. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

NGDC-digitized features 1,000

NAVOCEANO lidar 1,000

NED Topographic DEM 1,000

NGDC Multibeam surveys 100

Gaia Geo-Analytical Satellite Imagery Estimated Depths 100

Pre-surfaced bathymetric grid 1

NGDC Trackline 1

NOAA/CSC coastline 1

3.4 Quality Assessment of the DEMs

3.4.1. Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Pago Pago DEMs are dependent upon cell 

location and the datasets used to determine corresponding DEM cell values. Topographic features have an estimated 

accuracy of ~30 meters (NED). Bathymetric features are resolved only to within a few kilometers in deep-water 

areas. Shallow, near-coast regions have an accuracy approaching 25 meters for the Gaia estimated bathymetry and 

10 meters for the NAVOCEANO lidar. 

3.4.2 Vertical accuracy

Vertical accuracy of elevation values for the Pago Pago DEMs are also highly dependent upon the source datasets 

contributing to DEM cell values. The NED topographic DEM has an estimated vertical accuracy of up to 7 meters. 

NAVOCEANO lidar has an estimated accuracy of 1 to 2 meters. Bathymetric values have an estimated accuracy 

between 1 meter and 5% of water depth. The deep water values in the 3 arc-second DEM have an estimated accuracy 

of a few hundred meters due to gridding interpolation between sparse trackline soundings.

3.4.3 Slope maps and 3-D perspectives

ESRI ArcCatalog was used to generate slope grids from the Pago Pago DEMs to allow for visual inspection 

and identification of artificial slopes along boundaries between datasets (e.g., Figs. 16 and 18). The DEMs were 

transformed to UTM Zone 2 South 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 

revealed suspect data points, which were corrected before recompiling the DEMs. Three-dimensional viewing of 

the UTM-transformed DEMs was accomplished using ESRI ArcScene and QT Modeler. Figures 17 and 19 show 

perspective views of the 1/3 and 3 arc-second Pago Pago DEMs in their final versions, created by Persistence of 

VisionTM Raytracer (POV-Ray).
Digital Elevation Models of Pago Pago, American Samoa

21

Figure 16. Slope map of the 1/3 arc-second Pago Pago DEM. Flat-lying slopes are white; 

dark shading denotes steep slopes; coastline in red.

Figure 17. Perspective view from the northeast of the 1/3 arc-second Pago Pago DEM. 
Lim et al., 2009

22

Figure 18. Slope map of the 3 arc-second Pago Pago DEM. Flat-lying slopes are white; dark 

shading denotes steep slopes; coastline in red.

Figure 19. Perspective view from the northwest of the 3 arc-second Pago Pago DEM
Digital Elevation Models of Pago Pago, American Samoa

23

3.4.4	 Comparison with source data files

To ensure grid accuracy, the Pago Pago DEMs were 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 differences between a multibeam 

survey and the 3 arc-second Pago Pago DEM is shown in Figure 20. The greatest differences occurred due to averaging 

of overlapping surveys in deep-water swaths where the data are noisiest. 

Figure 20. Histogram of the differences between the Kilo Moana multibeam swath sonar survey and the 3 arc-second Pago 

Pago DEM. 

A histogram of the differences between the 1 arc-second NED topographic DEM and the 3 arc-second Pago Pago 

DEM is shown in Figure 21. Differences range from -131 meters to +125 meters. The greatest differences occur in 

areas of steep and rugged topography where multiple NED values contribute to each cell in the 3 arc-second DEM.

Figure 21. Histogram of the differences between NED data and the 3 arc-second Pago Pago DEM. 
Lim et al., 2009

24

3.4.5 Comparison with NGS geodetic monuments

The elevations of 232 NOAA NGS geodetic monuments (Fig. 22) were extracted from online shapefiles of 

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

geographic5 and elevations in American Samoa Vertical Datum of 2002 (ASVD 02). The basis for all ASVD 02 heights 

is mean sea level, for the epoch 1983-2001 (Zilkoski, 2009). Elevations were shifted from MSL/ASVD 02 to MHW 

vertical datum (see Table 9) for comparison with the 3 arc-second Pago Pago DEM. Differences between the 3 arcsecond Pago Pago DEM and the NGS geodetic monument elevations range from -130 to +68 meters (Fig. 23). The 

outliers are from horizontal inaccuracy (~ 20-30 meters) of topographic features derived from NED, due to the 705 

meter offset rectified and described in Section 3.1.3. 

Figure 22. Location of NGS geodetic monuments. NGS monuments were used to evaluate the 3 arc-second Pago Pago DEM.

Figure 23. Histogram of the differences between NGS geodetic monuments and the 3 arc-second Pago Pago DEM.

5. The horizontal difference between the North American Datum of 1983 (NAD 83) and World Geodetic System of 1984 (WGS 84) geographic 

horizontal datums is approximately one meter across the contiguous U.S., which is significantly less than the cell size of the DEM. Most GIS applications treat the two datums as identical, so do not actually transform data between them, and the error introduced by not converting between 

the datums is insignificant for our purposes. NAD 83 is restricted to North America, while WGS 84 is a global datum. As tsunamis may originate 

most anywhere around the world, tsunami modelers require a global datum, such as WGS 84 geographic, for their DEMs so that they can model the 

wave’s passage across ocean basins. At the scale of the DEMs, WGS 84 and NAD 83 geographic are identical and may be used interchangeably.
Digital Elevation Models of Pago Pago, American Samoa

25

4. Summary and Conclusions

Two integrated bathymetric–topographic digital elevation models of Pago Pago, American Samoa, with cell 

sizes of 3 arc-seconds 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 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 ArcGIS, FME, GMT, MB-System, and Quick Terrain Modeler software. 

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

•	 Conduct hydrographic surveys in shallow water areas.

•	 Conduct bathymetric–topographic lidar surveys along the coast of Tutuila.

•	 Complete multibeam swath sonar surveys of the deep ocean surrounding American Samoa.

•	 Correct NED topography offset.

5. Acknowledgments

The creation of the Pago Pago DEMs was funded by the NOAA Pacific Marine Environmental Laboratory. The 

authors thank Nazila Merati and Vasily Titov (PMEL). We would also like to thank Kyle Hogrefe and Dawn Wright 

(Gaia Geo-Analytical and Oregon State University) for providing us with estimated depths from Kyle Hogrefe’s thesis 

research, and the Naval Oceanographic Office for providing the bathymetric-topographic lidar survey of Pago Pago 

Harbor.

6. References

Hogrefe, Kyle R., 2008. Derivation of Near-shore Bathymetry from Multispectral Satellite Imagery used in a Coastal 

Terrain Model for the Topographic Analysis of Human Influence on Coral Reefs: MS Thesis Oregon State 

Unversity. 

Lim, E., L.A. Taylor, B.W. Eakins, K.S. Carignan, R.R. Warnken, and P.R. Medley, 2009. Digital Elevation Model of 

Portland, Maine: Procedures, Data Sources, and Analysis, NOAA Technical Memorandum NESDIS NGDC-30, 

March 2009, 29 pp, http://www.ngdc.noaa.gov/mgg/inundation/tsunami/data/portland_me/portland_me.pdf.

Nautical Chart #83484 (RNC), 11th Edition, 2006. Samoa Islands Pago Pago Harbor. Scale 1: 15,000. U.S. Department 

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

Zilkoski, D., 2009. Affirmation of Vertical Datum for Surveying and Mapping Activities for the Island of Tutuila, 

American Samoa. Federal Register, Notices, Vol. 74, No. 13, p. 3991, Thursday, January 22, 2009, http://www.

ngs.noaa.gov/PUBS_LIB/AffirmationOfVerticalDatumForSurveyingAndMappingGuamNMVD03_FRN.pdf.
Lim et al., 2009

26

7. Data Processing Software

ArcGIS v. 9.3, developed and licensed by ESRI, Redlands, Oregon, http://www.esri.com/

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

http://www.safe.com/

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

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

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

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

MB-System v. 5.1.0, free software 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/

Persistance of Vision Pty. Ltd,. (2004), Persistence of VisionTM Raytracer. Persistance of Vision Pty., Williamstown, 

Victoria, Australia, http://www.povray.org.

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

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