Digital Elevation Model of Shemya, Alaska:

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)

July 31, 2009

Carignan, K.S.1

, L.A. Taylor2, B.W. Eakins1, R.R. Warnken2, E. Lim1, P.R. Medley1, and R.J. Caldwell1

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 Model of Shemya, Alaska

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Contents

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

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

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 .................................................................................................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 DEM...............................................................................................20

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

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

3.4.3 Slope map, 3-D perspective, and data distribution plot.......................................21

3.4.4 Comparison with topographic and bathymetric contours....................................23

4. Summary and Conclusions...............................................................................................................26

5. Acknowledgments............................................................................................................................26

6. References........................................................................................................................................26

7. Data Processing Software ................................................................................................................27

List of Figures

Figure 1. Shaded-relief image of the Shemya, Alaska DEM .....................................................................1

Figure 2. Google Earth image of Alaska’s Aleutian Island Chain and the 

location of the Shemya DEM ..............................................................................................2

Figure 3. USGS Attu topographic quadrangle shown with DEM boundary..............................................3

Figure 4. Source and coverage of datasets used to compile the Shemya DEM .........................................5

Figure 5. Digital coastline datasets used in developing a ‘combined coastline’

for the Shemya region..........................................................................................................6

Figure 6. Spatial coverage of the bathymetric datasets used in compiling the Shemya DEM...................9

Figure 7. Digital NOS hydrographic survey coverage in the Shemya region..........................................11

Figure 8. Spatial coverage of NGDC trackline data used in compiling the Shemya DEM .....................13

Figure 9. Spatial coverage of digitized RNC sounding data used to compile the Shemya DEM............14

Figure 10. Spatial coverage of the topographic datasets used in compiling the Shemya DEM.................15

Figure 11. SRTM 3 arc-second DEM data coverage on Attu Island..........................................................16

Figure 12. Digitized RNC land elevation points on small islands in Shemya Pass...................................17

Figure 13. Histogram of the differences between NOS hydrographic survey H07088 and 

the 1 arc-second pre-surfaced bathymetric grid.................................................................19

Figure 14. Slope map of the Shemya DEM................................................................................................21

Figure 15. Perspective view from the south of the Shemya DEM.............................................................22

Figure 16. Data contribution plot of the Shemya DEM .............................................................................22

Figure 17. Comparison of USGS topographic map to Shemya DEM topographic contours.....................23

Figure 18. Comparison of ASTER DEM contours to Shemya DEM topographic contours......................24

Figure 19. Comparison of OCS nautical chart to Shemya DEM bathymetric contours ............................25
Carignan et al., 2009

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List of Tables

Table 1. PMEL specifications for the Shemya, Alaska DEM...................................................................4

Table 2. Shoreline datasets used in the Shemya, Alaska region ...............................................................6

Table 3. Electronic navigational charts and raster nautical charts available 

in the Shemya, Alaska region ..............................................................................................7

Table 4. Bathymetric datasets used in compiling the Shemya DEM........................................................8

Table 5. Digital NOS hydrographic surveys available within the Shemya DEM boundary ..................10

Table 6. NGDC multibeam sonar survey used in compiling the Shemya DEM region .........................12

Table 7. NGDC trackline surveys used in compiling the Shemya DEM region ....................................13

Table 8. Topographic datasets used in compiling the Shemya DEM .....................................................15

Table 9. Relationship between Mean High Water and other vertical datums 

used in the Shemya DEM ..................................................................................................18

Table 10. Data hierarchy used to assign gridding weight in MB-System.................................................20
Digital Elevation Model of Shemya, Alaska

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Digital Elevation Model of Shemya, Alaska:

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 a bathymetric–topographic digital elevation model (DEM) of Shemya, Alaska, (Fig. 1) for 

the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami Research (HUhttp://nctr.pmel.noaa.

gov/UH). The 1 arc-secondF

1

F coastal DEM will be used as input for the Method of Splitting Tsunami (MOST) model 

developed by PMEL to simulate tsunami generation, propagation and inundation. The DEM was generated from 

diverse digital datasets in the region (grid boundary and sources shown in Fig. 4) and will be used for tsunami 

inundation modeling, as part of the tsunami forecast system SIFT (Short-term Inundation Forecasting for Tsunamis) 

currently being developed by PMEL for the NOAA Tsunami Warning Centers. This report provides a summary of the 

data sources and methodology used in developing the Shemya DEM.

Figure 1. Shaded-relief image of the Shemya, Alaska DEM. Bathymetric contour interval is 100 meters and topographic contour intervals are 25 

meters at elevations below 600 meters and 100 meters for elevations above 600 meters.

1. The Shemya DEM is built upon a grid of cells that are square in geographic coordinates (latitude and longitude), however, the cells are not 

square when converted to projected coordinate systems, such as UTM zones (in meters). At the latitude of Shemya, Alaska, (52° 43′ 00′′ N, 174° 

06′ 00′′ E) 1 arc-second of latitude is equivalent to 30.91 meters; 1 arc-second of longitude equals 18.77 meters.
Carignan et al., 2009

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2. Study Area

Shemya, Alaska, is located on the eastern most island of the Semichi Island Group, part of the Near Islands, on 

the western end of the Aleutian Islands Chain of Alaska (Fig. 2). The DEM boundary encompasses: Shemya Island, 

Nizki Island, Alaid Island (the Semichi Islands); Agattu Island to the southwest; and the Attu Island to the west (Fig. 

3). The topography of the five islands varies from low-lying, wave-cut platforms on the Semichi Islands to high relief 

and mountainous on Attu and Agattu Islands (Coats, 1956).

Recent studies on the geology of the Western Aleutian Chain describe the area as a transitional zone between 

oblique subduction and strike-slip motion of the Pacific Plate and North American Plate (Wilson et al., 2006) resulting 

in frequent seismic events. In 1965, Shemya recorded a tsunami of over 10 meters resulting from an 8.7 magnitude 

earthquake, the sixth largest earthquake in recorded history. The epicenter was located about 500 kilometers to the 

southeast in the Rat Islands.

Shemya served as a military base during World War II until the late 1990s, when Shemya Air Force Base was 

renamed Eareckson Air Station. The station currently operates as a radar, surveillance, weather, and aircraft refueling 

station with a population of approximately 30 full time residents. A U.S. Coast Guard LORAN station is located on 

Attu Island and has a population of around 20 residents.

Figure 2. Google Earth image of Alaska’s Aleutian Island Chain. The Shemya DEM boundary, shown as red box, is in lower left. The yellow star 

to the south of the Rat Islands marks the approximate location of the 1965 earthquake. 
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Figure 3. USGS Attu topographic quadrangle with DEM boundary shown in red.
Carignan et al., 2009

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3. Methodology

The Shemya, Alaska DEM was constructed to meet PMEL specifications (Table 1), 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 Center 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), for modeling of 

maximum flooding, respectively. Data processing and evaluation, and DEM assembly and assessment are described 

in the following subsections.

Table 1: PMEL specifications for the Shemya, Alaska, DEM.

Grid Area Shemya, Alaska

Coverage Area 172.45o to 174.21o E; 52.27o to 53.06o 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 arc-second

Grid Format ESRI Arc ASCII grid
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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: NGDC; NOAA’s Office of Coast Survey (OCS); the U.S. Fish and Wildlife Service (USFWS); and the U.S. 

Geological Survey (USGS). Safe Software’s (HUhttp://www.safe.com/UH) FME2 data translation tool package was used to 

shift datasets to NAD 83 horizontal datum and to convert them into ESRI (HUhttp://www.esri.com/UH) 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, based upon data from the NOAA Shemya tide prediction 

table (http://co-ops.nos.noaa.gov/tide_pred.html). Applied Imagery’s (HUhttp://www.appliedimagery.com/UH) Quick 

Terrain Modeler software was used to evaluate processing and gridding techniques.

Figure 4. Source and coverage of datasets used to compile the Shemya DEM.

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

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

83 datum transformations.
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3.1.1 Shoreline

Coastline datasets for the Shemya region were obtained from NOAA’s Office of Coast Survey as Electronic 

Navigational Charts (ENCs; Table 2; Fig. 5) and were used to develop a ‘combined coastline’ of the Shemya region. 

NGDC reviewed but did not use the smaller scale U.S. Fish and Wildlife Service coastline, the National GeospatialIntelligence Agency Prototype Global Shoreline Data, and the Global Self-consistent, Hierarchical, High-resolution 

Shoreline (GSHHS).

Table 2: Shoreline datasets used in the Shemya DEM.

Source Year Data Type

Spatial 

Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Datum URL

OCS ENCs 1990 to 

1997

Extracted

coastline Various WGS 84 geographic (meters) Mean High Water

http://

w1.nauticalcharts.

noaa.gov/mcd/enc/

index.htm

Figure 5. Digital coastline datasets used in developing a ‘combined coastline’of the Shemya region.
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1) OCS extracted Electronic Navigational Chart coastline

Six electronic navigational charts (ENCs) were available for the Shemya area (Table 3) and downloaded 

from NOAA’s Office of Coast Survey website (HUhttp://nauticalcharts.noaa.gov/mcd/enc/index.htmUH). The 

ENCs are in S-57 format and include coastline data referenced to Mean High Water. The coastline shapefiles 

were extracted from ENCs #16430, 16431, and 16432 using FME and compared to large-scale RNCs and the 

SRTM topographic DEM. On the northern coast of Attu Island where large-scale ENCs or RNCs were not 

available, the smaller-scale ENC #16421 coastline was used; On Agattu and Shemya Islands, ENC #16423 

was used. The extracted coastlines were merged using ArcGIS and used to create a ‘combined coastline’ of 

the Shemya region. 

Table 3: NOAA nautical charts available in the Shemya, Alaska, region.

Chart Title Format Edition Issue Date Scale

16420 Near Islands Buldir Island to Attu Island ENC and RNC 2 2007 1,300,000

16421 Near Islands from Ingenstrem Rocks to Attu Island ENC and RNC 3 2008 1:160,000

16423 Shemya Island to Attu Island ENC and RNC 3 2007 1:100,000

16430 Attu Island Theodore Pt. to Cape Wrangell ENC and RNC 3 2005 1:40,000

16431 Temnac Bay ENC and RNC 1 2005 1:20,000

16432 Massacre Bay ENC and RNC 2 2005 1:25,000

16433 Sarana Bay to Holtz Bay;Chichagof Harbor RNC 8 2003 1:20,000

16434 Agattu Island RNC 6 2004 1:40,000

16435 Alaid and Nizki Islands RNC 6 2004 1:20,000

16436 Shemya Island RNC 10 1996 1:20,000

The ‘combined coastline’ was modified to include large offshore rocks and small islets shown on the larger-scale 

RNCs. An xyz file of the ‘combined coastline’ was generated using GEODAS (http://www.ngdc.noaa.gov/mgg/gdas/

gx_announce.html) for use in compiling the Shemya DEM.
Carignan et al., 2009

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3.1.2 Bathymetry

Bathymetric datasets available for use in the compilation of the Shemya DEM include 51 NOS hydrographic 

surveys; one multibeam survey downloaded from the NGDC multibeam database; four NGDC trackline surveys; and 

soundings digitized from NOAA/OCS’s RNCs (Table 4; Fig. 6).

Table 4: Bathymetric datasets used in compiling the Shemya DEM.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

 NGDC 1933 to 

2005

NOS 

hydrographic 

survey 

soundings

Ranges from less than 10 

m to 600 m (varies with 

scale of survey, depth, 

traffic, and probability of 

obstructions)

Early Alaska Mean Lower 

Low Water

HUhttp://www.ngdc.noaa.

gov/mgg/bathymetry/

hydro.htmlUH

NGDC 2005 Multibeam 

swath sonar 1 arc-second WGS 84 geographic

assumed 

Mean Sea 

Level

http://www.ngdc.noaa.

gov/mgg/bathymetry/

multibeam.html

NGDC 1989 to 

1998

Trackline 

bathymetry

Ranges from 150 meters 

to 1 km WGS 84 geographic

assumed 

Mean sea 

Level

http://www.ngdc.

noaa.gov/mgg/geodas/

trackline.html

OCS RNC 1954 to 

2000

digitized 

soundings

from several meters to 

tens of meters WGS 84 geographic Mean Lower Low Water

http://w1.nauticalcharts.

noaa.gov/mcd/Raster/

index.htm
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Figure 6. Spatial coverage of the bathymetric datasets used in compiling the Shemya DEM.
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1) NOS hydrographic survey data

A total of 51 NOS hydrographic surveys conducted between 1934 and 1952 were available for use in 

developing the Shemya DEM. The hydrographic survey data were originally vertically referenced to Mean 

Lower Low Water (MLLW) and horizontally referenced to Early Alaskan Datum. Forty-nine of the 51 surveys 

were used in building the Shemya DEM, as some older surveys have been superseded (Table 5; Fig. 7).

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

spacing than large scale surveys. Surveys were extracted from NGDC’s online NOS hydrographic database 

(HUhttp://www.ngdc.noaa.gov/mgg/bathymetry/hydro.htmlUH). The data were then converted to shapefiles using 

FME software, an integrated collection of spatial extract, transform, and load tools for data transformation 

(HUhttp://www.safe.comUH). The surveys were subsequently clipped to a polygon 0.05 degree (~5%) larger than 

the Shemya DEM area to support data interpolation along grid edges.

After converting all NOS survey data to MHW using a constant value based on the NOAA tide tables 

(see Section 3.2.1), the data were displayed in ESRI ArcMap and reviewed for digitizing errors against 

scanned original survey smooth sheets and edited as necessary. The surveys were also compared to the 

topographic and bathymetric datasets, the ‘combined coastline’, and NOS raster nautical charts (RNCs). 

Older surveys were clipped to remove soundings that have been superseded by more recent NOS surveys and 

multibeam data.

There is no defined transformation from Early Alaska to WGS 84. As a result, some surveys were 

offset from the ‘combined coastline’ by nearly 1 km. These offset surveys were manually adjusted to fit the 

‘combined coastline’.

Table 5: Digital NOS hydrographic surveys available within the Shemya DEM boundary.

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

H06926* 1934 5,000 mean lower low water Early Alaska Datums

H06939 1943 20,000 mean lower low water Early Alaska Datums

H06940 1943 10,000 mean lower low water Early Alaska Datums

H06975 1944 2,400 mean lower low water Early Alaska Datums

H06987 1944 10,000 mean lower low water Early Alaska Datums

H07012 1944 5,000 mean lower low water Early Alaska Datums

H07013 1944 10,000 mean lower low water Early Alaska Datums

H07014 1944 10,000 mean lower low water Early Alaska Datums

H07015 1944 20,000 mean lower low water Early Alaska Datums

H07016 1944 20,000 mean lower low water Early Alaska Datums

H07017 1944 20,000 mean lower low water Early Alaska Datums

H07018 1944 40,000 mean lower low water Early Alaska Datums

H06937 1944 20,000 mean lower low water Early Alaska Datums

H06988 1944 5,000 mean lower low water Early Alaska Datums

H06990 1944 10,000 mean lower low water Early Alaska Datums

H06991 1944 20,000 mean lower low water Early Alaska Datums

H06999 1944 10,000 mean lower low water Early Alaska Datums

H07000 1944 20,000 mean lower low water Early Alaska Datums

H06874 1945 40,000 mean lower low water Early Alaska Datums

H06875 1945 10,000 mean lower low water Early Alaska Datums

H07067 1945 2,500 mean lower low water Early Alaska Datums

H07068 1945 5,000 mean lower low water Early Alaska Datums

H07077 1945 2,500 mean lower low water Early Alaska Datums

H07088 1945 20,000 mean lower low water Early Alaska Datums

H06865 1945 10,000 mean lower low water Early Alaska Datums

H06866 1945 20,000 mean lower low water Early Alaska Datums

H06868 1945 20,000 mean lower low water Early Alaska Datums

H06869 1945 40,000 mean lower low water Early Alaska Datums
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H06870 1945 10,000 mean lower low water Early Alaska Datums

H06871 1945 40,000 mean lower low water Early Alaska Datums

H06872 1945 10,000 mean lower low water Early Alaska Datums

H06873 1945 2,400 mean lower low water Early Alaska Datums

H07019 1945 40,000 mean lower low water Early Alaska Datums

H06864 1946 20,000 mean lower low water Early Alaska Datums

H06867 1946 20,000 mean lower low water Early Alaska Datums

H07135 1946 80,000 mean lower low water Early Alaska Datums

H07137 1946 60,000 mean lower low water Early Alaska Datums

H07138 1946 10,000 mean lower low water Early Alaska Datums

H07139 1946 20,000 mean lower low water Early Alaska Datums

H07141 1946 80,000 mean lower low water Early Alaska Datums

H07146 1946 40,000 mean lower low water Early Alaska Datums

H07136 1946 60,000 mean lower low water Early Alaska Datums

H07142 1946 10,000 mean lower low water Early Alaska Datums

H07143* 1946 20,000 mean lower low water Early Alaska Datums

H07144 1946 20,000 mean lower low water Early Alaska Datums

H07145 1946 20,000 mean lower low water Early Alaska Datums

H07624 1947 40,000 mean lower low water Early Alaska Datums

H07626 1947 100,000 mean lower low water Early Alaska Datums

H07634 1947 5,000 mean lower low water Early Alaska Datums

H07597 1947 20,000 mean lower low water Early Alaska Datums

H07994 1952 2,500 mean lower low water Early Alaska Datums

* superceded survey not used in building the Shemya DEM

† digitized datum recorded as NAD 83 geographic or unknown

Figure 7. Digital NOS hydrographic survey coverage in the Shemya region. Some older surveys were not used as they have been 

superseded by more recent surveys. DEM boundary in red.
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2) NGDC Multibeam swath sonar survey

A 2005 multibeam swath sonar survey was downloaded from the NGDC multibeam bathymetry 

database (http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html; Table 6). The survey is located along 

the northern side of the island group (Fig. 6). The data were gridded using MB-System (http://www.ldeo.

columbia.edu/res/pi/MB-System) at 30 meter cell size and viewed in QT Modeler for quality analysis. The 

grid was then converted to xyz format and the elevations were transformed from MSL to MHW using FME

for use in the final gridding process.

Table 6: NGDC multibeam swath sonar survey available in the Shemya region.

Survey ID Year Original Vertical Datum Institute Ship

TN182 2005 assumed Mean Sea Level University of South Carolina Thomas 

Thompson
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Figure 8. Spatial coverage of NGDC trackline survey data used in compiling the Shemya DEM.

3) NGDC Trackline surveys

Four trackline surveys were downloaded from the NGDC trackline bathymetry database (http://www.

ngdc.noaa.gov/mgg/bathymetry/multibeam.html; Table 7; Fig. 8). The survey data were converted to 

shapefile format, transformed from MSL to MHW, and clipped to the 2005 multibeam swath sonar survey. 

The resulting shapefiles were converted to xyz format to be used in the final gridding process using FME.

Table 7: NGDC trackline surveys available in the Shemya DEM region.

Survey ID Year Original Vertical Datum Institute Ship

SI933001 1972 assumed Mean Sea Level U.S. Navy Naval Oceanographic 

Office Silas Bent

L981AA 1981 assumed Mean Sea Level USGS Silas P . Lee

FARN0687 1987 assumed Mean Sea Level U.K. Natural Environmental 

Research Council Farnella

FARN0787 1987 assumed Mean Sea Level U.K. Natural Environmental 

Research Council Farnella
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4) OCS Raster Nautical Chart digitized soundings

In Shemya’s Alcan Harbor, NGDC digitized the soundings on the 1:10,000 scale inset of RNC #16436, 

providing better data coverage and utilizing the more recent chart information (Figure 9). The NOS survey data 

in the harbor dated from the late 1940s and is sparse. The addition of the digitized soundings better defined the 

harbor. Sounding elevation values on the chart (fathoms) were converted to meters and the resulting shapefile 

was converted to xyz format for use in generating the bathymetric surface and in the final gridding process. 

Figure 9. Location of soundings digitized by NGDC from the 1:10,000 scale inset on RNC #16436.
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3.1.3 Topography

Three topographic datasets in the Shemya region were obtained from the U.S. Geological Survey (USGS) 

and NOAA’s Office of Coast Survey and used to build the Shemya DEM (Table 8; Fig. 10); NED 2 arc-second is 

not available from USGS within the DEM boundary. NGDC reviewed but did not use the 1 arc-second Advanced 

Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM; http://

www.gdem.aster.ersdac.or.jp/), a product of the Ministry of Economy, Trade, and Industry (METI) of Japan and the 

United States National Aeronautics and Space Administration (NASA), as low and zero elevations did not correspond 

to large scale nautical charts or SRTM topography. The ASTER GDEM was used to verify topographic contours at 

higher elevations (Sec. 3.4.4). NGDC digitized additional elevation points to supplement the available topographic 

datasets.

Table 8: Topographic datasets used in compiling the Shemya DEM.

Source Year Data Type

Spatial

Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical Datum URL

SRTM 2000 topographic 

DEM 3 arc-second WGS 84 geographic assumed MSL http://seamless.usgs.gov/

OCS 

ENC

1990 to 

1996

extracted 

land 

elevation 

points

varies WGS 84 geographic mean high 

water

http://w1.nauticalcharts.noaa.gov/

mcd/enc/index.htm

OCS 

RNC 1996

digitized 

land 

elevation 

points

varies WGS 84 geographic mean high 

water

http://w1.nauticalcharts.noaa.gov/

mcd/Raster/index.htm

Figure 10. Spatial coverage of the topographic datasets used in compiling the Shemya DEM.
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Figure 11. SRTM 3 arc-second DEM data coverage on the island of Attu shown in tan and gaps in the SRTM DEM shown in pink. Values 

over open ocean are shown in light blue.

1) NASA Shuttle Radar Topography Mission 3 arc-second DEM

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 Earth3. The SRTM consisted of 

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

in February 2000. Data from this mission have been processed into 1 degree × 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.

For the farthest west group of the Aleutian Islands, the data have 3 arc-second spacing and are referenced 

to the WGS 84/EGM96 Geoid. While providing complete coverage of the Shemya and Agattu Islands, there 

are numerous small areas with “no data” values on the island of Attu (Fig. 11). In order to create a 1 arcsecond Shemya DEM, NGDC has resampled the 3 arc-second data to 1 arc-second and transformed to ESRI

shapefile format removing points with elevations below zero. The SRTM data contained elevation values 

over the open ocean, which were deleted by clipping to the ‘combined coastline’.

3. 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 Model of Shemya, Alaska

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2) OCS Electronic Navigational Chart (ENC) extracted land elevation points

Many of the offshore rocks and islets were not fully resolved in the SRTM topographic elevation dataset. 

To include these features in the Shemya DEM, land elevation points were extracted from ENCs #16430, 

16431, and 16432 (Fig. 10). ENC land elevation data are referenced to MHW and WGS 84 geographic and 

are located along the southern coast of Attu Island. Data were converted to xyz format for use in the final 

gridding process using FME.

3) OCS Raster Nautical Chart (RNC) digitized land elevation points

Offshore rocks and islets not fully resolved in the SRTM topographic elevation dataset and not available 

as ENC land elevations were digitized by NGDC referencing the large scale RNCs (see Table 3). Elevations on 

the charts are referenced to MHW and in units of feet. ESRI ArcCatalog tools were used to assign geographic 

coordinates to the points; FME was used to convert data to xyz format and transform elevations to meters. 

Figure 12 shows three digitized points located on the smaller islands in Shemya Pass between Shemya and 

Nizki Islands.

Figure 12. Digitized RNC land elevation points, shown as red triangles, on the small islands in Shemya Pass. The assigned elevations in 

meters are shown in the light green boxes.
Carignan et al., 2009

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3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

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

vertical datums including Mean Lower Low Water (MLLW), Mean Sea Level (MSL), and WGS 84/EGM96 Geoid. 

All datasets were transformed to MHW to provide the maximum flooding for inundation modeling. Transformations 

to MHW were based on tide predictions at Alcan Harbor on Shemya Island4 (Stephen Gill, pers. comm.).

1) Bathymetric data

The NOS hydrographic surveys, NGDC trackline surveys, and the NGDC multibeam sonar survey were 

transformed from MLLW and MSL to MHW, using a constant based on a NOAA CO-OPS tide predictions 

for Alcan Harbor on Shemya Island (http://co-ops.nos.noaa.gov/tides09/; Table 9). 

2) Topographic data

The USGS SRTM 3 arc-second DEM was originally referenced to WGS 84/EGM96 Geoid vertical 

datum. There are no survey markers in the vicinity of Shemya that relate this geodetic datum to the local tidal 

datums. Thus, it was assumed that this datum is essentially equivalent to MSL in this area. Conversion to 

MHW, using ArcCatalog tools, was accomplished by adding a constant offset of -0.518 meters (Table 9), the 

difference between MSL and MHW using tidal predictions at Alcan Harbor.

Table 9. Relationship between Mean High Water and other vertical datums used in the Shemya DEM.

 

MSL to MHW -0.518 meters

MLLW to MHW -1.036 meters

3.2.2 Horizontal datum transformations

Datasets used to compile the Shemya DEM were originally referenced to Early Alaska and WGS 84 geographic 

horizontal datums. The NOS surveys referenced to Early Alaska horizontal datums were manually shifted in ArcGIS 

to fit the combined coastline and the raster nautical charts.

4. The Diurnal Range of tide is listed 3.4 ft. Since is is a primarily a Diurnal Tide, that means that Mean Higher High Water (MHHW) and Mean 

High Water (MHW) are approximately equivalent elevations. Thus, at Shemya, MHW is 3.4 ft. above MLLW. Local Mean Sea Level (LMSL) 

would be approximately at 1.7 ft., the same as MTL at this location. These data are based upon tide station measurements long ago - there have 

been no recent tide station occupations by NOAA.
Digital Elevation Model of Shemya, Alaska

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Figure 13. Histogram of the differences between NOS hydrographic survey H07088 and 

the 1 arc-second pre-surfaced bathymetric grid

3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

After horizontal and 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:

•	 Lack of any high resolution topographic data required resampling SRTM dataset. 

•	 Data values over the ocean in the SRTM topographic dataset. The SRTM dataset required automated clipping 

to the ‘combined coastline’.

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

the multibeam surveys, differed from older NOS data by as much as 100 meters vertically. The older NOS 

survey data were excised where more recent bathymetric data exists.

•	 NOS data in Early Alaska datum, which required manual shifting in ArcMap to align with ‘combined 

coastline’.

3.3.2 Smoothing of bathymetric data

The older NOS hydrographic survey data are generally sparse at the resolution of 1 arc-second in both deep water 

and in some areas close to shore. In order to reduce the effect of artifacts in the form of lines of “pimples” in the DEM 

due to this low resolution dataset, and to provide effective interpolation into the coastal zone, a 1 arc-second-spacing 

‘pre-surface’ bathymetric grid was generated using GMT, an NSF-funded share-ware software application designed to 

manipulate data for mapping purposes (HUhttp://gmt.soest.hawaii.edu/UH).

The NOS hydrographic point data, in xyz format, were clipped to remove overlap with the NGDC multibeam 

data and then combined with trackline data, digitized soundings, and points extracted from the combined coastline—to 

provide a buffer along the entire coastline. The coastline elevation value was set to -1.0 meter to ensure a bathymetric 

surface below zero in areas where bathymetric data are sparse or non-existent.

The point data were median-averaged using the GMT tool ‘blockmedian’ to create a 1 arc-second grid 0.05 

degrees (~5%) larger than the Shemya 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 and exported as an xyz 

file for use in the final gridding process (Table 10). Figure 13 shows a histogram of NOS survey H07088 compared 

to the 1 arc-second pre-surfaced bathymetric grid. Differences cluster around zero with only 20 out of 5869 points 

varying more than 2 meters from the bathymetric surface. These few points are located at the coastline and in shallow, 

rocky areas.

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

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

either edited or not used.
Carignan et al., 2009

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3.3.3 Gridding the data with MB-System

MB-System (HUhttp://www.ldeo.columbia.edu/res/pi/MB-System/UH) was used to create the 1 arc-second Shemya 

DEM. MB-System is an NSF-funded share-ware software application specifically designed to manipulate submarine 

multibeam sonar data, though it can utilize a wide variety of data types, including generic xyz data. The MB-System 

tool ‘mbgrid’ 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 RNC digitized sounding data and NGDC multibeam survey. Least weight was given to the 

combined coastline and pre-surfaced 1 arc-second bathymetric grid.

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

Dataset Relative Gridding Weight

RNC digitized soundings 100

NGDC multibeam survey 100

NGDC trackline bathymetry 10

ENC extracted land elevations 10

NOS hydrographic surveys 10

USGS SRTM topographic DEM 10

RNC digitized land elevations 10

Pre-surfaced bathymetric grid 1

Combined Coastline 1

3.4 Quality Assessment of the DEM

3.4.1. Horizontal accuracy

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

datasets used to determine corresponding DEM cell values. Topographic features in island interiors have an estimated 

horizontal accuracy of 50 to 75 meters, based on the documented accuracy of the SRTM DEM. Bathymetric features 

are resolved only to within a few hundred meters in deep-water areas. Shallow, near-coastal regions, rivers, and harbor 

surveys have an accuracy approaching that of sub aerial topographic features. Positional accuracy is limited by: the 

sparseness of deep-water soundings; and potentially large positional uncertainty of pre-satellite navigated (e.g., GPS) 

NOS hydrographic surveys.

3.4.2 Vertical accuracy

Vertical accuracy of elevation values for the Shemya DEM is highly dependent upon the source datasets 

contributing to DEM cell values. Elevation values within the island interiors, derived from the SRTM topographic 

data, have vertical accuracies of between 10 and 15 meters. Bathymetric values were derived from the wide range of 

sounding measurements from the early 20th century to recent, GPS-navigated multibeam swath sonar survey. Gridding 

interpolation to determine bathymetric values between sparse, poorly located NOS soundings degrades the vertical 

accuracy of elevations in deep water to about 5% of water depth.
Digital Elevation Model of Shemya, Alaska

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Figure 14. Slope map of the Shemya DEM. Flat-lying slopes are white; dark shading denotes steep 

slopes; combined coastline in red.

3.4.3 Slope map, 3-D perspective, and data contribution plot

ESRI ArcCatalog was used to generate a slope grid from the Shemya DEM to allow for visual inspection and 

identification of artificial slopes along boundaries between datasets (Fig. 14). The DEM was transformed to UTM Zone 

59 North coordinates (horizontal units in meters) in ArcCatalog for derivation of the slope grid; equivalent horizontal 

and vertical units are required for effective slope analysis. Analysis of preliminary grids using Quick Terrain Modeler 

and Fledermaus revealed suspect data points, which were corrected before recompiling the DEM. Figure 1 shows a 

color image of the 1 arc-second Shemya DEM in its final version. Figure 15 shows a perspective rendering of the final 

DEM. Figure 16 shows a data contribution plot of the Shemya DEM.
Carignan et al., 2009

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Figure 16. Data contribution plot of the Shemya DEM. Gray depicts DEM cells constrained by source data. White depicts cells with elevation 

values derived from interpolation. Coastline in red.

Figure 15. Perspective view from the south of the Shemya DEM. Vertical exaggeration 2 times.
Digital Elevation Model of Shemya, Alaska

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3.4.4 Comparison with topographic and bathymetric contours

Contours generated from the Shemya DEM were compared to bathymetric and topographic contours from different 

maps to qualitatively assess elevation values. Figure 17 shows the Attu USGS 1:250,000 topographic quadrangle map 

of western Attu Island displayed with the Shemya DEM topographic contours overlaid. Land features on the map are 

well represented in the DEM contours.

Contours derived from the ASTER global digital elevation model (GDEM) were used to evaluate the topography 

of the Shemya DEM. The ASTER GDEM is referenced to WGS 84/EGM96 geoid and can be downloaded in 1° by 

1° tiles from the WIST website (https://lpdaac.usgs.gov/lpdaac/get_data/wist). Figure 18 shows a comparison of the 

contours derived from each. 

Bathymetric contours from the DEM were also compared to contours extracted from 1:160,000 scale ENC #16421 

(Fig. 19) and were generally consistent.

Figure 17. Comparison of the Attu USGS topographic quadrangle contours, shown in green, and the Shemya DEM topographic 

contours, shown in brown.
Carignan et al., 2009

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Figure 18. Comparison of ASTER GDEM contours, shown in red, to Shemya DEM topographic contours, shown in blue, on the eastern half of 

Attu Island. Note discrepencies occur principally at the coast , where the ASTER GDEM is particularly noisy.
Digital Elevation Model of Shemya, Alaska

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Figure 19. Comparison of the extracted bathymetric contours from the 1:160,000 scale ENC #16421, shown as blue dashed lines, and the 

Shemya DEM bathymetric contours, shown in red. The DEM boundary is shown in yellow.
Carignan et al., 2009

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4. Summary and Conclusions

A bathymetric–topographic digital elevation model of the Shemya, Alaska, region, with cell size of 1 arc-second, 

was developed for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami Research. The 

best available digital data from U.S. federal 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, Fledermaus, GMT, MB-System and Quick Terrain Modeler software. 

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

•	 Conduct hydrographic surveys in near-shore areas, especially in Alcan Harbor on Shemya Island and 

Massacre Bay on Attu Island.

•	 Conduct bathymetric–topographic coastal LiDAR surveys of Semichi Islands.

•	 Conduct deep water multibeam surveys for region south of Attu and Agattu Islands.

5. Acknowledgments

The creation of the Shemya DEM was funded by the NOAA Pacific Marine Environmental Laboratory. The 

authors thank Nazila Merati, Chris Chamberlin and Vasily Titov (PMEL) and Stephen Gill (NOAA/NOS).

6. References

Coats, Robert R. Reconnaissance Geology of Some Western Aleutian Islands, Alaska. Investigations of Alaskan 

Volcanoes. 1956. U.S. Geological Survey Bulletin 1028-E. 

Nautical Chart #16420 (ENC and RNC), 2nd Edition, 2007. Near Islands, Buldir Island to Attu Island. Scale 1: 

1,300,000. U.S. Department of Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16421 (ENC and RNC), 3rd Edition, 2008. Near Islands from Ingenstrem Rocks to Attu Island. Scale 

1:160,000. U.S. Department of Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16423 (ENC and RNC), 3rd Edition, 2007. Shemya Island to Attu Island. Scale 1:100,000. U.S. 

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

Nautical Chart #16430 (ENC and RNC), 3rd Edition, 2005. Attu Island, Theodore Point to Cape Wrangell. Scale 

1:40,000. U.S. Department of Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16431 (ENC and RNC), 1st Edition, 2005. Tenmac Bay. Scale 1:20,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16432 (ENC and RNC), 2nd Edition, 2005. Massacre Bay. Scale 1:25,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16433 (RNC), 8th Edition, 2003. Sarana Bay to Holtz Bay, Chichagof Harbor. Scale 1:20,000. U.S. 

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

Nautical Chart #16434 (RNC), 6th Edition, 2004. Agattu Island. Scale 1:40,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16435 (RNC), 6th Edition, 2004. Alaid and Nizki Islands. Scale 1:20,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16436 (RNC), 10th Edition, 1996. Shemya Island. Scale 1:20,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Wilson, Frederic H., S. Mohadjer, and D. M. Grey, Reconnaissance Geologic Map of Western Aleutian Islands, Alaska, 

2006. U.S. Geological Survey Open-File Report 2006-1302.
Digital Elevation Model of Shemya, Alaska

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7. Data Processing Software

ArcGIS v. 9.2 – developed and licensed by ESRI, Redlands, Alaska, HUhttp://www.esri.com/UH

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

HUhttp://www.safe.com/UH

Fledermaus v. 6.7.0 – developed and licensed by Interactive Visualization Systems (IVS 3D), Fredericton, New 

Brunswick, Canada, (http://www.ivs3d.com/).

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

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

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

by the National Science Foundation, HUhttp://gmt.soest.hawaii.edu/UH

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

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

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, HUhttp://www.appliedimagery.com/UH