Digital Elevation Models for Midway Atoll:

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)

June 30, 2009

Medley, P.R., Taylor, L.A., B.W. Eakins, K.S. Carignan, E.D. Lim and R.R. Warnken

NOAA, National Geophysical Data Center, Boulder, Colorado

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/
Medley et al., 2009

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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

3.2 Establishing Common Datums .................................................................................................17

3.2.1 Vertical datum transformations ...........................................................................17

3.2.2 Horizontal datum transformations.......................................................................17

3.3 Digital Elevation Model Development .....................................................................................18

3.3.1 Verifying consistency between datasets..............................................................18

3.3.2 Smoothing of bathymetric data ...........................................................................18

3.3.3 Gridding the data with MB-System.....................................................................19

3.4 Quality Assessment of the DEMs.............................................................................................19

3.4.1 Horizontal accuracy.............................................................................................19

3.4.2 Vertical accuracy .................................................................................................19

3.4.3 Slope maps and 3-D perspectives........................................................................19

3.4.4 Comparison with source data files.......................................................................22

4. Summary and Conclusions...............................................................................................................23

5. Acknowledgments............................................................................................................................23

6. References........................................................................................................................................23

7. Data Processing Software ................................................................................................................24

List of Figures

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

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

Figure 3. Location of Midway Atoll in the Northwestern Hawaiian Island Chain ....................................3

Figure 4. Source and coverage of datasets available in the Midway Atoll region .....................................5

Figure 5. Satellite image of Midway Island, extracted from Google Earth...............................................6

Figure 6. Example of coastal morphological change since 2000...............................................................7

Figure 7. Spatial coverage of bathymetric datasets used to compile the Midway Atoll DEMs.................9

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

 Midway Atoll DEMs.....................................................................................................................................10

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

Figure 10. Spatial coverage of estimated depths from the Naval Postgraduate School.............................13

Figure 11. Soundings on Nero Seamount digitized from NOAA RNC #19840 ........................................14

Figure 12. SRTM data coverage for Midway Island..................................................................................15

Figure 13. Digitized points on the breakwaters around the boat harbor ....................................................16

Figure 14. Histogram of the differences between Naval Postgraduate School estimated 

depths from the coral layer and the 1 arc-second bathymetric grid...................................18

Figure 15. Slope map of the 1/3 arc-second Midway Atoll DEM..............................................................20

Figure 16. Perspective view from the south of the 1/3 arc-second Midway Atoll.....................................21

Figure 17. Histogram of the differences between the NOAA National Marine Fisheries Service 

multibeam swath sonar surveys the 3 arc-second Midway Atoll DEM.............................22

Figure 18. Histogram of the differences between SRTM data and the 1/3 arc-second 

Midway Atoll DEM...........................................................................................................22
Digital Elevation Models of Midway Island

iii

List of Tables

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

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

Table 2. Shoreline dataset used in the Midway Atoll DEMs....................................................................6

Table 3. NOAA nautical charts available for Midway Atoll.....................................................................7

Table 4. Bathymetric datasets used in compiling the Midway Atoll DEMs.............................................8

Table 5. Multibeam swath sonar surveys used in compiling the Midway Atoll DEMs .........................10

Table 6. Trackline surveys available in the Midway Atoll region ..........................................................11

Table 7. Topographic dataset used in compiling the Midway Atoll DEMs............................................15

Table 8. Relationships between mean high water and other vertical datums in the Midway 

 Atoll region ........................................................................................................................17

Table 9. Data hierarchy used to assign gridding weight in MB-System.................................................19
Digital Elevation Models of Midway Atoll:

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 

Midway Atoll 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 Midway Atoll, where high-resolution 

bathymetry multibeam data were available. This report provides a summary of the data sources and methodology used 

in developing the Midway Atoll DEMs.

Figure 1. Shaded-relief image of the 3 arc-second Midway Atoll DEM. Contour interval is 500 meters for bathymetry and 20 meters for 

topography.

1. The Midway Atoll 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 Midway Atoll (28°13’N, -177°21’W) 

1/3 arc-second of latitude is equivalent to 10.26 meters; 1/3 arc-second longitude equals 9.09 meters. Three arc-seconds of latitude is equivalent to 

92.35 meters; 3 arc-seconds of longitude is equivalent to 81.80 meters.
Medley et al., 2009

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Figure 2. Shaded-relief image of the 1/3 arc-second Midway Atoll DEM. Contour interval is 500 meters for the bathymetry and 20 meters for the 

topography. 
Digital Elevation Models of Midway Island

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2. Study Area

Midway Atoll is a circular atoll that consists of two islands, Sand Island and Eastern Island, collectively called 

Midway Island. The atoll is located in the Hawaiian archipelago approximately 1,150 miles west-northwest of Hawaii 

(Fig. 3). In 1940, an air and submarine base were built on the island. The island gained notoriety in 1942 after the 

Battle of Midway proved to be the turning point of WWII. In 2006, Midway Island was designated as a national 

monument with the other Northwestern Hawaiian Islands, and is managed by the U.S. Fish and Wildlife Service and 

NOAA.

Midway Atoll formed from a shield volcano that developed around 28 million years ago from the same hotspot 

that is located under Hawaii today. Pacific plate motion has moved the volcano northwest from the hotspot, where 

it has slowly subsided. A coral reef has grown around the rim of the atoll, which is currently 160 meters thick and is 

comprised of post-Miocene limestones and upper-Miocene sediments overlying basalt.

Figure 3: Location of Midway Atoll in the Northwestern Hawaiian Island Chain. 

(http://www.soest.hawaii.edu/pibhmc/pibhmc_nwhi.htm)
Medley et al., 2009

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3. Methodology

The Midway Atoll DEMs were constructed to meet PMEL specifications (Table 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: World Geodetic System 1984 (WGS 84) and Mean High Water (MHW), respectively, 

for modeling of maximum flooding. Data processing and evaluation, and DEM assembly and assessment are described 

in the following subsections. 

Table 1a: PMEL specifications for the 3 arc-second Midway Atoll DEM.

Grid Area Midway Atoll

Coverage Area 177.88o to 176.87o W; 27.77o to 28.70o N

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 Midway Atoll DEM.

Grid Area Midway Atoll

Coverage Area 177.62o to 177.11o W; 28.04o to 28.47o N

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 Midway Island

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3.1 Data Sources and Processing

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

including: NOAA’s National Geophysical Data Center (NGDC), Office of Coast Survey (OCS), and the Center of 

Coastal Monitoring and Assessment (CCMA); the U.S. Geological Survey (USGS); and the Naval Postgraduate 

School. 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 shape files. The shape files 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 #1619910 Sand Island, Midway 

Island. 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 in the Midway Atoll region. Areas of no data are white. 

NOS and CCMA surveys were not used in DEM development.
Medley et al., 2009

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3.1.1 Shoreline

There were no NOAA Electronic Nautical Charts (ENCs) or high-resolution coastline datasets available for 

Midway Island, therefore, NGDC digitized the coastline based on NOAA Raster Nautical Chart (RNC) #19481 (Table 

2). 

Table 2: Shoreline dataset used in the Midway Atoll DEMs. 

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Coordinate System URL

NOAA RNC 

#19481 2009 vector 1:32,500 WGS 84 geographic Mean High Water

http://www.

nauticalcharts.noaa.

gov/mcd/Raster/index.

htm

Figure 5. Satellite image (2007) of Midway Island, extracted from Google Earth. Final coastline in pink.
Digital Elevation Models of Midway Island

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1) NGDC Digitized Coastline

 NGDC digitized the Midway Island coastline from NOAA RNC #19481 (Table 3), which has a resolution 

of 1:32,500. NOAA RNC #19482 has higher resolution but was not used in digitizing the final coastline 

because significant coastal morphologic changes have occurred since 2000 (Fig. 6). Google Earth satellite 

imagery (2007) and the shallow-water Naval Postgraduate School’s estimated depths (2006) were used to 

adjust the final coastline to match present morphology. 

Table 3: NOAA nautical charts available for Midway Atoll.

Chart Title Edition Edition Date Format Scale

19480 Gambia Shoal to Kure Atoll 9th 2007 RNC 1:180,00

19481 Midway Islands 11th 2005 RNC 1:32,500

19482 Midway Islands 9th 2000 RNC 1:10,000

Figure 6. Example of coastal morphological change since 2000. NOAA RNC #19482 (2000) in background shows major 

differences in the sand spit from the final coastline (in pink), which was derived from NOAA RNC #19481 (2005) and Google 

Earth Satellite imagery (2007).
Medley et al., 2009

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3.1.2 Bathymetry

Bathymetric datasets available for the compilation of the Midway Atoll DEMs include five multibeam swath 

sonar surveys from the NGDC multibeam sonar database, three NOS hydrographic surveys, 28 trackline geophysics 

surveys, CCMA bathymetric LiDAR surveys and estimated depths extracted from satellite imagery, and the Naval 

Postgraduate School’s (NPS) estimated depths extracted from satellite imagery (Table 4; see Figs. 4, 7;). The NOS 

hydrographic soundings (1941) and CCMA’s estimated depths from satellite imagery (2002) and bathymetric LiDAR 

data (2001) were not used in DEM development because they have been superseded by more recent, higher-resolution 

data.

Table 4: Bathymetric datasets used in compiling the Midway Atoll DEMs.

Source Year Data Type Spatial Resolution

Original 

Horizontal Datum/

Coordinate System

Original 

Vertical 

Datum

URL

NGDC

1992 

to 

2005

Multibeam sonar 

swath files

raw MB files gridded 

to 1/3 arc-second

WGS 84 

geographic

assumed 

MSL

http://www.ngdc.noaa.

gov/mgg/bathymetry/

multibeam.html

NGDC

1961 

to 

1991

Trackline (single 

beam echo-sounder)

soundings up to 

100’s of meters along 

profile with profiles 

spaced kms apart

WGS 84 

geographic

assumed 

MSL

http://www.ngdc.

noaa.gov/mgg/geodas/

trackline.html

Naval 

Postgraduate 

School

2006

Estimated Depths 

from Satellite 

Imagery

3 to 5 meters WGS 84 

geographic

assumed 

MSL

NOAA RNC 

19480 2009 Digitized Points 1:180,000 WGS 84 geographic MHW

http://www.

nauticalcharts.noaa.

gov/mcd/Raster/index.

htm
Digital Elevation Models of Midway Island

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Figure 7. Spatial coverage of bathymetric datasets used to compile the Midway Atoll DEMs. 

Areas of no data are white. Final coastline is illustrated in black.
Medley et al., 2009

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1) Multibeam swath sonar surveys

Five multibeam swath sonar surveys were available from the NGDC multibeam bathymetry database 

(http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html) for use in the Midway Atoll DEMs (Table 

5, Fig. 8). This database is comprised of the original swath sonar surveys conducted mostly by the U.S. 

academic fleet. The data were gridded to 1/3 arc-second resolution using MB-System. MB-System is an NSFfunded free software application specifically designed to manipulate multibeam swath sonar data (http://

www.ldeo.columbia.edu/res/pi/MB-System/). After assessing individual survey quality, the gridded data 

were transformed to MHW (see section 3.2.1) and shape files for editing in ArcMap. NGDC edited out 

noise along the swath edges and then converted to xyz format using FME for gridding of the preliminary 

bathymetric surface and final grid.

Surveys HI-05-03 and AHI-03-06 provide high-resolution coverage surrounding Midway Atoll but do 

not include the shallower region inside the atoll. The other three surveys are transits rather than dedicated 

sea-floor surveys. All have a horizontal datum of WGS 84 geographic and undefined vertical datum, which 

was assumed to be essentially mean sea level (MSL).

 Table 5: Multibeam swath sonar surveys used in compiling the Midway Atoll DEMs.

Cruise ID Ship Year

Original Vertical 

Datum

Original Horizontal 

Datum Institution

AVON06MV Melville 1999 Assumed MSL WGS 84 geographic University of California, Scripps Institution of Oceanography (UC/SIO)

AHI-03-06 Ahi 2003 Assumed MSL WGS 84 geographic NOAA National Marine Fisheries Service (NMFS)

HI-05-03 Hi'ialakai 2005 Assumed MSL WGS 84 geographic NOAA National Marine Fisheries Service (NMFS)

TUNE09WT Thomas Washington 1992 Assumed MSL WGS 84 geographic University of California, Scripps Institution of Oceanography (UC/SIO)

KOK0319 Ka'imikai-O-Kanaloa 2003 Assumed MSL WGS 84 geographic NOAA National Marine Fisheries Service (NMFS)

 

Figure 8. Ship tracks of multibeam swath sonar surveys used in compiling the Midway Atoll DEMs. 
Digital Elevation Models of Midway Island

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2) Trackline surveys

Twenty-eight 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 the Midway Atoll DEMs 

(Table 6, 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 data were downloaded as xyz files 

in WGS 84 and converted to MHW using FME.

The tracklines are spaced 10’s of kilometers apart and were only used where no high-resolution multibeam 

bathymetric data exist. 

Table 6: Trackline surveys available in the Midway Atoll region.

Cruise ID Ship Year Original Vertical Datum

Original Horizontal 

Datum Institution

73102500 Kana Keoki 1973 assumed MSL WGS 84 geographic University of Hawaii, HIG

75072600 Kana Keoki 1975 assumed MSL WGS 84 geographic University of Hawaii, HIG

76080601 Kana Keoki 1976 assumed MSL WGS 84 geographic University of Hawaii, HIG

76080602 Kana Keoki 1976 assumed MSL WGS 84 geographic University of Hawaii, HIG

77031705 Kana Keoki 1977 assumed MSL WGS 84 geographic University of Hawaii, HIG

77031706* Kana Keoki 1977 assumed MSL WGS 84 geographic University of Hawaii, HIG

80041401 Kana Keoki 1980 assumed MSL WGS 84 geographic University of Hawaii, HIG

80041402 Kana Keoki 1980 assumed MSL WGS 84 geographic University of Hawaii, HIG

84042805 Kana Keoki 1984 assumed MSL WGS 84 geographic University of Hawaii, HIG

KK720702 Kana Keoki 1972 assumed MSL WGS 84 geographic University of Hawaii, HIG

KH7402 Hakuho Maru 1974 assumed MSL WGS 84 geographic Ocean Research Institute, University 

of Tokyo

V2005 Vema 1964 assumed MSL WGS 84 geographic Lamont-Doherty Geological 

Observatory

V2006 Vema 1964 assumed MSL WGS 84 geographic Lamont-Doherty Geological 

Observatory

V2404 Vema 1967 assumed MSL WGS 84 geographic Lamont-Doherty Geological 

Observatory

V2405 Vema 1967 assumed MSL WGS 84 geographic Lamont-Doherty Geological 

Observatory

POL7201 Oceanographer 1972 assumed MSL WGS 84 geographic NOAA

CMAPPI7E Pioneer 1961 assumed MSL WGS 84 geographic NOAA Nation Ocean Service

CMAPSU7E Surveyor 1963 assumed MSL WGS 84 geographic NOAA Nation Ocean Service

POL7004 Oceanographer 1970 assumed MSL WGS 84 geographic NOAA POL

POL6829 Surveyor 1968 assumed MSL WGS 84 geographic NOAA POL

STYX07AZ Agassiz 1968 assumed MSL WGS 84 geographic Scripps Institution of Oceanography

NOVA01AR Argo 1967 assumed MSL WGS 84 geographic Scripps Institution of Oceanography

TSDY03WT Thomas Washington 1973 assumed MSL WGS 84 geographic Scripps Institution of Oceanography

FARN0691 Farnella 1991 assumed MSL WGS 84 geographic Natural Environment Research 

Council

FARN1091 Farnella 1991 assumed MSL WGS 84 geographic Natural Environment Research 

Council

SI932003 Silas Bent 1971 assumed MSL WGS 84 geographic US Navy Naval Oceanographic 

Office

SI932009 Silas Bent 1972 assumed MSL WGS 84 geographic US Navy Naval Oceanographic 

Office

L876NP Samuel P. Lee 1976 assumed MSL WGS 84 geographic USGS Branch of Pacific Marine 

Geology
Medley et al., 2009

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Figure 9. Spatial coverage of trackline surveys in the Midway Atoll region. Trackline data shown in red were not used in the final DEM gridding, 

as there were high-resolution multibeam coverage or newer, more accurate trackline surveys available. Trackline data shown in blue were used 

for gridding of the final DEM.
Digital Elevation Models of Midway Island

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3) Naval Postgraduate School Estimated Depths 

Graduate student Mark A. Camacho of the Naval Postgraduate School (NPS), completed his masters 

thesis on a depth analysis of Midway Atoll using high-resolution satellite imaging. Mr. Camacho’s co-advisor, 

Dr. Richard Olsen, graciously provided NGDC with the resulting data. 

Comancho used a high-resolution multispectral image taken from the QuickBird satellite in October 

2004. Environment for Visualizing Images 4.2 was used to analyze and process the image. Corrections were 

made for atmospheric absorption and scattering, the water column, and reflection from waves due to wind. 

Two separate bathymetric images were produced from two different algorithms for the main benthic classes: 

sand and coral.

Data were provided to NGDC as two xyz files, coral bottom and sand bottom. No vertical datum was 

documented, so NGDC assumed the data were essentially MSL. The xyz files were transformed to shape 

files and MHW using FME for assessing the data (Fig. 10). The data have a correlation of 86.5% between 

predicted depth and actual depths based on NOAA nautical chart values.

NPS estimated depth data are the most recent, high-resolution data, so were used instead of the CCMA 

bathymetric LiDAR (2001) and estimated depths (2002), or the 1941 NOS surveys.

Figure 10. Spatial coverage of estimated depths from the Naval Postgraduate School. Yellow represents a sandy bottom; pink 

represents coral cover.
Medley et al., 2009

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4) NGDC Digitized Depths

NGDC digitized Nero Seamount from the NOAA RNC #19840 in the southwestern area of the 3 arc-second 

DEM (Figure 11). Multibeam swath sonar survey KOK0319 covered the area, but had bad returns that did not 

accurately reflect the seamount, so was not used.

Figure 11. Soundings on Nero Seamount digitized from NOAA RNC #19840.
Digital Elevation Models of Midway Island

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3.1.3 Topography

The NASA Space Shuttle Radar Topography (SRTM) 1 arc-second DEM provided full coverage of the islands 

and was used to build the Midway Atoll DEMs (Table 7; Fig. 12). 

Table 7: Topographic dataset used in compiling the Midway Atoll DEMs.

Source Year Data Type Spatial

Resolution

Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum URL

NASA 2000 Topographic

DEM

1 arcsecond WGS 84 Geographic WGS 84/EGM 

96 Geoid http://srtm.usgs.gov/

NGDC 2009 Digitized 

Points WGS 84 Geographic MHW

Figure 12. SRTM data coverage for Midway Island. Coastline in black.
Medley et al., 2009

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1) NASA space shuttle radar topography

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

generate the most complete high-resolution digital topographic database of Earth2. The SRTM consisted of a 

specially modified radar system that flew onboard the Space Shuttle Endeavour during an 11-day mission in 

February of 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 USGS Seamless web site (http://seamless.usgs.gov/) 

as raster DEMs. The data have not been processed to bare earth, and have absolute horizontal and vertical 

accuracies of 20 and 16 meters, respectively.

A topographic surface was created with GMT to resample the SRMT from 1 arc-second to 1/3 arcsecond. This created a smoother fit along the coastline when the SRTM DEM was clipped to the coast to 

delete values over the open ocean. The clipped data were converted into an xyz format for the final gridding 

process.

2) NGDC Digitized Breakwater

 NGDC digitized elevation points at 0.5 meters along the breakwater to the harbor (Figure 13). The width 

of the breakwater is ~5-6 meters, less than the width of one cell size for a 1/3 arc-second grid. A few points 

were digitized to create a continuous wall above MHW in the DEM, to avoid “channels” of water between 

cells representing the breakwater.

Figure 13. Digitized points (in pink) on the breakwaters around the boat harbor. Coastline in black.

2. The SRTM data sets result from a collaborative effort by the National Aeronautics and Space Administration (NASA) and the National GeospatialIntelligence 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]
Digital Elevation Models of Midway Island

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3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation and evaluation of the Midway Atoll DEMs were originally referenced to vertical 

datums of mean lower low water (MLLW), mean sea level (MSL), and WGS 84/EGM 96 Geoid. All datasets were 

transformed to MHW to provide the maximum flooding scenario for inundation modeling. 

1) Bathymetric data

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

FME software, by adding a constant offset of -0.132 meters measured at the Midway Island NOAA tidal 

station #1619910 (Table 8; http://tidesandcurrents.noaa.gov/). 

2) Topographic data

The SRTM topographic DEM was originally in WGS 84/EGM 96 Geoid vertical datum. There are no 

survey markers near Midway Island that relate this datum to the local tidal datum. Thus, we assumed the 

datum to be essentially equivalent to MSL in this area (Table 8). Conversion to MHW was accomplished by 

adding a constant value of -0.132 meters. 

Table 8: Relationship between mean high water and other vertical datums in the Midway Atoll region.*

Vertical Datums Difference to MHW

MSL 0.132 meters

MLLW 0.328 meters

*Datum relationships determined by NOAA tidal station 1619910 at Midway Island

3.2.2 Horizontal datum transformations

Datasets used to compile the Midway Atoll DEMs were originally referenced to WGS 84. No conversion was 

needed for the datasets in WGS 84. Datasets that were available but were not used for the final gridding process were 

referenced to UTM Zone 2 (CCMA) and undetermined horizontal datum (NOS). The relationship and transformational 

equations between WGS 84 and UTM Zone 2 are well established. These data were converted to WGS 84 using FME

software. There was no defined relationship between the undetermined and WGS 84 datums, so smoothsheets of the 

NOS surveys were georeferenced in ArcMap and the data were manually shifted to visually match the smoothsheets.
Medley et al., 2009

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3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

After vertical transformations were applied, the resulting ESRI shape files 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 shape files were then converted to xyz files in preparation for gridding. Problems 

included:

•	 Old NOS surveys in the shallow water with an undetermined horizontal datum

•	 Estimated depths were the only recent data available for the shallow water region

•	 Recent morphological changes due to the jetties at the boat harbor on Sand Island, constructed after surveys 

were conducted in the area.

•	 Limited high-resolution bathymetric measurements in the deep ocean surrounding the atoll

•	 Lack of metadata for the LiDAR surveys and estimated depths from the Naval Postgraduate school, leading 

us to assume MSL vertical datum

3.3.2 Smoothing of bathymetric data

Two ‘pre-surface’ bathymetric grids were generated for the Midway Atoll 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, a marked contrast with the trackline surveys spaced 1 to 15 kilometers apart. NGDC 

created a 1 arc-second cell size bathymetric grid slightly larger than the 1/3 arc-second DEM extent, and a 3 arcsecond cell size bathymetric grid slightly larger than the 3 arc-second DEM extent. The grids were generated using 

GMT, an NSF-funded share-ware software application designed to manipulate data for mapping purposes (http://gmt.

soest.hawaii.edu/).

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 final Midway Atoll DEM gridding extents. 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 original soundings to ensure grid accuracy 

(Fig. 14) and exported as an xyz file for use in the final gridding process (see Table 9). The statistical analysis of the 

differences between the 1 arc-second bathymetric surface and the estimated depths from the coral layer show that the 

majority of the soundings are in agreement with the bathymetric surface. Depths with the greatest difference (-10 and 

+10 meters) are due to averaging of multiple, closely spaced depths.

Figure 14. Histogram of the differences between Naval Postgraduate School estimated depths from the coral layer and the 

1 arc-second bathymetric grid.
Digital Elevation Models of Midway Island

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3.3.3 Gridding the data with MB-System

MB-System (http://www.ldeo.columbia.edu/res/pi/MB-System/) was used to create the Midway Atoll 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 

9. Greatest weight was given to the digitized features. Least weight was given to the coastline, trackline surveys, and 

the pre-surfaced bathymetric grid. 

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

Dataset Relative Gridding Weight

NGDC digitized features 1,000

NGDC Multibeam surveys 100

NPS Satellite Imagery Depths 100

SRTM Topographic DEM 10

1 arc-second Pre-surfaced bathymetric grid 1

NGDC Trackline 1

Midway Island coastline 1

3.4 Quality Assessment of the DEMs

3.4.1. Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Midway Atoll DEMs are dependent upon 

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

accuracy of no better than 30 meters (SRMT). Bathymetric features are resolved only to within a few kilometers in 

deep-water areas. Shallow, near-coastal regions have an accuracy approaching 10 meters (NPS). Positional accuracy 

is limited by: the paucity of deep-water soundings and by recent morphologic change.

3.4.2 Vertical accuracy

Vertical accuracy of elevation values for the Midway Atoll DEMs are also highly dependent upon the source 

datasets contributing to DEM cell values. The SRTM topographic DEM has an estimated vertical accuracy between 10 

and 16 meters. Bathymetric areas have an estimated accuracy of between 0.1 meters and 1% of water depth for the 1/3 

arc-second DEM. The deep water values in the 3 arc-second DEM have an estimated accuracy of hundreds of meters 

due to gridding interpolation between sparse soundings.

3.4.3 Slope maps and 3-D perspectives

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

and identification of artificial slopes along boundaries between datasets (e.g., Fig. 15). The DEMs were transformed 

to UTM Zone 2 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. Figure 16 shows a perspective view of the 1/3 arcsecond Midway Atoll DEM in its final version, created by Persistence of VisionTM Raytracer (POV-Ray).
Medley et al., 2009

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Figure 15. Slope map of the 1/3 arc-second Midway Atoll DEM. Flat-lying slopes are white; dark 

shading denotes steep slopes; Midway Island coastline in red.
Digital Elevation Models of Midway Island

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Figure 16. Perspective view from the south of the 1/3 arc-second Midway Atoll DEM. 

1.5x vertical exaggeration.
Medley et al., 2009

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3.4.4	 Comparison with source data files

To ensure grid accuracy, the Midway Atoll 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 multibeam 

surveys and the 3 arc-second Midway Atoll DEM is shown in Figure 17. The greatest differences occurred along the 

edges of the deep-water swaths where the data are nosiest. 

Figure 17. Histogram of the differences between the NOAA National Marine Fisheries Service multibeam swath sonar 

surveys and the 3 arc-second Midway Atoll DEM.

A histogram of the differences between the SRTM topographic DEM gridded to 1/3 arc-second and the 1/3 arcsecond Midway DEM is shown in Figure 18. Differences range from -25 meters to 3 meters, where negative values 

indicate that elevations of the SRTM data are higher than the DEM elevations. The greatest differences occurred along 

the harbor coastline where the SRTM data (2000) were clipped to the present coastline.

Figure 18. Histogram of the differences between SRTM data and the 1/3 arc-second Midway Atoll DEM.
Digital Elevation Models of Midway Island

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4. Summary and Conclusions

Two integrated bathymetric–topographic digital elevation models of Midway Atoll, with cell spacing of 3 arcseconds and 1/3 arc-seconds, were developed for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center 

for Tsunami Research. The best available digital data from U.S. federal 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 ArcGIS, FME, GMT, MB-System, and Quick Terrain Modeler software. 

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

•	 Conduct hydrographic surveys in shallow-water areas inside the atoll.

•	 Conduct bathymetric–topographic LiDAR surveys of the entire atoll.

•	 Conduct multibeam swath sonar surveys of the deep ocean surrounding the atoll.

•	 Digitize NOAA Raster Nautical Charts as Electronic Nautical Charts (ENCs).

5. Acknowledgments

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

The authors thank Nazila Merati and Vasily Titov (PMEL); and Ann Gibbs (USGS Pacific Science Center). We would 

also like to thank Richard Olsen and Mark Camacho (Naval Postgraduate School) for providing us with estimated 

depths data from Mark Camacho’s thesis research, and Tim Battista and Richard Stumpf (CCMA) for providing the 

bathymetric LiDAR data and estimated depths from their studies.

6. References

Camacho Mark A., 2006. Depth analysis of Midway Atoll using quickbird multi-spectral imaging over variable 

substrates: MS Thesis Naval Postgraduate School. 

Nautical Chart #19480 (RNC), 9th Edition, 2007. Gambia Shoal to Kure Atoll. Scale 1: 180,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #19481 (RNC), 11th Edition, 2005. Midway Islands. Scale 1: 32,500. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #19482 (RNC), 9th Edition, 2000. Midway Islands. Scale 1: 10,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.
Medley et al., 2009

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7. Data Processing Software

ArcGIS v. 9.2, developed and licensed by ESRI, Redlands, Oregon, http://www.esri.com/

FME 2008 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/