noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-56

DIGITAL ELEVATION MODEL OF CHENEGA BAY, ALASKA:

PROCEDURES, DATA SOURCES AND ANALYSIS 

R.J. Caldwell

L.A. Taylor

B.W. Eakins

K.S. Carignan

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

August 2012

UNITED STATES

DEPARTMENT OF COMMERCE

Rebecca Blank

Acting Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

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-56

DIGITAL ELEVATION MODEL OF CHENEGA BAY, ALASKA:

PROCEDURES, DATA SOURCES AND ANALYSIS 

R. Jason Caldwell1 

Lisa A. Taylor2

Barry W. Eakins1

Kelly S. Carignan1

1Cooperative 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

August 2012
Caldwell et al., 2010

ii

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/coastal/

Also available from the National Technical Information Service (NTIS)

(http://www.ntis.gov)
Digital Elevation Model of Chenega Bay, Alaska

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Contents

1..Introduction 1

2..Study Area 2

3..Methodology 4

3.1 Supplemental Data Sources and Processing 5

3.1.1 Shoreline 6

3.1.2 Bathymetry 6

3.1.3 Topography 6

3.2 Establishing Common Datums 11

3.2.1 Vertical datum transformations 11

3.2.2 Horizontal datum transformations 11

3.3 Digital Elevation Model Development 12

3.3.1 Verifying consistency between datasets 12

3.3.2 Smoothing of bathymetric data 12

3.3.3 Quality Assessment of the Bathymetric Surface 12

3.3.4 Building the DEM using MB-System 13

3.4 Quality Assessment of the DEM 14

3.4.1 Horizontal accuracy 14

3.4.2 Vertical accuracy 14

3.4.3 Slope map and 3-D perspectives 14

3.4.4 Comparison with UAF GPS measurements 16

3.4.5 Comparison with NOAA Raster Nautical Charts 17

4..Summary and Conclusions 18

5..Acknowledgments 18

6..References 18

7..Data Processing Software 19

List of Figures

Figure 1. Contour map of the Chenega Bay 8/15 arc-second DEM 1

Figure 2. Photograph of Chenega Bay looking southwest (2006) 2

Figure 3. Photograph of Chenega Bay looking west-southwest (2006) 2

Figure 4. Map of the Prince William Sound region 3

Figure 5. Extents of the Chenega Bay 8/15 arc-second DEM 4

Figure 6. Source and coverage of supplemental datasets used in compiling and evaluating the

Chenega Bay DEM 5

Figure 7. Example of georeferenced DCRA aerial imagery used in developing the final coastline

of the Chenega Bay region 6

Figure 8. USFS SPOT DEM coverage and elevations in the Chenega Bay region 7

Figure 9. Histogram of the differences between the UAF GPS elevation measurements and the

NED topographic DEM 8

Figure 10. Histogram of the differences between the UAF GPS elevation measurements and the

ASTER topographic DEM 8

Figure 11. Histogram of the differences between the UAF GPS elevation measurements and the

SPOT topographic DEM 9

Figure 12. Problems with available topographic datasets 9

Figure 13. Interpolated points at Chenega Bay 10

Figure 14. Histogram of the differences between NOS hydrographic surveys and the 8/15 arcsecond pre-surfaced bathymetric grid 13

Figure 15. Slope map of the 8/15 arc-second Chenega Bay DEM 14

Figure 16. Perspective view from the northeast of the 8/15 arc-second Chenega Bay DEM 15

Figure 17. Location of UAF GPS elevation measurements near Chenega Bay 16

Figure 18. Histogram of the differences between UAF GPS elevations and the 8/15 arc-second

Chenega Bay DEM 17
Caldwell et al., 2010

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List of Tables

Table 1. Specifications for the Chenega Bay, Alaska DEM 4

Table 2. Data hierarchy used to assign gridding weight in MB-System 13
Digital Elevation Model of Chenega Bay, 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) centered on Chenega

Bay, Alaska (Fig. 1). The DEM was developed for the National Tsunami Hazard Mitigation Program (NTHMP; http://

nthmp.tsunami.gov/) in support of the State of Alaska’s tsunami inundation modeling efforts led by the Geophysical

Institute at the University of Alaska at Fairbanks (UAF). The 8/15 arc-second1 DEM is nested within the extents of the

previously developed Prince William Sound 8 arc-second and 8/3 arc-second DEMs (see Fig. 4; Caldwell et al., 2009).

The coastal DEM will be used as input for the university-developed modeling system to simulate tsunami generation,

propagation, and inundation (http://www.aeic.alaska.edu/tsunami/). The DEM was generated from diverse digital datasets in the region and was designed to represent modern morphology. Primary data sources and grid development

methodology are described in the technical report for the Prince William Sound DEMs (Caldwell et al., 2009; see link

above). Additional datasets used in developing the Chenega Bay DEM are described in this report, which provides a

summary of the supplemental data sources and quality control procedures used in developing the Chenega Bay DEM.

1. The Chenega Bay 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 Chenega Bay, Alaska, (60° 03' 59"N, 148°

00' 40"W) 1 arc-second of latitude is equivalent to 30.94 meters; 1 arc-second of longitude is equivalent to 15.47 meters.

Figure 1. Contour map of the 

Chenega Bay 8/15 arc-second 

DEM. Contour interval is 100 

meters. Image is in Mercator 

projection.
Caldwell et al., 2010

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2. Study Area

The community of Chenega Bay is currently located on Evans Island in the southwest portion of Prince

William Sound. Chenega Bay is situated on a generally flat plain with the adjacent mountains climbing to over 300

meters (Figs. 2 and 3). Chenega Bay is located in the Chugach National Forest approximately 50 kilometers southeast

of Whittier (Fig. 4). The population of Chenega Bay according to the U.S. Census Bureau in 2000 was 86, primarily

Alaska Natives.

Chenega Bay is in an earthquake prone region, which makes the area highly vulnerable to tsunamis. The

second most powerful earthquake in the twentieth century occurred on March 27, 1964. Its epicenter was located

approximately 90 miles west of Valdez and measured 9.2 on the Richter scale (http://www.drgeorgepc.com/Earthquake1964Alaska.html). The town of Chenega was originally located on Chenega Island to the north, but was relocated and renamed to Chenega Bay after the village was destroyed by the tsunami from the 1964 earthquake. An estimated one-third to one-half of the population was killed and the remaining population migrated to the current location

on Evans Island in the late 1970s and early 1980s.

Figure 2. Photograph of Chenega Bay looking southwest (2006). Source: Alaska Division of Community and Regional Affairs (DCRA; http://

www.commerce.state.ak.us/dca/photos/comm_list.cfm).

Figure 3. Photograph of Chenega Bay looking west-southwest (2006). Source: DCRA; http://www.commerce.state.ak.us/dca/photos/comm_list.

cfm.
Digital Elevation Model of Chenega Bay, Alaska

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Figure 4. Map of the Prince William Sound region with extents of the Chenega Bay DEM and previous Prince William Sound DEMs (e.g., 

Cordova , Whittier, and Tatitlek) shown. ESRI World 2D Imagery shown in the background. 
Caldwell et al., 2010

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3. Methodology

The Chenega Bay DEM was developed in the World Geodetic System 1984 (WGS 84) geographic horizontal

datum and Mean Higher High Water (MHHW) vertical datum in vertical units of meters. NGDC developed the DEM

at extents slightly larger (~ 5 percent) than required by UAF. The best available bathymetric and topographic digital

data were obtained by NGDC and shifted to common horizontal and vertical datums: North American Datum of 19832

(NAD 83) and MHHW, for modeling of maximum flooding, respectively.

Table 1. Specifications for the Chenega Bay, Alaska DEM. 

Grid Area Chenega Bay, Alaska

Coverage Area 148.17o to 147.89o W; 59.97o to 60.19o N

Grid Spacing 8/15 arc-second

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System 1984 (WGS 84)

Vertical Datum Mean Higher High Water (MHHW)

Vertical Units Meters

Grid Format netCDF

Figure 5. Extents of the Chenega Bay 8/15 arc-second DEM. ESRI U.S. Topographic Maps layer in background.

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.
Digital Elevation Model of Chenega Bay, Alaska

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3.1 Supplemental Data Sources and Processing

Source datasets for the Chenega Bay DEM were comprised primarily of the datasets listed in the Prince

William Sound technical report (Caldwell et al., 2009). Supplemental data included a recent U.S. Forestry Service

(USFS) topographic DEM; point elevation measurements in the immediate vicinity of Chenega Bay from UAF that

were used for quality assurance; and high-resolution aerial photographs of the Chenega Bay shoreline in 2005 from

the State of Alaska’s Division of Community and Regional Affairs (DCRA) that were used to digitize a final coastline

(see Figs. 6 and 7).

Safe Software’s Feature Manipulation Engine (FME)3 data translation tool and Proj44 were used to shift

datasets to NAD 83 geographic horizontal datum. FME, GDAL5, and OGR6 were used to convert the datasets into

ESRI ArcGIS shapefiles and xyz format. The shapefiles and xyz files were then displayed with ArcGIS and Applied

Imagery’s Quick Terrain Modeler (QT Modeler) to assess data quality and manually edit datasets. QT Modeler and

Interactive Visualization System’s Fledermaus software were used to evaluate processing and gridding techniques.

Figure 6. Source and coverage of supplemental datasets used in compiling and evaluating the Chenega Bay DEM. 

3. FME uses the North American Datum Conversion Utility (NADCON; http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.shtml) 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.

4. Proj4 is a cartographic projections library, originally written by Gerald Evenden, then of the USGS. The software is released under an MIT style

Open Source license. Proj4 was used to horizontally transform datasets that originated in State Plane datums before vertical transformations were

performed .

5. GDAL is a translator library for raster geospatial data formats that is released under an X/MIT style Open Source license by the Open Source

Geospatial Foundation. As a library, it presents a single abstract data model to the calling application for all supported formats. It also comes with

a variety of useful commandline utilities for data translation and processing.

6. The OGR Simple Features Library is a C++ open source library and commandline tools providing read and write access to a variety of vector file

formats, including ESRI shapefiles. OGR is a part of the GDAL library.
Caldwell et al., 2010

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3.1.1 Shoreline

Three high-resolution aerial photographs of Chenega Bay were downloaded from the DCRA (http://www.

commerce.state.ak.us/dca/profiles/profile-maps.htm). These photographs were georeferenced to the Prince William

Sound coastline using ArcMap (e.g., Fig 7). The Prince William Sound coastline was adjusted using the digital photographs as reference to produce a final coastline for Chenega Bay (see Fig. 6). The final coastline was subsequently

modified to include large offshore rocks and small islets shown on the larger-scale NOAA raster nautical charts

(RNCs) and clipped to 0.05 degrees larger than the 8/15 arc-second DEM boundary. Piers and docks were deleted

from the coastline.

Figure 7. Example of georeferenced DCRA aerial imagery used in developing the final coastline of the Chenega Bay region. Final coastline in 

blue.

3.1.2 Bathymetry

No additional bathymetric surveys were available for use in the compilation of the Chenega Bay DEM.

3.1.3 Topography

A USFS high-resolution stereo digital elevation model (SPOT5) covering the Chugach National Forest area

was downloaded from Alaska’s Statewide Digital Mapping Initiative website (Fig. 8; http://www.alaskamapped.org).

The non-bare-earth, Chugach SPOT5 DEM7 was downloaded at 20 meter resolution with a vertical datum of altitude

above Earth’s Gravitational Model of 1996 (EGM 96) Geoid, and horizontal datum of WGS 84/UTM Zone 6 North.

The vertical and horizontal accuracies of the DEM are 10 and 15 meters, respectively, at the 90 percent confidence

level.

The Chugach SPOT5 DEM was shifted to WGS 84 and MHHW horizontal and vertical datums, respectively,

before clipping to the final coastline and converting the grid to xyz using GDAL. Values less than 1 meter were then

set to 1 meter to eliminate negative values on land. NGDC also reviewed the United States Geological Survey National Elevation Dataset (NED; http://ned.usgs.gov/) 2 arc-second gridded topography, the National Aeronautics and

Space Administration (NASA) Shuttle Radar Topography Mission (SRTM; http://srtm.usgs.gov) 1 arc-second gridded

topography, and the 1 arc-second NASA/Japan’s Ministry of Economy, Trade, and Industry Advanced Spaceborne

Thermal Emission and Reflection Radiometer (ASTER; http://asterweb.jpl.nasa.gov/gdem.asp/) global digital elevation model.

7. The SPOT5 (Satellite Pour l’Observation de la Terre) is a high-resolution, optical imaging Earth observation satellite system operating from

space. SPOT5 was launched on May 4, 2002 and features a high-resolution stereo (HRS) imaging instrument operating in panchromatic mode.

The HRS instrument points forward and backward of the satellite. Thus, it is able to take stereopair images almost simultaneously to map relief.
Digital Elevation Model of Chenega Bay, Alaska

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In addition, UAF provided GPS point elevation values from a field survey in Summer 2010 at MLLW vertical

datum. These points were converted to MHHW using the ‘MLLW to MHHW’ vertical datum offset grid described in

Caldwell et al. (2009). To select the most representative topographic dataset for the Chenega Bay DEM, NGDC performed an initial analysis to compare the GPS measurements to each topographic dataset. The quality check revealed

biases in both the NED and ASTER datasets (Figs. 9 -10). Positive/negative values in Figures 9 -11 indicate under/

over-estimated elevations in the topographic DEM.

The USGS NED dataset was derived from USGS quadrangle maps and aerial photographs based on topographic surveys conducted in the 1970s and 1980s. The NED data were not used in the development of the Chenega

Bay DEM due to: morphological changes in regions of rapid deglaciation across Alaska; lateral shifts in the NED discovered during prior DEM development in Alaska (see Caldwell et al., 2009 for further details); and lower resolution

than other available topographic datasets. The SRTM only provided coverage of a very small part of the Chenega Bay

region; and, the ASTER data showed a distinct bias to under-estimate elevations along the coast. Figure 12 displays

the different issues observed in the NED and ASTER datasets.

The quality assessment of the SPOT data indicated a near-normal distribution of differences in the histogram

with clustering around zero and most errors +/- 4 meters (Fig. 11). Qualitative analysis of the SPOT data indicated

gridding artifacts in the form of artificial steps in elevation that were oriented generally from north to south (see

Caldwell et al., 2010). This did not appear to impact the quality of the DEM and most steps were on the order of several meters. The SPOT data fit the coastline well and did not exhibit any significant horizontal or vertical biases as did

the ASTER and NED (Fig. 12c)

Figure 8. USFS SPOT DEM coverage and elevations in the Chenega Bay region. Final coastline in blue. DEM extents in red.
Caldwell et al., 2010

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Figure 9. Histogram of the differences between the UAF GPS elevation measurements and the NED topographic DEM. Positive/negative 

values indicate under/over-estimated elevations in the topographic DEM. The NED generally over-estimates elevations relative to the UAF GPS 

measurements.

Figure 10. Histogram of the differences between the UAF GPS elevation measurements and the ASTER topographic DEM. Positive/negative 

values indicate under/over-estimated elevations in the topographic DEM. The ASTER generally under-estimates elevations relative to the UAF 

GPS measurements.
Digital Elevation Model of Chenega Bay, Alaska

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Figure 11. Histogram of the differences between the UAF GPS elevation measurements and the SPOT topographic DEM. Positive/negative 

values indicate under/over-estimated elevations in the topographic DEM. Differences with the SPOT data cluster around zero with no obvious 

bias as in the NED and ASTER data.

Figure 12. Problems with available topographic datasets. (a) Coastal elevations in the ASTER data are regularly below zero (light blue shading). 

(b) Elevations in the NED appear to be shifted southward and smoothed horizontally, possibly due to interpolation. (c) The SPOT data best fits 

the final coastline (black line) with no obvious horizontal or vertical bias. 
Caldwell et al., 2010

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Due to the steep slopes along the coastline in some regions of the Chenega Bay DEM, the spline interpolation

generated small regions of negative elevations onshore. To correct this issue, NGDC digitized points (see Fig. 6) with

elevation values of 1 meter to better represent the actual morphology in these locations by mitigating the introduction

of interpolation artifacts in the grid.

NGDC also digitized points to represent the airport and a region near the ferry landing in Chenega Bay (Fig.

13). The GPS elevations provided by UAF were used to generate a 1 arc-second interpolation grid using the inverse

distance weighting (IDW) tool in ArcMap. The IDW grid was converted to xyz format and then clipped to polygons

representing the airport and ferry landing regions using GDAL. The SPOT data were subsequently removed from the

same regions to avoid gridding artifacts due to overlapping datasets of different resolutions and quality.

Figure 13. Interpolated points at Chenega Bay. Elevation points from the inverse distance weighting (triangles) and UAF GPS measurements 

(squares) are shown in same color scale. The interpolated points have been clipped to the respective polygons (light blue). These polygons were 

also used to remove the SPOT data from the final gridding. DCRA imagery is in the background. Coastline is shown in black.
Digital Elevation Model of Chenega Bay, Alaska

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3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation and evaluation of the Chenega Bay DEM were originally referenced to a

number of vertical datums including MLLW (meters) and EGM 96 Geoid (assumed to be mean sea level (MSL)). All

datasets were transformed to MHHW for modeling of maximum flooding. Vertical datum transformations to MHHW

were accomplished using FME and GDAL, and the offset grids developed for the Prince William Sound DEMs.

1) Bathymetric data

No additional bathymetric datasets were used in the compilation of the Chenega Bay DEM.

2) Topographic data

The SPOT topographic dataset was originally referenced to the EGM 96 Geoid. There are no survey

markers in the vicinity of Chenega Bay that relate the geodetic datum to local tidal datums. Therefore, it was

assumed that the datum is essentially equivalent to MSL in this area. Conversion to MHHW, using GDAL,

was accomplished by adding the ‘MSL to MHHW’ offset grid. Values less than 1 meter following the conversion were set equal to 1 meter, as the SPOT data are in integer format.

The UAF GPS measurements were converted from MLLW to MHHW using the ‘MLLW to MHHW’

offset grid and GDAL.

3.2.2 Horizontal datum transformations

Supplemental datasets used to compile the Chenega Bay DEM were originally referenced to WGS 84/UTM

Zone 6 North. The relationships and transformational equations between the horizontal datums are well established.

Transformations to NAD 83 geographic were accomplished using Proj4.
Caldwell et al., 2010

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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 and QT Modeler 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:

•	 Inconsistent, overlapping high-resolution bathymetric datasets. Older datasets were clipped to newer datasets

when possible. Datasets were weighted based on quality and year during the gridding process.

•	 Data values over the ocean in the SPOT DEM dataset. The dataset required automated clipping to the final

coastline or were edited manually.

•	 Steep slopes in the SPOT DEM along coastal areas led to interpolation artifacts in the DEM. NGDC digitized

elevation points to provide additional control to mitigate negative values in onshore locations.

3.3.2 Smoothing of bathymetric data 

The older hydrographic survey data are generally sparse at the resolution of the DEM in both deep water and

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

to the low resolution datasets, and to provide effective interpolation into the coastal zone, ‘pre-surface’ bathymetric

grids in MHHW vertical datum were generated using GMT8, an NSF-funded software application designed to manipulate data for mapping purposes (HUhttp://gmt.soest.hawaii.edu/UH). Additional information on the methodology to develop

a pre-surface bathymetric grid can be found in Caldwell et al. (2009).

3.3.3 Quality Assessment of the Bathymetric Surface

The bathymetric surface was compared with the originalsoundings to ensure grid accuracy, and then exported

as an xyz file for use in the final gridding process. The statistical analyses of the differences between the 8/15 arcsecond bathymetric surface at Chenega Bay with the NOS hydrographic surveys show that the majority of the surveys

are in good agreement (Fig. 14) with the bathymetric surface. The few exceptions where the differences reached up

to 113.30 meters are attributed primarily to overlapping datasets. These differences are fairly consistent with the

chart comparison in the NOS descriptive report for survey H11387 (http://surveys.ngdc.noaa.gov/mgg/NOS/coast/

H10001-H12000/H11387/DR/H11387.pdf), which indicates differences up to 29 fathoms (~54 meters) between older

and newer surveys. In addition, large differences also occurred where two or more closely positioned points were

averaged to obtain the elevation of one grid cell. Some inconsistencies were identified while merging the bathymetric

datasets due to the range in ages and resolutions of the surveys. In areas where more recent data were available, the

older surveys were either edited or not used. The gridded bathymetric surface was then converted to an xyz file for use

in building the final DEM.

8. GMT is an open source collection of ~60 tools for manipulating geographic and Cartesian data sets (including filtering, trend fitting, gridding,

projecting, etc.) and producing Encapsulated PostScript File (EPS) illustrations ranging from simple x-y plots via contour maps to artificially

illuminated surfaces and 3-D perspective views. GMT supports ~30 map projections and transformations and comes with support data such as

GSHHS coastlines, rivers, and political boundaries. GMT is developed and maintained by Paul Wessel and Walter H. F. Smith with help from a

global set of volunteers, and is supported by the National Science Foundation. It is released under the GNU General Public License. URL: http://

gmt.soest.hawaii.edu/ [Extracted from GMT web site.]
Digital Elevation Model of Chenega Bay, Alaska

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Figure 14. Histogram of the differences between NOS hydrographic surveys and the 8/15 arc-second pre-surfaced bathymetric grid. 

3.3.4 Building the DEM using MB-System

MB-System9 was used to create the Chenega Bay DEM. 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 2. For supplemental data of Chenega Bay, the

greatest weight was given to the NGDC digitized features. Least weight was given to the pre-surfaced bathymetric

grid. Only those datasets denoted with an ‘*’ were used to compile the Chenega Bay DEM.

Table 2. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

USACE surveys 100

USGS NED topographic DEM 100

ENC soundings 100

NGDC digitized features * 100

UAF Valdez DEM 100

NOS BAG H11743 100

SPOT topographic DEM * 10

SRTM topographic DEM 10

NOS hydrographic surveys * 10

Final coastline * 10

Higher resolution DEMs 10

NGDC hydrographic sonar multibeam 10

Pre-surfaced bathymetric grid * 1

Trackline soundings 0.1

* indicates supplemental datasets used in compiling the Chenega Bay DEM.

9. MB-System is an open source software package for the processing and display of bathymetry and backscatter imagery data derived from

multibeam, interferometry, and sidescan sonars. The source code for MB-System is freely available (for free) by anonymous ftp (including “point

and click” access through these web pages). A complete description is provided in web pages accessed through the web site. MB-System was

originally developed at the Lamont-Doherty Earth Observatory of Columbia University (L-DEO) and is now a collaborative effort between

the Monterey Bay Aquarium Research Institute (MBARI) and L-DEO. The National Science Foundation has provided the primary support for

MB-System development since 1993. The Packard Foundation has provided significant support through MBARI since 1998. Additional support

has derived from SeaBeam Instruments (1994-1997), NOAA (2002-2004), and others. URL: http://www.ldeo.columbia.edu/res/pi/MB-System/

[Extracted from MB-System web site.]
Caldwell et al., 2010

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3.4 Quality Assessment of the DEM

3.4.1 Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Chenega Bay DEM is dependent

upon DEM cell size and source datasets. Topographic features have an estimated horizontal accuracy equivalent to

that of the SPOT data, ~15 meters. Bathymetric features are resolved only to within a few hundreds of meters in deepwater 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 in the Chenega Bay DEM is also dependent upon the source datasets

contributing to DEM cell values. Topographic data have a stated vertical accuracy of approximately 10 meters for the

SPOT DEM, though a comparison with the UAF GPS elevations indicated an accuracy of 1-2 meters. Bathymetric

values have an estimated accuracy between 0.1 meters and 5% of water depth. Those 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.

3.4.3 Slope map and 3-D perspectives

ESRI ArcCatalog was used to generate a slope grid from the 8/15 arc-second Chenega Bay DEM to allow

for visual inspection and identification of artificial slopes along boundaries between datasets (Fig. 15). The DEM was

transformed to NAD 83 UTM Zone 6 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 QT Modeler and Fledermaus revealed suspect data points, which were corrected before recompiling the

DEM. Again, the north-south oriented, artificial steps in the SPOT data are clearly visible in the slope analysis. Figure

1 shows a color image of the 8/15 arc-second Chenega Bay DEM in its final version. Figure 16 shows a perspective

rendering of the final 8/15 arc-second Chenega Bay DEM.

Figure 15. Slope map of the 8/15 arc-second Chenega Bay DEM. Flat-lying slopes are shown in white; dark shading denotes steep slopes; final 

coastline indicated in blue. Steepest slopes exist over land.
Digital Elevation Model of Chenega Bay, Alaska

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Figure 16. Perspective view from the northeast of the 8/15 arc-second Chenega Bay DEM. Vertical exaggeration times four.
Caldwell et al., 2010

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3.4.4 Comparison with UAF GPS measurements 

The locations of 230 GPS-collected elevation measurements were provided to NGDC by UAF (Fig. 17). Elevations were given relative to MLLW, but were converted using the ‘MLLW to MHHW’ offset grid from the Prince

William Sound report (Caldwell et al., 2009). Elevations were compared to the 8/15 arc-second Chenega Bay DEM

(Fig. 18). Differences between the DEM and the UAF GPS elevations range from -10.02 to 9.21 meters, with half of

the differences between -1.28 and 1.96 meters. Large differences in elevations occurred where measurements were

taken on piers and docks or in regions of steep topography. In addition, the SPOT topographic data used in the Chenega Bay DEM are not bare earth, which contributes to large differences in forested regions.

Figure 17. Location of UAF GPS elevation measurements near Chenega Bay. Aerial imagery from DCRA is in the background. Coastline is in 

blue.
Digital Elevation Model of Chenega Bay, Alaska

17

Figure 18. Histogram of the differences between UAF GPS elevations and the 8/15 arc-second Chenega Bay DEM. 

3.4.5 Comparison with NOAA Raster Nautical Charts 

Elevations were also compared against five raster nautical charts (RNCs) in the area, including: 531, 16013,

16700, 16701, and 16702. Manual inspection of the DEM elevations revealed only small differences from elevations

indicated on the RNCs. For complete reference information on these RNCs, see the Caldwell et al. (2009) report on

the Prince William Sound DEMs.
Caldwell et al., 2010

18

4. Summary and Conclusions

An integrated bathymetric–topographic digital elevation model of the Chenega Bay, Alaska region, with a

cell size of 8/15 arc-second, was developed for the National Tsunami Hazard Mitigation Program in support of the

State of Alaska’s tsunami inundation modeling efforts led by the Geophysical Institute at the University of Alaska

at Fairbanks. The best available digital data from U.S. federal, state, local, and academic 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, ESRI ArcGIS World Imagery 2-D, FME, Fledermaus, GMT, MB-System, QT Modeler, GDAL, OGR, Proj4, and VDatum software.

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

•	 Conduct high-resolution topographic surveys, including lidar, of the Chenega Bay region.

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

•	 Correct horizontal shift evident in the NED data and process SPOT data to bare earth.

5. Acknowledgments

The creation of the Chenega Bay DEM was funded by the National Tsunami Hazard Mitigation Program.

The authors thank Elena Sulemani and Dmitry Nikolski (UAF); Bret Christensen (USFWS); and, Hans Andersen and

Mark Riley (U.S. Forest Service).

6. References

Caldwell, R.J., B.W. Eakins, and E. Lim, 2011. Digital Elevation Models of Prince William Sound,Alaska: Procedures,

Data Sources, and Analysis, U.S. Department of Commerce, NOAA Technical Memorandum, NESDIS,

NGDC-40.

Caldwell, R.J., B.W. Eakins, L.A. Taylor, and K.S. Carignan, 2010. Digital Elevation Model of Tatitlek, Alaska:

Procedures, Data Sources, and Analysis, U.S. Department of Commerce, NOAA, National Geophysical Data

Center.

Descriptive Report to Accompany Hydrographic Survey H11387, 2004. Northern Prince of Wales Passage. Scale

1:10,000. U.S. Department of Commerce, NOAA, National Ocean Service, Project OPR-P139-TC-04, http://

surveys.ngdc.noaa.gov/mgg/NOS/coast/H10001-H12000/H11351/DR/H11387.pdf.
Digital Elevation Model of Chenega Bay, Alaska

19

7. Data Processing Software

ArcGIS v. 9.3.1 – developed and licensed by ESRI, Redlands, California, HUhttp://www.esri.com/UH

ESRI World Imagery (ESRI_Imagery_World_2D) – ESRI ArcGIS Resource Centers http://resources.esri.com/

arcgisonlineservices/.

FME 2010 GB – Feature Manipulation Engine, developed and licensed by Safe Software, Vancouver, BC, Canada,

HUhttp://www.safe.com/UH

Fledermaus v. 7.0.0 – developed and licensed by Interactive Visualization Systems (IVS 3D), Fredericton, New

Brunswick, Canada, http://www.ivs3d.com/products/fledermaus/

GDAL v. 1.7.1 – Geographic Data Abstraction Library is a translator library maintained by Frank Warmerdam, http://

gdal.org/.

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. 4.3.4 – 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

Gnuplot v. 4.2, free software developed and maintained by Thomas Williams, Colin Kelley, Russell Lang, Dave Kotz,

John Campbell, Gershon Elber, Alexander Woo, http://www.gnuplot.info/.

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/

Proj4 v. 4.7.0, free software developed by Gerald Evenden and maintained by Frank Warmerdam, http://trac.osgeo.

org/proj/.

Quick Terrain Modeler v. 7.0.0 – LiDAR processing software developed by John Hopkins University’sApplied Physics

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

VDatum Transformation Tool, developed and maintained by NOAA’s National Geodetic Survey (NGS), Office of

CoastSurvey (OCS), and Center for Operational Oceanographic Products and Services (CO-OPS), http://

vdatum.noaa.gov/.
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