noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-28

DIGITAL ELEVATION MODELS OF CHIGNIK, ALASKA:

PROCEDURES, DATA SOURCES AND ANALYSIS 

P.R. Medley

L.A. Taylor

B.W. Eakins

K.S. Carignan

R.R. Warnken

E. Lim 

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

March 2009

UNITED STATES

DEPARTMENT OF COMMERCE

Gary Locke

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Dr. Jane Lubchenco

Under Secretary for Oceans

and Atmosphere/Administrator

National Environmental Satellite,

Data, and Information Service

Mary E. Kicza

Assistant Administrator

NOAA Technical Memorandum NESDIS NGDC-28

DIGITAL ELEVATION MODELS OF CHIGNIK, ALASKA:

PROCEDURES, DATA SOURCES AND ANALYSIS

Pamela R. Medley1

Lisa A. Taylor2

Barry W. Eakins1

Kelly S. Carignan1

Robin R. Warnken2

Elliot Lim1

1

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

2

NOAA, National Geophysical Data Center, Boulder, Colorado 

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

March 2009
ii

Medley et al., 2009

NOTICE

Mention of a commercial company or product does not constitute an endorsement by the 

NOAA National Environmental Satellite, Data, and Information Service. Use of information 

from this publication concerning proprietary products or the test of such products for publicity

or advertising purposes is not authorized. 

Corresponding project contact:

Lisa A. Taylor

NOAA National Geophysical Data Center

Marine Geology and Geophysics Division

325 Broadway, E/GC 3

Boulder, Colorado 80305

Phone: 303-497-6767

Fax: 303-497-6513

E-mail: Lisa.A.Taylor@noaa.gov

http://www.ngdc.noaa.gov/mgg/inundation/tsunami/inundation.html

Also available from the National Technical Information Service (NTIS)

(http://www.ntis.gov)
iii

Digital Elevation Models of Chignik, Alaska

Contents

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

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

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

3.1 Data Sources and Processing ......................................................................................................4

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

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 Building the DEMs with MB-System ..................................................................20

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

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

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

3.4.3 Slope map and 3-D perspective...........................................................................21

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

3.4.5 Comparison with U.S. Geological Survey topographic elevations.....................23

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

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

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

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

List of Figures

Figure 1. Shaded-relief image of the Chignik region, derived from the 1 arc-second DEM .....................1

Figure 2. Chignik Bay during the summer months.....................................................................................2

Figure 3. Source and coverage of datasets used in compiling the Chignik DEMs.....................................4

Figure 4. Digital coastline datasets in the Chignik vicinity shown with SRTM topographic data.............5

Figure 5. Chignik’s small boat harbor ........................................................................................................7

Figure 6. Digital NOS hydrographic survey coverage in the Chignik region ............................................9

Figure 7. NOS shallow-water multibeam swath sonar survey H11065....................................................12

Figure 8. Lidar survey data coverage in the southwestern portion of the 1 arc-second DEM .................13

Figure 9. Bathymetry surrounding Mitrofania Island...............................................................................14

Figure 10. Source and coverage of topographic datasets used in compiling the Chignik DEMs...............15

Figure 11. Example of gaps in SRTM data coverage .................................................................................17

Figure 12. Detail of Chignik small boat harbor with georeferenced aerial image underlying SRTM 

topographic data.................................................................................................................17

Figure 13. Locations of the 8 NOAA tide stations used to average tidal datum relationships...................18

Figure 14. Histogram of the differences between NOS hydrographic survey H10759 and the 

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

Figure 15. Slope map of the 1 arc-second Chignik DEM...........................................................................21

Figure 16. Perspective views of the Chignik DEMs...................................................................................22

Figure 17. Histogram of the differences between the SRTM topographic dataset and the 1/3 arc-second 

Chignik DEM.....................................................................................................................23

Figure 18. Comparison between USGS topographic contours and the 1/3 arc-second Chignik DEM 

topographic contours .........................................................................................................24
iv

Medley et al., 2009

List of Tables

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

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

Table 2. Shoreline datasets used in compiling the Chignik DEMs...........................................................5

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

Table 4. Digital NOS hydrographic surveys used in compiling the Chignik DEMs ..............................10

Table 5. Topographic datasets used in compiling the Chignik DEMs....................................................15

Table 6. Relationship between MHW and other vertical datums in the Chignik region.........................18

Table 7. Data hierarchy used to assign gridding weight in MB-System..................................................20
Digital Elevation Models of Chignik, Alaska:

Procedures, Data Sources and Analysis

1. Introduction

 In October 2008, the National Geophysical Data Center (NGDC), an office of the National Oceanic and

Atmospheric Administration (NOAA), developed two integrated bathymetric–topographic digital elevation models 

(DEMs) centered on Chignik, Alaska 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 developed by PMEL to simulate tsunami generation, propagation and inundation. A 1 arcsecond1

 DEM (Fig. 1) was generated from diverse digital datasets in the region (see Fig. 3). It will be used for tsunami 

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

being developed by PMEL for the NOAA Tsunami Warning Centers. A smaller 1/3 arc-second DEM (grid boundary 

shown in Fig. 3) was generated for the area surrounding Chignik Bay to increase forecasting accuracy around the 

harbor. This report provides a summary of the data sources and methodology used in developing the Chignik DEMs.

Figure 1. Shaded-relief image of the Chignik region, derived from the 1 arc-second DEM. Contour interval is 500 meters.

1. In polar latitudes, longitude lines are spaced significantly closer together than latitude lines, approaching zero at the poles. While the DEM is

built upon grids of square cells in geographic coordinates, they are not square cells when converted to meters. At the latitude of Chignik, Alaska 

(56°17′43′′ N, 158°24′08′′ W) 1 arc-second of latitude is equivalent to 30.78 meters; 1 arc-second of longitude equals 17.60 meters.
2

Medley et al., 2009

2. Study Area

 Chignik, Alaska (Fig. 2), population 80, is located approximately 450 miles southwest of Anchorage and 

250 miles southwest of Kodiak Island at 56°17’42”N 158°24’7”W. Chignik is a fishing village where many residents

have dual residency in either Kodiak or Anchorage and only live in Chignik during the summer for the salmon season 

(http://www.chignikbay.com/).

 Chignik is approximately 40 miles east of Mt. Veniaminol and contains many deposits of volcanic ash and 

cinders. The geology is predominately brown to tan sandstone with coal seams and conglomerate. Rocks exposed 

in Chignik Bay are part of the Chignik Formation and are approximately 77 to 68 million years old. The Chignik 

Formation represents a cyclic sequence of predominately shallow to nearshore marine environments.

 Chignik’s land features are characterized by long mountain slopes with many streams. Mountain runoff from 

waterfalls occurs after heavy rains and floods the town’s roads. The steep topography has well-drained soils on the

slopes of loamy volcanic ash overlying sandy and cindery ash. The soil in the community of Chignik consists mostly 

of wetlands, pebble rock, and sand. The beach consists of rock, pebble, and sand, and the banks are mud, rock, and a 

sand mixture. 

Figure 2. Chignik Bay during the summer months. [Photo credit: U.S. Fish and Wildlife Service; http://images.fws.gov/]
3

Digital Elevation Models of Chignik, Alaska

3. Methodology

 The Chignik 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 Centers use of SIFT to provide real-time tsunami forecasts in an operational 

environment. The best available digital data were obtained by NGDC and shifted to common horizontal and vertical 

datums: North America Datum of 1983 (NAD 83) and mean high water (MHW), for modeling of maximum flooding,

respectively2. Data processing and evaluation, and DEM assembly and assessment are described in the following 

subsections. The 1 arc-second DEM fully encompasses the 1/3 arc-second DEM, but extends out to a much broader 

boundary for tsunami propagation modeling (Fig. 3).

Table 1a: PMEL specifications for the 1 arc-second Chignik DEM.

Grid Area Chignik, Alaska

Coverage Area 157.56o to 159.26o W; 55.79o to 56.72o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984 (WGS 84)

Vertical Datum MHW

Vertical Units Meters

Cell Size 1 arc-second

Grid Format ESRI ASCII raster grid

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

Grid Area Chignik, Alaska

Coverage Area 158.54o to 158.21o W; 56.23o to 56.49o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984 (WGS 84)

Vertical Datum MHW

Vertical Units Meters

Cell Size 1/3 arc-second

Grid Format ESRI ASCII raster grid

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

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

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

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

wave’s passage across ocean basins. This DEM is identified as having a WGS 84 geographic horizontal datum even though the underlying elevation

data were typically transformed to NAD 83 geographic. At the scale of the DEM, WGS 84 and NAD 83 geographic are identical and may be used 

interchangeably.
4

Medley et al., 2009

3.1 Data Sources and Processing

Shoreline, bathymetric, and topographic digital datasets (Fig. 3) were obtained from several U.S. federal 

agencies, including: NOAA’s Office of Coast Survey (OCS) and NGDC; the National Geospatial-Intelligence Agency

(NGA); the U.S. Fish and Wildlife Service (USFWS); and the U.S. Geological Survey (USGS). Safe Software’s FME

data translation tool package was used to shift datasets to NAD 83 horizontal datum and to convert into ESRI ArcGIS

shapefiles3. The shapefiles were then displayed with ArcGIS to assess them and manually edit datasets. Vertical datum 

transformations to MHW were also accomplished using FME, based upon data from several NOAA tide stations in the 

Chignik region, as no VDatum model software was available for this area. 

Figure 3. Source and coverage of datasets used in compiling the Chignik DEMs. 

White areas denote data gaps.

3. 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.
5

Digital Elevation Models of Chignik, Alaska

3.1.1 Shoreline

Three digital coastline datasets available for the Chignik region were analyzed for inclusion in the Chignik 

DEMs: NOAA Electronic Navigational Chart (ENC)4 #16011; NGA High Water Line; and USFWS statewide Alaska 

digital coastline. Comparisons between the different coastline datasets, NOS hydrographic surveys, Space Shuttle 

Radar Topography (SRTM) topographic DEM, Raster Navigational Charts (RNCs), and Google Earth satellite 

imagery showed that the USFWS coastline (Table 2) best fit the topographic and bathymetric data (Figs. 4 and 5) and

was used to create a “final coastline” for the Chignik DEMs.

Table 2. Shoreline datasets used in compiling the Chignik DEMs.

Source Year Data Type Spatial Resolution Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum

USFWS 2006 Compiled 

coastline Various WGS 84 geographic Undefined

Figure 4. Digital coastline datasets in the Chignik vicinity shown with SRTM topographic data.

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

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

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

other critical changes. NOAA ENC®s are available for free download on the OCS web site. [Extracted from NOAA OCS web site: http://nauticalcharts.noaa.gov/mcd/enc/]
6

Medley et al., 2009

1) National Geospatial-Intelligence Agency High Water Line

The NGA5 Office of Global Navigation, Maritime Division developed the Global Shoreline Data set

from digitized orthorectified NASA, 2000 era, LANDSAT GeoCover (multi-spectral imagery). This new

shoreline is an approximation of the High Water Line with a resolution of 1:75,000 or smaller. The NGA 

coastline provides complete coverage of the 1 arc-second DEM area but was not used in the final coastline,

as the USFWS was more accurate.

2) U.S. Fish and Wildlife Service

USFWS compiled a seamless digital coastline of the State of Alaska from a variety of sources, including: 

the National Hydrography Dataset, NOAA nautical charts, USFWS, National Geographic Topo Software, 

U.S. Army Corps of Engineers, and Alaska Department of Natural Resources. This dataset was provided to 

NGDC by Bret Christensen, USFWS. Though efforts were made to obtain the highest resolution coastlines 

available, vertical datums were not determined nor controlled in any way in compiling the USFWS coastline; 

the horizontal datum of the compiled USFWS coastline is WGS 84. The USFWS coastline provides complete 

coverage of the 1 arc-second Chignik DEM area and most closely matched the SRTM topographic data and 

raster nautical charts.

3) NOAA Electronic Navigational Chart #16011 extracted coastlines

ENC #16011 provides complete coverage of the 1 arc-second Chignik DEM, however, the coastline 

extracted from the chart is not as detailed as the NGA and the USFWS coastlines, so was not used in building 

the final coastline for the DEM.

To obtain the best digital MHW coastline, NGDC edited the USFWS coastline in ArcGIS to fit recent NOS

hydrographic survey data, SRTM data and Google Earth satellite imagery. Recent breakwater construction for a small 

boat harbor in Chignik Bay is not represented in any of the three available coastlines. Therefore, an aerial image of 

the small boat harbor was georeferenced in ArcGIS and the breakwater was manually digitized by NGDC and added 

to the final coastline (Figs. 5 and 12). The final coastline was subsampled to 10-meter spacing and converted to point

data for use in the gridding process. It was also used as a coastal buffer for the bathymetric pre-surfacing algorithm 

(see Sec. 3.3.2) to ensure that interpolated bathymetric values reached “zero” at the coast. The final coastline was also

used to clip the SRTM and National Elevation Dataset (NED) topographic DEMs, which contained elevation values, 

typically zero, over the open ocean (Sec. 3.1.3).

5. The NGA Office of Global Navigation, Maritime Division is in the process of developing a new version of World Vector Shoreline (WVS®)

and in support of this effort has acquired a prototype Global Shoreline Data set. This new shoreline is an approximation of the High Water Line; 

it is NOT a Mean High Water Line since the source data have not been tide coordinated (http://www.nga.mil/portal/site/nga01/index.jsp?epi-con

tent=GENERIC&itemID=9328fbd8dcc4a010VgnVCMServer3c02010aRCRD&beanID=1629630080&viewID=Article). The prototype Global 

Shoreline Data set (satellite derived High Water Line) in work at NGA has been acquired from orthorectified NASA, 2000 era, LANDSAT GeoCover (multi-spectral imagery). [Extracted from metadata]
7

Digital Elevation Models of Chignik, Alaska

Figure 5. Chignik’s small boat harbor. A) NGDC manually digitized the coastline to include the 

boat harbor, and added 1 meter elevations approximately every 4 meters. The final coastline is in 

black. B) Aerial photo of the small boat harbor taken from a plane.

[Photo credit: West Construction Company, Inc.; http://www.bwcc.us]

A

B
8

Medley et al., 2009

3.1.2 Bathymetry

Bathymetric datasets used in the compilation of the Chignik DEMs include 60 NOS hydrographic surveys, a 

shallow-water multibeam swath sonar survey, hydrographic lidar surveys, ENC soundings, and subsets extracted from 

the ETOPO1 Global Relief Model (Table 3). Datasets were originally referenced to mean lower low water (MLLW) 

or mean sea level (MSL).

Table 3. Bathymetric datasets used in compiling the Chignik DEMs.

Source Year Data Type Spatial 

Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum 

(meters)

URL

NGDC

1914 

to 

2005

NOS 

hydrographic 

survey 

soundings

1:10,00 to 

1:120,000

Unalaska Datum, 

Early Alaskan Datum, 

Undefined Datum,

NAD 83 geographic

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

hydro.html

NOS 2001

Shallow-water 

multibeam 

sonar

10 meters NAD 83 geographic MLLW http://www.ngdc.noaa.gov/mgg/bathymetry/

hydro.html

NOS 2004 Hydrographic 

lidar surveys 1:10,000 NAD 83 geographic MLLW http://www.ngdc.noaa.gov/mgg/bathymetry/

hydro.html

NOAA 

ENC

#16561

Extracted ENC 

sounding data 1:80,000 WGS 84 geographic MLLW http://nauticalcharts.noaa.gov/mcd/enc

ETOPO1 2008 Global Relief 

Model 1 arc-minute WGS 84 geographic MSL http://www.ngdc.noaa.gov/mgg/global/

global.html

1) National Ocean Service hydrographic survey data

A total of 60 NOS hydrographic surveys conducted between 1914 and 2005 were used in the development 

of the Chignik DEMs (Fig. 6; Table 4); a survey containing sparse data from 1910 was excluded (H03194). 

The hydrographic survey data were originally vertically referenced to MLLW and horizontally referenced to 

either Early Alaska, Unalaska, Undetermined, or NAD 83 datums. 

Data point spacing for the surveys ranged from about 10 meters in shallow water to 1.2 kilometers in 

deep water. All surveys were extracted from NGDC’s NOS Hydrographic Survey Database (http://www.

ngdc.noaa.gov/mgg/bathymetry/hydro.html) in their original datums (Table 4). The data were then converted 

to NAD 83 using FME software, an integrated collection of spatial extract, transform, and load tools for data 

transformation; some NOS surveys were manually shifted in ArcGIS to fit the final coastline. The surveys

were subsequently clipped to a polygon 0.05 degrees (~5%) larger than the 1 arc-second gridding area to 

support data interpolation across DEM boundaries.

After converting all NOS survey data to MHW (see Sec. 3.2.1), the data were displayed in ESRI ArcMap

and reviewed for digitizing errors against scanned original survey smooth sheets and compared to the SRTM 

and NED topographic data, the final coastline and Google Earth satellite imagery.
9

Digital Elevation Models of Chignik, Alaska

Figure 6. Digital NOS hydrographic survey coverage in the Chignik region. Red denotes boundary of the 1 arc-second DEM, 

blue denotes boundary of the 1/3 arc-second DEM. The combined coastline is in black.
10

Medley et al., 2009

Table 4. Digital NOS hydrographic surveys used in compiling the Chignik DEMs.

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

H03722* 1914 100,000 MLLW Unalaska Datum

H03796 1915 100,000 MLLW Undetermined Horizontal Datum

H04388* 1924 20,000 MLLW Undetermined Horizontal Datum

H04389* 1924 20,000 MLLW Undetermined Horizontal Datum

H04397* 1924 20,000 MLLW Undetermined Horizontal Datum

H04433* 1924 20,000 MLLW Undetermined Horizontal Datum

H04502* 1925 20,000 MLLW Undetermined Horizontal Datum

H04506 1925 60,000 MLLW Undetermined Horizontal Datum

H04509 1925 60,000 MLLW Undetermined Horizontal Datum

H04510* 1925 20,000 MLLW Undetermined Horizontal Datum

H06880 1943 120,000 MLLW Early Alaska Datums

H06929 1943 120,000 MLLW Early Alaska Datums

H07923 1953 20,000 MLLW Early Alaska Datums

H07924 1951 20,000 MLLW Early Alaska Datums

H07927 1951 40,000 MLLW Early Alaska Datums

H07928 1951 40,000 MLLW Early Alaska Datums

H10490 1993 10,000 MLLW NAD 83

H10557 1994 10,000 MLLW NAD 83

H10692 1996 10,000 MLLW NAD 83

H10693 1996 10,000 MLLW NAD 83

H10694 1996 10,000 MLLW NAD 83

H10695 1996 10,000 MLLW NAD 83

H10696 1996 20,000 MLLW NAD 83

H10697 1996 10,000 MLLW NAD 83

H10697A 1997 10,000 MLLW NAD 83

H10698 1996 10,000 MLLW NAD 83

H10699 1996 10,000 MLLW NAD 83

H10699A 1996 10,000 MLLW NAD 83

H10701 1996 20,000 MLLW NAD 83

H10701A 1997 20,000 MLLW NAD 83

H10702 1996 10,000 MLLW NAD 83

H10705 1996 10,000 MLLW NAD 83

H10759 1997 10,000 MLLW NAD 83

H10760 1997 10,000 MLLW NAD 83

H10761 1997 10,000 MLLW NAD 83

H10762 1997 10,000 MLLW NAD 83

H10765 1997 10,000 MLLW NAD 83

H10767 1997 10,000 MLLW NAD 83

H10768 1997 10,000 MLLW NAD 83

H10770 1997 10,000 MLLW NAD 83

H11021 2002 40,000 MLLW NAD 83

H11066 2001 40,000 MLLW NAD 83

H11191 2003 10,000 MLLW NAD 83

H11192 2003 10,000 MLLW NAD 83

H11193 2003 10,000 MLLW NAD 83
11

Digital Elevation Models of Chignik, Alaska

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

H11194 2003 10,000 MLLW NAD 83

H11195 2003 10,000 MLLW NAD 83

H11229 2003 10,000 MLLW NAD 83

H11230 2003 10,000 MLLW NAD 83

H11231 2003 10,000 MLLW NAD 83

H11232 2003 10,000 MLLW NAD 83

H11233 2003 10,000 MLLW NAD 83

H11325 2004 40,000 MLLW NAD 83

H11459 2005 10,000 MLLW NAD 83

H11460 2005 10,000 MLLW NAD 83

H11461 2005 10,000 MLLW NAD 83

H11462 2005 10,000 MLLW NAD 83

H11463 2005 20,000 MLLW NAD 83

H11464 2005 10,000 MLLW NAD 83

H11465 2005 40,000 MLLW NAD 83

* Geographic position manually adjusted in ArcGIS to fit combined coastline.
12

Medley et al., 2009

2) National Ocean Service shallow-water multibeam sonar

 NOS conducted a shallow-water multibeam swath sonar survey just south of Nakchamik Island (Fig. 

7). The survey was downloaded from NGDC’s NOS Hydrographic Survey Database (http://www.ngdc.noaa.

gov/mgg/bathymetry/hydro.html) in ASCII xyz gridded format in NAD 83 geographic at 10 meter resolution 

and referenced to MLLW. This dataset provided dense bathymetric coverage in the area south of Nakchamik 

Island.

Figure 7. NOS shallow-water multibeam swath sonar survey H11065. The high-resolution survey has soundings 10 meters apart and 

covers the near-shore region south of Nakchamik Island.
13

Digital Elevation Models of Chignik, Alaska

3) National Ocean Service hydrographic lidar surveys

NOS’s Pacific Hydrographic Branch provided NGDC with seven recent hydrographic lidar surveys

located in the southwestern part of the 1 arc-second DEM (Fig. 8). The lidar surveys are referenced to NAD 

83 and MLLW. These surveys range from -40 to 46 meters in elevation and have a point spacing of 5 meters. 

The elevations on and near the shoreline are generally consistent with the NOS and SRTM datasets; higher 

elevations are less consistent.

Figure 8: Lidar survey data coverage in the southwestern portion of the 1 arc-second DEM. 

Final coastline is in black. Red line denotes the bottom of the DEM extent.

4) NOAA Electronic Navigational Chart soundings

NOAA nautical charts #16011 and #16561 were available in ENC format and, as no bathymetric survey 

data were available for the area surrounding Mitrofania Island and other southern portions of the DEM, 

sounding data were extracted from these charts using FME. Chart #16011 covers the complete DEM but 

has a 1:1,023,188, resolution creating sparse soundings and was not used in the final gridding process. Chart

#16561 covers most of the southern portion of the DEM and has a 1:80,000 resolution. Soundings were only 

used where no NOS data were available. Soundings range from ~0.5 kilometers to ~2 kilometers apart, and 

depths range from -2 meters to -150 meters at MHW. The “pimple” pattern evident in the 1 arc-second DEM 

between Mitrofania Island and the mainland is due to the low point density of these soundings. No other data 

were available in this area (Fig. 9).
14

Medley et al., 2009

5) ETOPO1 Global Relief Model

Three areas within the DEM boundary where no survey data were available are located in the southwestern 

section of the DEM around Mitrofania Island, the south central region, and the southeastern region (see Fig. 

3; ETOPO1 source data). The 1 arc-minute ETOPO1 data were extracted from the NGDC web site (http://

www.ngdc.noaa.gov/mgg/global) and converted to point shapefiles using FME. 

The ETOPO1 grid (Amante and Eakins, 2009) has a 1 arc-minute cell size with data in WGS 84 

geographic coordinates and MSL. These data are exceptionally coarse at the resolution of the 1 arc-second 

Chignik DEM, however, they provide the only digital constraints on the bathymetry in the three southern 

regions. Extracted bathymetric soundings are generally shallower than overlapping measured bathymetric 

values (e.g., NOS hydrographic soundings and ENC soundings). These values are considered to be of low 

accuracy and result in the expression of isolated highs in the vicinity of Mitrofania Island on the 1 arc-second 

DEM (Fig. 9).

Figure 9. Bathymetry surrounding Mitrofania Island. A) Pregridded bathysurface illustrating the expression of isolated highs due to sparse, 

low accuracy data in this region. B) ETOPO1 data and ENC data surrounded by NOS polygons overlying a raster nautical chart. 
15

Digital Elevation Models of Chignik, Alaska

3.1.3 Topography

Topographic datasets for Chignik were obtained from the USGS: NED 2 arc-second gridded topography 

and 1 arc-second NASA SRTM (Fig. 10; Table 5). NGDC also digitized harbor features not represented in either 

topographic dataset.

Table 5. Topographic datasets used in compiling the Chignik DEMs.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical Datum URL

USGS 

NED 2006 Topographic DEM 2 arc-second grid NAD 27 geographic NGVD29(meters) http://ned.usgs.gov/

NASA

SRTM 2000 Topographic DEM 1 arc-second grid WGS 84 geographic

WGS 84/

EGM96 Geoid 

(meters)

http://srtm.usgs.gov/

NGDC 2008 Digitized harbor 

features 5 meter point spacing WGS 84 geographic MHW

Figure 10. Source and coverage of topographic datasets used in compiling the Chignik DEMs. 1 arc-second DEM boundary in red; 

1/3 arc-second DEM boundary in blue; combined coastline in black.
16

Medley et al., 2009

1) U.S. Geological Survey National Elevation Dataset topography

USGS’s NED provides complete 2 arc-second coverage of Alaska6. Data are in NAD 27 Alaska 

geographic coordinates and North American Vertical Datum of 1929 (NGVD29) vertical datum (meters), 

and are available for download as raster DEMs. The extracted bare-earth elevations have a vertical accuracy 

of +/- 7 to 15 meters depending on source data resolution (see the USGS Seamless web site for specific source

information: http://seamless.usgs.gov). The dataset was derived from USGS quad maps and aerial photos 

based on surveys conducted in the 1970s and 1980s. The NED data were used only to fill in gaps within the

SRTM data (e.g., Fig. 7).

2) NASA Space Shuttle Radar Topography Mission

NASA’s SRTM obtained elevation data on a near-global scale to generate the most complete highresolution digital topographic database of Earth7

. 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 × 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 U.S. regions, the data have 1 arc-second spacing and are referenced to the WGS 84/EGM96 Geoid. 

While providing near complete coverage of the Aleutian Islands in the vicinity of Chignik, there are numerous 

small areas with “no data” values (e.g., Fig. 11), necessitating use of the lower-resolution NED topographic 

data in these areas. The SRTM DEM also contains values over the open ocean, which were deleted by 

clipping to the final coastline. Negative SRTM values on land were converted to 0.9 meter elevation using

FME to prevent terrestrial areas from being flooded at MHW.

6. The USGS National Elevation Dataset (NED) has been developed by merging the highest-resolution, best quality elevation data available across 

the United States into a seamless raster format. NED is the result of the maturation of the USGS effort to provide 1:24,000-scale Digital Elevation 

Model (DEM) data for the conterminous U.S. and 1:63,360-scale DEM data for Alaska. The dataset provides seamless coverage of the United 

States, HI, AK, and the island territories. NED has a consistent projection (Geographic), resolution (1 arc-second), and elevation units (meters). The 

horizontal datum is NAD 83, except for AK, which is NAD 27. The vertical datum is NAVD88, except for AK, which is NGVD29. NED is a living 

dataset that is updated bimonthly to incorporate the “best available” DEM data. As more 1/3 arc-second (10 m) data covers the U.S., then this will 

also be a seamless dataset. [Extracted from USGS NED web site, http://ned.usgs.gov/]

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

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

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

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

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

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

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

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

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

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

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

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

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

south and 60 degrees north latitude, which comprises almost exactly 80% of Earth’s total landmass. [Extracted from SRTM online documentation]
17

Digital Elevation Models of Chignik, Alaska

Figure 11. Example of gaps (white areas) in SRTM data coverage. Gaps were filled with topographic 

values from the NED DEM. Final coastline in black.

3) NGDC digitized harbor features

Using an aerial view image as a reference, NGDC digitized a point shapefile to represent the newly

constructed small boat harbor in Chignik Bay. The breakwater is just south of the spit at the entrance to the 

harbor and was not represented in any available coastline or topographic data. Elevations applied to digitized 

points were one meter above MHW.

Figure 12. Detail of Chignik small boat harbor with georeferenced aerial image underlying SRTM topographic data. 

(http://www.commerce.state.ak.us/dca/profiles/profile-maps.htm)
18

Medley et al., 2009

Figure 13. Locations of the 8 NOAA tide stations 

used to average tidal datum relationships. 

(http://tidesandcurrents.noaa.gov/)

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation and evaluation of the Chignik DEMs were originally referenced to a number 

of vertical datums including: MLLW, MSL, WGS 84/EGM96 Geoid, and NGVD29. All datasets were transformed to 

MHW to provide the maximum flooding for inundation modeling. Conversions were accomplished by averaging the

datum values of 8 NOAA tide stations in the region (Fig. 13).

1) Bathymetric data

The NOS hydrographic surveys, shallow-water multibeam swath sonar survey, and lidar surveys were 

transformed from MLLW to MHW, using FME software, by adding a constant offset measured at NOAA tide 

stations in the Chignik region (see Table 6). The 8 tide stations in the region used to calculate an average tidal 

measurement only had one, two, and four months of data records; there were no tide stations in the region 

with records that spanned multiple years. 

2) Topographic data

The NED and SRTM DEMs were originally in NGVD29 and WGS 84/EGM96 Geoid vertical datums, 

respectively. There are no survey markers in the vicinity of Chignik that relate these two geodetic datums to 

the local tidal datums. Thus, it was assumed that both datums are essentially equivalent to MSL in this area 

(Table 6). Conversion to MHW was accomplished by adding a constant value of -1.061 meters.

Table 6. Relationship between MHW and other vertical datums in the Chignik region.*

Vertical datum Difference to MHW

MTL -0.983

NGVD29 + -1.061

MSL -1.061

MLW -1.966

MLLW -2.434

* Datum relationships determined by tidal station #9459881 at Chignik.

+ Assumed to be equivalent to MSL.

3.2.2 Horizontal datum transformations

Datasets used in compiling the Chignik DEMs were originally referenced to Early Alaska, “undetermined”, 

Unalaska, NAD 83, or WGS 84 horizontal datums. The NOS surveys referenced to Early Alaska, “undetermined” or 

Unalaska horizontal datums were manually shifted in ArcGIS to fit the final coastline.
19

Digital Elevation Models of Chignik, Alaska

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

ESRI ArcMap for inter-dataset consistency. Problems and errors were identified and resolved before proceeding with

subsequent gridding steps. The evaluated and edited ESRI shapefiles were then converted to xyz files in preparation

for gridding. Problems included:

•	 Data values over the open ocean in the NED and SRTM topographic DEMs. Each dataset required 

automated clipping to the final coastline.

•	 Holes in the SRTM DEM which were filled in with lower-resolution data from the NED DEM.	

•	 Lack of accurate bathymetric data in the southern region of the DEM, especially around Mitrofania Island.

•	 Lack of accurate bathymetric and topographic data near the coastline throughout the region.

3.3.2 Smoothing of bathymetric data

The NOS hydrographic survey spacings generally depend on the collection year: older surveys (1914-1951) 

are more sparse with point spacing up to 1.2 kilometers in deep water; newer surveys (1993-2005) have point spacings 

generally less than 40 meters. ETOPO1 and ENC data used to fill gaps in the NOS data have a much lower resolution

with spacing up to 2 kilometers apart. In order to reduce the effect of artifacts in the form of lines of “pimples” in the 

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

arc-second spacing “pre-surface” or grid was generated using GMT, an NSF-funded shareware software application 

designed to manipulate data for mapping purposes.

The NOS hydrographic point data, in xyz format, were combined with the shallow-water multibeam data, 

hydrographic coastal lidarsurveys, ENC soundings, and the ETOPO1 data into a single file, along with points extracted

every 10 meters from the final coastline. A -1 meter value was assigned to the coastline to make sure that the offshore

elevations remained negative, which was necessary due to the sparseness of bathymetric data near the coast. These 

point data were then smoothed onto a 1 arc-second grid using the GMT tool “blockmedian”. The GMT tool “surface” 

was then applied to interpolate values for cells without data. The GMT grid created by “surface” was converted into 

an ESRI Arc ASCII grid file using the MB-System tool “mbm_grd2arc”. Conversion of this Arc ASCII grid file into an

Arc raster format permitted clipping of the grid with the final coastline (to eliminate data interpolation into land areas).

The resulting surface was compared with the original soundings to ensure grid accuracy (e.g., Fig. 14), converted to a 

shapefile, and exported as an xyz file for use in the final gridding process (see Table 7). The statistical analysis of the

differences between the 1 arc-second bathymetric surface and one of the NOS surveys (see Fig. 14) showed that the 

majority of the NOS soundings are in a good agreement with the bathymetric surface. The few exceptions where the 

difference reached tens of meters are attributed to the rugged terrain when two or more closely positioned points were 

averaged to obtain the elevation of one grid cell.

Figure 14. Histogram of the differences between NOS hydrographic survey H10759 (a relatively dense survey near 

Chignik Harbor) and the 1 arc-second pre-surfaced bathymetric grid. The greatest differences resulted from averaging 

several closely spaced soundings from overlapping surveys.
20

Medley et al., 2009

3.3.3 Building the DEMs with MB-System

MB-System was used to create a 1 arc-second Chignik DEM and a 1/3 arc-second DEM centered on Chignik 

Bay. MB-System is an NSF-funded shareware software application specifically designed to manipulate submarine

multibeam sonar data, though it can utilize a wide variety of data types, including generic xyz data, and interpolate values 

for cells without data. The MB-System tool “mbgrid” applied a tight spline tension to the xyz data, and interpolated 

values for cells without data. The data hierarchy used in the “mbgrid” gridding algorithm, as relative gridding weights, 

is listed in Table 7. Greatest weight was given to NGDC digitized harbor feature datasets. Least weight was given to 

the pre-surfaced 1 arc-second bathymetric grid, ETOPO1 bathymetric grids, ENC #16561 soundings, final coastline,

and USGS NED DEM.

 Table 7. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

NGDC digitized features 1,000

 SRTM topographic DEM 100

Shallow-water multibeam survey 100

NOS lidar surveys 100

Recent NOS hydrographic surveys 100

Older NOS hydrographic surveys 10

USGS NED topographic DEM 1

Final coastline 1

ENC #16561 soundings 1

ETOPO1 bathymetric DEM 1

Pre-surfaced bathymetric grid 1

3.4 Quality Assessment of the DEMs

3.4.1. Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Chignik DEMs is dependent upon 

the datasets used to determine corresponding DEM cell values. Topographic features have a horizontal accuracy no 

better than 30 meters. Bathymetric features are resolved only to within a few tens of meters to a few hundred meters 

in deep-water areas; shallow, near-coastal regions have an accuracy approaching the sub aerial topographic features. 

3.4.2 Vertical accuracy

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

contributing to grid cell values. Topographic values are derived from the SRTM DEM, which has a vertical accuracy 

typically better than 16 meters, and from the USGS NED DEM, which has an estimated vertical accuracy of 7 to 15 

meters. Bathymetric values are derived from a wide range of input data. Modern NOS standards are 0.3 meters in 0 

to 20 meters of water, 1.0 meters in 20 to 100 meters of water, and 1% of the water depth in 100 meters of water. The 

ENC soundings and ETOPO1 data have a very low resolution and vertical accuracy of 5% and 10% of water depth. 
21

Digital Elevation Models of Chignik, Alaska

3.4.3 Slope map and 3-D perspective

ESRI ArcCatalog was used to generate a slope grid from the 1 arc-second Chignik DEM to allow for visual 

inspection and identification of artificial slopes along boundaries between datasets (e.g., Fig. 15). The DEM was

transformed to UTM zone 4 coordinates (horizontal units in meters) in ArcCatalog for derivation of the slope grid; 

equivalent horizontal and vertical units are required for effective slope analysis. Three-dimensional viewing of the 

UTM-transformed DEM (e.g., Fig. 16) was accomplished using ESRI ArcScene and Applied Imagery’s Quick Terrain 

Modeler. Analysis of preliminary grids revealed suspect data points, which were corrected before recompiling the 

DEM. 

Figure 15. Slope map of the 1 arc-second Chignik DEM. 

Flat-lying slopes are white; dark shading denotes steep slopes; final coastline in red.
22

Medley et al., 2009

Figure 16. Perspective views of the Chignik DEMs. A)1 arc-second Chignik DEM from the northeast. Vertical 

exaggeration–times 2. B) 1/3 arc-second Chignik DEM from the northeast. Vertical exaggeration–times 1.

A

B
23

Digital Elevation Models of Chignik, Alaska

3.4.4	 Comparison with source data files

To ensure grid accuracy, the 1/3 arc-second Chignik DEM was compared to select source data files. Files

were chosen on the basis of their contribution to the grid-cell values in their coverage areas. A histogram of the 

difference between the SRTM topographic data points and the 1/3 arc-second DEM is shown in Figure 17. 

Figure 17. Histogram of the differences between the SRTM topographic dataset and the 1/3 arc-second Chignik DEM. 

3.4.5 Comparison with U.S. Geological Survey topographic elevations

NGDC used USGS digital topographic quadrangles for qualitatively assessing the 1/3 arc-second Chignik 

DEM. Quadrangles were downloaded as georeferenced TIFFs from http://agdc.usgs.gov/data/usgs/ and loaded into 

ArcMap.

To be consistent with the USGS Chignik quadrangle, the 1/3 arc-second Chignik DEM was converted from 

meters to feet and 100 ft contours were created. Figure 17 displays the DEM contours on top of the USGS quadrangle 

for the populated regions around the town of Chignik and around Chignik Lake. The two sets of contour lines are in 

good agreement with each other, with only minor differences. Wavy areas in the DEM contours that do not match the 

USGS contours are from the lower resolution NED 2 arc-second DEM that was used to fill in holes from the SRTM 1

arc-second DEM.

Topographic elevations at localized high points in the DEM are lower than the USGS topographic quadrangle 

elevations (Fig. 18). These differences may be attributed to the fact that the SRTM and NED topographic data, used to 

constrain the sub aerial parts of the DEM, represent averages of land elevations over 30 x 30 meter and 60 x 60 meter 

square areas, respectively, while the topographic quadrangle elevations represent local maxima.
24

Medley et al., 2009

Figure 18. Comparison between USGS topographic contours (brown lines) and the 1/3 arc-second Chignik DEM topographic contours (black 

lines). Contour lines are in 100 foot intervals. A) The town of Chignik. B) Chignik Lake. 

Areas were chosen based on population. (http://agdc.usgs.gov/data/usgs/)

A

B
25

Digital Elevation Models of Chignik, Alaska

4. Summary and Conclusions

 Two integrated topographic–bathymetric DEMs of the Chignik, Alaska area, with cell sizes of 1 arc-second 

and 1/3 arc-second, were developed for the PMEL NOAA Center for Tsunami Research. The best available digital data 

from U.S. federal agencies were obtained by NGDC, shifted to common horizontal and vertical datums, and evaluated 

and edited before DEM generation. The data were quality checked, processed and gridded using ArcGIS, FME, GMT,

Quick Terrain Modeler, and MB-System software. 

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

•	 Conduct bathymetric surveys in the southern portion of the DEM that currently have sparse or no digital 

measured bathymetric data.

•	 Conduct additional lidar surveys along the coast to establish a more accurate coastline.

•	 Establish, via survey, the relationships between tidal and geodetic datums in the Chignik region.

•	 Complete survey work in Chignik Harbor to accurately reflect the small boat harbor.

5. Acknowledgments

 The creation of the DEMs was funded by NOAA Center for Tsunami Research. The authors thank Nazila 

Meratia and Vasily Titov (PMEL), Bret Christensen (USFWS), Gary Nelson and Brooke McMahon (NOS Pacific

Hydrographic Branch), David Scharff (NOS), Jeff Freymueller (University of Alaska, Fairbanks) and Greg Lee 

(USGS).

6. References

Electronic Navigational Chart #16011, 37th Edition, 2007. Alaska Peninsula and Aleutian Islands to Segum Pass. 

Scale 1:1,023,188. U.S. Department of Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16561, 3rd Edition, 2007. Mitrofania Bay and Kuiukta Bay. Scale 1:80,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16587, 1st Edition, 2006. Semidi Islands and Vicinity. Scale 1:135,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16566, 11th Edition, 2007. Chignik and Kujulik Bays. Scale 1:77,477. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16566, 11th Edition, 2007. Anchorage and Mud Bays. Scale 1:38,730. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Amante, C. and B. W. Eakins, ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis, 

NOAA Technical Memorandum NESDIS NGDC-24, 19 pp., March 2009.
26

Medley et al., 2009

7. Data Processing Software

ArcGIS v. 9.2, developed and licensed by ESRI, Redlands, California, 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, shareware developed and maintained by Dan Metzger, NOAA National 

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

GMT v. 4.3.0 – Generic Mapping Tools, shareware developed and maintained by Paul Wessel and Walter Smith, 

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

MB-System v. 5.1.0, shareware 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, developed by Johns Hopkins University Applied Physics Laboratory, licensed by 

Applied Imagery, Silver Spring, Maryland, http://www.appliedimagery.com/