Digital Elevation Models of Wake Island:

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)

April 3, 2009

Medley, P.R.1

, L.A. Taylor2, B.W. Eakins1, K.S. Carignan1, R.R. Warnken2, and E. Lim1

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

2. 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/ 
Digital Elevation Models of Wake Island

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Contents

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

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

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

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

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

3.1.2 Bathymetry ............................................................................................................7

3.1.3 Topography..........................................................................................................13

3.2 Establishing Common Datums .................................................................................................15

3.2.1 Vertical datum transformations ...........................................................................15

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

3.3 Digital Elevation Model Development .....................................................................................16

3.3.1 Estimating bathymetry from satellite imagery ....................................................16

3.3.2 Verifying consistency between datasets..............................................................17

3.3.3 Smoothing of bathymetric data ...........................................................................17

3.3.4 Gridding the data with MB-System.....................................................................18

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.......................................................................21

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

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

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

7. Data Processing Software ................................................................................................................23

List of Figures

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

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

Figure 3. Aerial image of the Wake island airstrip.....................................................................................2

Figure 4. Source and coverage of datasets used to compile the Wake Island DEMs.................................4

Figure 5. Wake Island coastline digitized from the NOAA RNC #81664 .................................................5

Figure 6. World Vector coastline not used in the final coastline. ...............................................................6

Figure 7. Spatial coverage of bathymetric datasets used to compile the Wake Island DEMs....................7

Figure 8. Spatial coverage of multibeam swath sonar surveys from NGDC multibeam 

database................................................................................................................................8

Figure 9. Perspective image of the atoll’s flanks as revealed in AH-07-01 and HI-07-01...............................9

Figure 10. Spatial coverage of trackline surveys in the Wake Island region..............................................10

Figure 11. Digital Globe satellite image used for satellite imagery depth analysis...................................11

Figure 12. Estimated depths of the Wake Island lagoon from satellite imagery........................................11

Figure 13. Digitized reef ............................................................................................................................12

Figure 14. SRTM data coverage for Wake Island ......................................................................................13

Figure 15. SRTM clipped to the coastline at 1/3 arc-second and 3 arc-seconds.......................................14

Figure 16. Satellite pixel value plotted against depth ................................................................................16

Figure 17. Buffer established between the high-resolution NGDC multibeam sonar surveys 

and the low-resolution ETOPO1........................................................................................17

Figure 18. Histogram of the differences between a NGDC multibeam sonar survey AH-0701 

and the 1 arc-second bathymetric grid...............................................................................18

Figure 19. Slope map of the 1/3 arc-second Wake Island DEM ................................................................19

Figure 20. Perspective view from the east of the 1/3 arc-second DEM. 5x vertical exaggeration................20

Figure 21. Histogram of the differences between a NGDC multibeam survey AHI-7-01 and the 

1/3 arc-second DEM..........................................................................................................21
Medley et al., 2009

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Figure 22. Histogram of the differences between ETOPO1 and the 3 arc-second Wake Island 

DEM ..................................................................................................................................21

Figure 23. Histogram of the differences between SRTM and the 1/3 arc-second Wake Island 

DEM ..................................................................................................................................22

Figure 24. Histogram of the differences between the estimated depths from Digital Globe Satellite 

Imagery and the 1/3 arc-second Wake Island DEM ..........................................................22

List of Tables

Table 1a. PMEL specifications for the 3 arc-second Wake Island DEM....................................................3

Table 1b. PMEL specifications for the 1/3 arc-second Wake Island DEM.................................................3

Table 2. Shoreline datasets used in the Wake Island DEMs.....................................................................5

Table 3. NOAA nautical chart available in the Wake Island region .........................................................6

Table 4. Bathymetric datasets used in compiling the Wake Island DEMs ...............................................7

Table 5. Multibeam swath sonar surveys used in compiling the Wake Island DEMs..............................8

Table 6. Trackline surveys used in compiling the Wake Island DEMs ..................................................10

Table 7. Topographic dataset used in compiling the Wake Island DEMs...............................................13

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

 Island region.......................................................................................................................15

Table 9. Data hierarchy used to assign gridding weight in MB-System.................................................18
Digital Elevation Models of Wake Island:

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 

Wake Island 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. A smaller 1/3 arc-second DEM (Fig. 2) was generated 

for the immediate area surrounding Wake Island, where high-resolution multibeam data were available, to increase 

forecasting accuracy. This report provides a summary of the data sources and methodology used in developing the 

Wake Island DEMs.

Figure 1. Shaded-relief image of the 3 arc-second Wake Island DEM. Contour interval is 500 meters.

1. The Wake Island 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 Wake Island (19°16’N, 166°35’E) 

1/3 arc-second of latitude is equivalent to 10.35 meters; 1/3 arc-second longitude equals 9.87 meters.. Three arc-seconds is equivalent to 91.14 

meters; 3 arc-seconds longitude equals 88.35 meters
Medley et al., 2009

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

2. Study Area

Wake Island is a coral atoll with only twelve miles of coastline, located in the North Pacific Ocean approximately 

two-thirds of the way from Hawaii to the Northern Mariana Islands. The atoll consists of three low coral islands, Peale, 

Wake, and Wilkes, built upon an underwater volcano, with a central lagoon that was its former crater.

In 1940 and 1941 an air and naval base was constructed on the island, but the island was captured in December 

1941 by the Japanese and held until the end of the war. Later, Wake Island was used as a stopover and refueling site 

for military and commercial aircraft transiting the Pacific. Since 1974, the island has been used by the US military 

and as an emergency landing strip (Fig. 3).

Figure 3: Airstrip on Wake Island, currently only for military use or emergency landings. 

(http://upload.wikimedia.org/wikipedia/commons/7/75/Ioke-wake-damage002.jpg)
Digital Elevation Models of Wake Island

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3. Methodology

The Wake Island DEMs were developed to meet PMEL specifications (Table 1), based on input requirements 

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

horizontal and vertical datums: World Geodetic System 1984 (WGS84) and Mean High Water (MHW), respectively, 

for modeling of “worst-case scenario” 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 Wake Island DEM.

Grid Area Wake Island

Coverage Area 166.15o to 167.10o E; 18.84o to 19.76o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System 1984 (WGS84)

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 Wake Island DEM.

Grid Area Wake Island

Coverage Area 166.42o to 166.82o E; 19.17o to 19.40o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System 1984 (WGS84)

Vertical Datum Mean High Water (MHW)

Vertical Units Meters

Cell Size 1/3 arc-second

Grid Format ESRI Arc ASCII grid

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 Ocean Service (NOS), Office of Coast Survey (OCS), and the U.S. Geological Survey 

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

WGS84 horizontal datum and to convert 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 a NOAA tide station near Wake Island. 

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

datasets before the final gridding process.
Medley et al., 2009

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Figure 4. Source and coverage of datasets used to compile the Wake Island DEMs.
Digital Elevation Models of Wake Island

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3.1.1 Shoreline

The World Vector coastline dataset for the Wake Island region is of low resolution and was not used in the DEMs. 

There is no electronic navigational chart (ENC) available for Wake Island, therefore NGDC digitized the coastline 

based on the NOAA raster nautical chart (RNC) #81664, digital images from Google Earth, and the 3 arc-second 

SRTM topographic DEM. (Table 2; Fig. 5). 

Table 2: Shoreline datasets available for the Wake Island DEMs. 

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Coordinate System URL

NOAA RNC 2009 vector 1:15,000 WGS84 geographic Mean High Water

http://www.

nauticalcharts.noaa.

gov/mcd/Raster/index.

htm

NGDC 

World Vector 

Shoreline

2004 vector 1:250,000 WGS84 geographic Mean High Water

http://www.ngdc.noaa.

gov/mgg/shorelines/

shorelines.html

Figure 5. Wake Island coastline (in green), based on NOAA RNC #81664.
Medley et al., 2009

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1) NGDC Digitized Coastline

 NGDC digitized the Wake Island coastline from the RNC #81664 (Table 3), which has a resolution of 

1:15,000 and 1:3,500. The 3 arc-second SRTM DEM, and Google Earth satellite imagery were used to assess 

the final coastline. 

Table 3: NOAA nautical charts available for Wake Island.

Chart Title Edition Edition Date Format Scale

81664 Wake Island with inset of Wake Island Boat 

Basin 6th 1998 RNC 1:15,000 and 1:3,500

2) NGDC World Vector Shoreline

NGDC did not use the Wold Vector Shoreline (http://www.ngdc.noaa.gov/mgg/shorelines/shorelines.

html) in the DEM. The low-resolution dataset (1:250,000) did not align with the 3 arc-second SRTM DEM 

or the NOAA RNC (Figure 6).

Figure 6. World Vector Coastline at a scale of 1:250,000 is not consistent with NOAA RNC #81664, and was not used in the final coastline.
Digital Elevation Models of Wake Island

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3.1.2 Bathymetry

Bathymetric datasets used in the compilation of the Wake Island DEMs include three multibeam swath sonar 

surveys downloaded from the NGDC multibeam sonar database, seven trackline surveys, depths extracted from 

satellite imagery, NGDC digitized depths, and extracted data from ETOPO1 Global Relief Model (Table 4; see Fig. 

4). Figure 7 shows the spatial coverage of data used in the 1/3 arc-second DEM.

Table 4: Bathymetric datasets used in compiling the Wake Island DEMs.

Source Year Data Type Spatial Resolution

Original 

Horizontal Datum/

Coordinate System

Original 

Vertical 

Datum

URL

NGDC

1992 

to 

2007

Multibeam 

sonar swath 

files

raw MB files gridded 

to 1/3 arc-second

WGS84 

geographic

assumed 

Mean Sea 

Level

http://www.ngdc.noaa.

gov/mgg/bathymetry/

multibeam.html

NGDC 

1968 

to 

1982

Trackline 100’s of

 meters along profile

WGS84 

geographic

assumed 

Mean Sea 

Level

http://www.ngdc.

noaa.gov/mgg/geodas/

trackline.html

Digital Globe 2008 Satellite 

Imagery 1 to 2 meters undefined undefined

NGDC 2009 Digitized 

Depths several meters WGS84 

geographic

MHW 

(meters)

ETOPO1 2008 Global Relief 

Grid 1 arc-minute WGS84 geographic MSL

http://www.ngdc.

noaa.gov/mgg/global/

global.html

Figure 7. Spatial coverage of bathymetric datasets used to compile the 1/3 arc-second Wake Island DEM. Final coastline is shown in black.
Medley et al., 2009

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1) Multibeam swath sonar surveys

Three 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 Wake Island DEMs (Table 5, Fig. 

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

fleet. The downloaded data were gridded to 1/3 arc-second resolution using MB-System. MB-System is an 

NSF-funded free software application specifically designed to manipulate submarine multibeam sonar data 

(http://www.ldeo.columbia.edu/res/pi/MB-System/).

Surveys by the Ahi and Hi’ialakai ships provide excellent shallow-water bathymetry, surrounding the 

island to approximately 10-15 km offshore (Fig. 9). The flanks of the atoll dip steeply to abyssal depths. After 

assessing individual survey quality, the gridded data were transformed to MHW (see section 3.2.1) and xyz 

format using FME, and edited using ArcMap. 

 

Table 5: Multibeam swath sonar surveys used in compiling the Wake Island DEMs.

Cruise ID Ship Year

Original Vertical 

Datum

Original Horizontal Datum Institution

AHI-07-01 Ahi 2007 assumed mean sea 

level WGS84 geographic National Oceanic and Atmospheric Administration (NOAA/NMES)

HI-07-01 Hi'ialakai 2007 assumed mean sea 

level WGS84 geographic National Oceanic and Atmospheric Administration (NOAA/NMES)

TUNE09WT Thomas Washington 1992 assumed mean sea 

level WGS84 geographic University of California, Scripps Institute of Oceanography (UC/SIO)

 

Figure 8. Spatial coverage of multibeam swath sonar files used in compiling the Wake Island DEMs. Coastline is shown in black.
Digital Elevation Models of Wake Island

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Figure 9. Perspective image of the atoll’s flanks as revealed in the AHI-07-01 and HI-07-01 surveys. Image from http://www.soest.

hawaii.edu/pibhmc/PRIAs_images/Jarvis_60m3D_.jpg.
Medley et al., 2009

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2) Trackline surveys

Seven single-beam trackline surveys were available from the NGDC marine geophysical trackline 

database (http://www.ngdc.noaa.gov/mgg/geodas/trackline.html) for use in the Wake Island DEMs (Table 6, 

Fig. 10). This database is comprised of bathymetry, magnetics, gravity, and seismic navigation data collected 

during marine cruises from 1953 to present. The data was downloaded as xyz files in WGS84 and converted 

to mean high water.

The trackline surveys are at a scale of 100’s of meters and therefore were only used where no high 

resolution multibeam surveys existed. 

Table 6: Trackline surveys in the Wake Island region.

Cruise ID Ship Year

Original Vertical 

Datum

Original Horizontal Datum Institution

STYX07AZ Agassiz 1968 assumed MSL WGS84 Scripps Institution of Oceanography

76010301 Kana Keoki 1976 assumed MSL WGS84 University of Hawaii, HIG

MARA11WT Thomas Washington 1979 assumed MSL WGS84 Scripps Institution of Oceanography

81062604 Kana Keoki 1981 assumed MSL WGS84 University of Hawaii, HIG

GH814-A Hakurei Maru 1981 assumed MSL WGS84 Geological Survey of Japan

GH814-B Hakurei Maru 1981 assumed MSL WGS84 Geological Survey of Japan

L882NP Samuel P. Lee 1982 assumed MSL WGS84 USGS Branch of Pacific Marine 

Geology

Figure 10. Spatial coverage of trackline surveys in the Wake Island region.
Digital Elevation Models of Wake Island

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3) Estimated bathymetry from satellite imagery 

The lagoon on Wake Island does not contain bathymetric surveys. The only available data are sparse 

soundings from NOAA RNC #81664. To create high-resolution data for the lagoon, NGDC developed 

a methodology to convert satellite image ocean color into estimated bathymetric depths. A Digital Globe 

Satellite image from Google Earth was downloaded and georeferenced in Arcmap and converted to gray 

scale (Fig. 11). The depths from the RNC were used as control points to establish the relationship between 

depths and pixel values from the gray scale image (Fig. 12)(see section 3.3.1).

Figure 11. Digital Globe satellite image used to estimate depths for the Wake Island lagoon. A) Color satellite image 

georeferenced in ArcMap B) a gray scale geotiff used for pixel vs. depth analysis.

Figure 12. Estimated Depths of the Wake Island lagoon calculated from satellite imagery.

A B
Medley et al., 2009

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4) NGDC digitized depths 

NGDC digitized depths from RNC #81664 where no data were available (Fig. 13). The depths inside the 

lagoon were not used in the final gridding but were used for the control in the satellite depth analysis. The reef 

that surrounds the perimeter of Wake Island was not included in the satellite imagery depth analysis as the 

bottom substrate of coral reflected light differently than the lagoon. NGDC digitized the reef that surrounds 

the perimeter of the lagoon, as identified in the RNCs, to approximately 120 meters offshore where the highresolution multibeam bathymetry data exists. Values between 0.5 to 2.5 meters were used as depths. 

Figure 13. Digitized reef. The reef surrounds the perimeter of Wake Island approximately 120 meters offsore with depths of 0.5 to 2.5 

meters.

5) ETOPO1

The majority of the Wake Island region contains no bathymetric measurements. NGDC’s ETOPO1 Global 

Relief Model, which is derived from satellite altimetry, was used to fill this region.

The ETOPO1 grid has 1 minute cell size with data in WGS84 geographic coordinates and MSL. These data 

are exceptionally coarse at the resolution of the 1/3 arc-second and 3 arc-second Wake Island DEMs, however, 

they provide the only digital constraint on the bathymetry in deep water of the Wake Island DEMs. Extracted 

bathymetric data are generally shallower than overlapping measured bathymetric values and are considered to be 

of low accuracy.
Digital Elevation Models of Wake Island

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3.1.3 Topography

The NASA Space Shuttle Radar Topography (SRTM) 3 arc-second DEM provided full coverage of the Island and 

was obtained and used to build the Wake Island DEMs (Table 7; Fig. 14). 

Table 7: Topographic dataset used in compiling the Wake Island DEMs.

Source Year Data Type Spatial

Resolution

Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum URL

NASA 2000 Topographic

DEM

3 arcsecond WGS84 Geographic WGS84 Geoid http://srtm.usgs.gov/

Figure 14. SRTM data coverage for Wake Island (green). 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, but meet the absolute horizontal and vertical 

accuracies of 20 and 16 meters, respectively.

The SRTM DEM is at 3 arc-seconds and referenced to the WGS84 Geoid. To create a 1/3 arc-second 

grid, NGDC sampled the data to 1/3 arc-seconds to create a smooth interpolation of the land (Fig. 15). The 

SRTM data were converted into a shapefile using FME. Using ArcMap, the kriging tool was used to smooth 

the 3 arc-second surface, which was converted to an xyz file, and gridded into a 1/3 arc-second grid using 

GMT (see section 3.3.3). 

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]

Figure 15. SRTM clipped to the coastline, A) original 3 arc-second B) smoothed 1/3 arc-second
Digital Elevation Models of Wake 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 Wake Island DEMs were originally referenced to vertical 

datums of Mean Lower Low Water (MLLW), Mean Sea Level (MSL), and WGS84 Geoid. All datasets were transformed 

to MHW to provide the worst-case scenario for inundation modeling. Depth values in the lagoon on the RNC were 

converted from feet to meters as appropriate.

1) Bathymetric data

The shallow-water multibeam, trackline surveys, and ETOPO1 were transformed from MSL to MHW, 

using FME software, by adding a constant offset of -0.307 meters measured at the Wake Island NOAA 

tidal station (see table 9) (http://tidesandcurrents.noaa.gov/). NGDC digitized points from RNC #81664 were 

converted from feet to meters, and then from MLLW to MHW by adding a constant offset of -0.653 meters. 

2) Topographic data

The SRTM topographic DEM was originally in WGS84 Geoid vertical datum. There are no survey 

markers near Wake 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 9). Conversion to MHW was accomplished by adding a 

constant value of -0.307 meters.

Table 8: Relationship between Mean High Water and other vertical datums in the Wake Island region.*

Vertical Datums Difference to MHW

MSL 0.307

MLLW 0.653

*Datum relationships determined by tidal station 1890000 at Wake Island

3.2.2 Horizontal datum transformations

Datasets used to compile the Wake Island DEM were originally referenced to WGS84 geographic and needed no 

conversion. 
Medley et al., 2009

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3.3 Digital Elevation Model Development

3.3.1 Estimating bathymetry from satellite imagery

There are no digital soundings for the lagoon at Wake Island. The only available data were sparse RNC soundings. 

Depths for the lagoon were estimated by calculating shallow-water depths from a satellite image. The image was 

extracted from Google Earth Pro, converted to tif format and orthorectified in ArcMap using stable, identifiable 

features, such as the airstrip. The image was then converted to a 16-bit greyscale image (single value per pixel) and 

compared to the sparse RNC soundings. An approximate logarithmic relationship between pixel value and sounding 

(Fig. 16) was calculated using MatLab, a numerical computing environment and programming language maintained by 

MathWorks (http://www.mathworks.com/), and this equation was applied to the grayscale image to obtain estimated 

depths. The resulting Arc raster was resampled to 1/3 arc-second, clipped to the combined coastline, then converted 

to a ESRI point shapefile, which was edited to remove depths from the land, coral reefs, waves, and clouds identifiable 

in the satellite image.

Figure 16. Satellite greyscale pixel value plotted against depth. A logarithmic equation relating the two parameters was derived and used to 

estimate depths in the lagoon.
Digital Elevation Models of Wake Island

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3.3.2 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:

•	 No digital survey soundings of the lagoon or of the surrounding coral reef

•	 No bathymetric measurements in the deep ocean

•	 Different bottom substrate reflections for satellite depth analysis 

•	 Only 3 arc-scond SRTM topographic data were available

3.3.3 Smoothing of bathymetric data

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

data coverage. A 1 arc-second-spacing bathymetric grid was created for the 1/3 arc-second DEM extent, and a 3 arcsecond-spacing bathymetric grid was created for the 3 arc-second DEM extent because of the low-resolution datasets 

in the deep ocean. 

The NGDC multibeam surveys are high resolution with beam spacing 10 meters apart, a marked contrast with 

the 1 arc-minute (2 km) ETOPO1 data. A buffer was created between the multibeam and the ETOPO1 data (Fig. 17). 

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/).

Figure 17. Buffer established between the high-resolution NGDC multibeam sonar surveys and the low-resolution ETOPO1.

The 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 Wake Island DEM gridding region. 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 (Fig. 18) and 

exported as an xyz file for use in the final gridding process (see Table 10).
Medley et al., 2009

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Figure 18. Histogram of the differences between NGDC multbeam sonar survey, AHI-07-01 and the 1 arc-second 

bathymetric grid.

3.3.4 Gridding the data with MB-System

MB-System (http://www.ldeo.columbia.edu/res/pi/MB-System/) was used to create the Wake Island 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. Least weight was given to the coastline SRTM topographic 

DEM, and ETOPO1. 

 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

Satellite Imagery Depths 100

Trackline 10

Pre-surfaced bathymetric grid 10

1/3 arc-second DEM (for the 3 arc-second DEM only) 10

SRTM Topographic DEM 1

Wake Island coastline 1

ETOPO1 1
Digital Elevation Models of Wake Island

19

3.4 Quality Assessment of the DEMs

3.4.1. Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Wake Island DEMs are dependent upon 

cell location and the datasets used to determine corresponding DEM cell values. The SRTM topographic DEM has 

an estimated accuracy of 90 to 100 meters. Bathymetric features are resolved only to within a few thousand meters 

in deep-water areas. Shallow, near-coastal regions, and harbor surveys have an accuracy of 10 meters. Positional 

accuracy is limited by: the lack of deep-water soundings, simplified methodology for extracting depths from satellite 

images, and by man made morphologic change (i.e., channel dredging).

3.4.2 Vertical accuracy

Vertical accuracy of elevation values for the Wake Island 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 and course estimated depths from ETOPO1.

3.4.3 Slope maps and 3-D perspectives

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

and identification of artificial slopes along boundaries between datasets (e.g., Fig. 19). The DEMs were transformed 

to UTM Zone 58 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. Figure 20 shows a perspective view of the 1/3 arc-second Wake Island 

DEM in its final version.

Figure 19. Slope map of the 1/3 arc-second Wake Island DEM. Flat-lying slopes are white; dark 

shading denotes steep slopes; Wake Island coastline in red.
Medley et al., 2009

20

Figure 20. Perspective view from the east of the 1/3 arc-second Wake Island DEM. 5x 

vertical exaggeration.
Digital Elevation Models of Wake Island

21

3.4.4	 Comparison with source data files

To ensure grid accuracy, the Wake Island 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 the NGDC 

multibeam survey AHI-07-01 and the 1/3 arc-second Wake Island DEM is shown in Figure 21. Differences range 

from -200 to 20 meters, where negative values indicate that elevations of the multibeam data are higher than the DEM 

elevations. The area where the greatest difference occurred is where the two multibeam surveys, AHI-07-01 and Hi07-01, overlap and have variations in depth.

Figure 21. Histogram of the differences between the NGDC multibeam survey AHI-07-01 and the 1/3 arc-second Wake 

Island DEM.

A histogram of the differences between ETOPO1 and the 3 arc-second DEM is shown in Figure 22. Differences 

range from -687 meters to 638 meters, where negative values indicate that elevations of the ETOPO1 data are higher 

than the DEM elevations. The area where the greatest difference occurred is along the buffer zone between the high 

resolution multibeam data and the ETOPO1 DEM. Values along the edge of the buffer zone are affected by gridding 

interpolation to create a smooth surface between the datasets. There is also a difference between the ETOPO1 and the 

3 arc-second DEM where trackline data exists. Trackline data was given a heavier weight than the ETOPO1 data in 

the gridding process, so have a stronger influence over the cell value. 

Figure 22. Histogram of the differences between ETOPO1 and the 3 arc-second Wake Island DEM.
Medley et al., 2009

22

A histogram of the differences between the SRTM topographic DEM and the 1/3 arc-second DEM is shown in 

Figure 23. Differences range from -3.5 meters to 0.98 meters, where negative values indicate that elevations of the 

SRTM data are higher than the DEM elevations. These minor differences are most likely due to resampling of the 3 

arc-second SRTM to 1/3 arc-second.

Figure 23. Histogram of the differences between SRTM data and the 1/3 arc-second Wake Island DEM.

A histogram of the differences between the estimated depths from Digital Globe Satellite and the 1/3 arcsecond DEM is shown in Figure 24. Differences range from -0.74 meters to 2.05 meters, where negative values 

indicate that elevations of the estimated depths are higher than the DEM elevations. These minor differences are most 

likely do to the averaging of cell values when the coastline falls in the same cell as an estimated depth.

Figure 23. Histogram of the differences between the Digital Globe Satellite estimated depths and the 1/3 arc-second Wake 

Island DEM.
Digital Elevation Models of Wake Island

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4. Summary and Conclusions

Two integrated bathymetric–topographic digital elevation models of the Wake Island region, with cell spacing of 

3 arc-second 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 agencies were obtained by NGDC, 

shifted to common horizontal and vertical datums, and evaluated and edited before DEM generation. The data were 

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

software. 

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

•	 Conduct hydrographic surveys for near-shore areas, especially in the lagoon and of the reefs.

•	 Complete bathymetric–topographic LiDAR surveying of the island and near shore region.

•	 Conduct hydrographic surveys for the deep ocean.

•	 Digitize the Raster Nautical Chart as an Electronic Nautical Chart (ENC).

5. Acknowledgments

The creation of the Wake Island DEM 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).

6. References

Nautical Chart #81664 (RNC), 6th Edition, 1998. Wake Island. Scale 1: 15,000 and 1:3,500. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

7. Data Processing Software

ArcGIS v. 9.2, developed and licensed by ESRI, Redlands, 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/

MATLAB v. 7.2.1,developed and licensed by hWorks Inc., 2005, http://www.mathworks.com/

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/

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/