noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-58

DIGITAL ELEVATION MODEL OF KACHEMAK BAY, ALASKA:

PROCEDURES, DATA SOURCES AND ANALYSIS 

D.Z. Friday 

L.A. Taylor

B.W. Eakins

K.S. Carignan

R.J. Caldwell 

P.R. Grothe 

E. Lim

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

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-58

DIGITAL ELEVATION MODEL OF KACHEMAK BAY, ALASKA:

PROCEDURES, DATA SOURCES AND ANALYSIS

Dorothy Z. Friday1

Lisa A. Taylor2

Barry W. Eakins1

Kelly S. Carignan1 

R.J. Caldwell1 

Pam R. Grothe1

Elliot Lim1

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

2NOAA, National Geophysical Data Center, Boulder, Colorado 

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

August 2012
Friday et al., 2012

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 Models of Kachemak Bay, Alaska

iii

Contents

1..Introduction 1

2..Study Area 2

3..Methodology 3

3.1 Data Sources and Processing 4

3.1.1 Coastline 5

3.1.2 Bathymetry 6

3.1.3 Topography 10

3.2 Establishing Common Datums 12

3.2.1 Vertical datum transformations 12

3.2.2 Horizontal datum transformations 13

3.3 Digital Elevation Model Development 13

3.3.1 Verifying consistency between datasets 13

3.3.2 Smoothing of bathymetric data 13

3.3.3 Building the NAVD 88 DEM 15

3.4 Quality Assessment of the DEM 15

3.4.1 Horizontal accuracy 15

3.4.2 Vertical accuracy 15

3.4.3 Slope map and 3-D perspectives 15

3.4.4 Comparison with National Geodetic Survey geodetic monuments 18

3.4.5 NAVD 88 DEM comparison with source data files 18

4..Summary and Conclusions 22

5..Acknowledgments 22

6..References 22

7..Data Processing Software 22

List of Figures

Figure 1. Shaded-relief image of the Kachemak Bay 1/3 arc-second DEM 1

Figure 2. Overview of the Kachemak Bay DEM region 2

Figure 3. Aerial photograph showing the Homer spit 3

Figure 4. Source and coverage of datasets used in compiling the Kachemak Bay DEM 4

Figure 5. Overview of final coastline 5

Figure 6. Early NOS hydrographic survey coverage in the Kachemak Bay region 7

Figure 7. Recent NOS hydrographic survey coverage in the Kachemak Bay region 8

Figure 8. Spatial coverage of USACE hydrographic survey data in Kachemak Bay 9

Figure 9. Source and coverage of topographic datasets used in compiling the Kachemak Bay

DEM 10

Figure 10. Locations of NOAA tide stations in the Kachemak Bay area. 12

Figure 11. Histogram of the differences between the early NOS bathymetric data and the 1/3

arc-second pre-surfaced bathymetric grid 14

Figure 12. Histogram of the differences between the recent NOS bathymetric data and the

1/3 arc-second pre-surfaced bathymetric grid 14

Figure 13. Slope map of the Kachemak Bay DEM 16

Figure 14. Data contribution plot of the Kachemak Bay DEM 16

Figure 15. Perspective view from the west of the 1/3 arc-second Kachemak Bay DEM 17

Figure 16. Locations of NGS geodetic monuments in the Kachemak Bay region 18

Figure 17. Histogram of the differences between NGS geodetic monument elevations and the

Kachemak Bay DEM 18

Figure 18. Comparisons of 2010 Kachemak Bay DEM and 2004 PMEL Homer DEMs 19

Figure 19. Histogram of the differences between a portion of the USGS Kenai lidar dataset and

the Kachemak Bay DEM. 20

Figure 20. Histogram of the differences between the NOAA coastal lidar data and the

Kachemak Bay DEM. 21

Figure 21. Histogram of the differences between the USACE hydrographic survey data

points and the Kachemak Bay DEM 21
Friday et al., 2012

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List of Tables

Table 1. Specifications for the Kachemak Bay DEM............................................................................... 3

Table 2. Shoreline dataset used in developing the Kachemak Bay DEM. 5

Table 3. Bathymetric datasets used in compiling the Kachemak Bay DEM. 6

Table 4. Digital NOS hydrographic surveys used in developing the Kachemak Bay DEM. 7

Table 5. Digital NOS hydrographic surveys used in developing the Kachemak DEM. 8

Table 6. Topographic datasets used in compiling the Kachemak DEM. 10

Table 7. Tide stations in the Kachemak Bay area and relationships between vertical datums 12

Table 8. Data hierarchy used to assign gridding weight in MB-System 15
Digital Elevation Models of Kachemak Bay, Alaska

1

Digital Elevation Model of Kachemak 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) of Kachemak Bay,

Alaska (Fig. 1). The 1/3 arc-second1 DEM, referenced to Mean High Water (MHW), was developed and evaluated

using diverse digital datasets available for the region (grid boundary and sources shown in Fig. 4). The DEM will be

used as input for the Method of Splitting Tsunami (MOST) model developed by the Pacific Marine Environmental

Laboratory (PMEL) NOAA Center for Tsunami Research (HUhttp://nctr.pmel.noaa.gov/UH) to simulate tsunami generation,

propagation and inundation as part of the tsunami forecast system Short-term Inundation Forecasting for Tsunamis

(SIFT) currently being developed by PMEL for the NOAATsunami Warning Centers. This report provides a summary

of the data sources and methodology used in developing the Kachemak Bay DEM.

Figure 1. Shaded-relief image of the Kachemak Bay 1/3 arc-second DEM. 

Contour interval is 15 meters for bathymetry and 100 meters for topography.

1. The Kachemak Bay DEM is built upon a grid of cells that are square in geographic coordinates (latitiude and longitude), however, the cells

are not square when converted to projected coordinate systems such as UTM zones (in meters). At the latitude of Homer, Alaska, (59°38'35"N,

151°31'33"W) 1/3 arc-second of latitude is equivalent to 10.32 meters; 1/3 arc-second of longitude equals 5.22 meters.
Friday et al., 2012

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2. Study Area

The Kachemak Bay DEM provides coverage of the area surrounding Homer, Alaska, including all of

Kachemak Bay and the town of Seldovia. The DEM area is located along the southwestern Kenai Peninsula, west of

the Kenai mountains, and approximately 100 miles south of Anchorage (Fig. 2). The area is tectonically active and at

high risk of future tsunamis. The 9.2 magnitude Alaskan earthquake of 1964, for example, affected the area significantly; it changed the shoreline terrain and caused numerous tsunamis in the area.

The town of Homer was founded in the late nineteenth century as a mining town, and is currently known for

its renowned fishing industry and its natural beauty. The town has about 4,000 year-round residents. From the town of

Homer, a four mile long spit extendsinto Kachemak Bay, which affectsthe water circulation patternsin the bay (Fig. 3).

Figure 2. Overview of the Kachemak Bay DEM region. Black box represents approximate Kachemak Bay DEM extent. 

ESRI’s online World 2D Imagery in background. 
Digital Elevation Models of Kachemak Bay, Alaska

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Figure 3. Aerial photograph showing the Homer spit. (Wikipedia Commons http://en.wikipedia.org/wiki/Homer_Spit)

3. Methodology

The Kachemak Bay 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 (Short-term Inundation Forecasting for

Tsunamis) to provide real-time tsunami forecasts in an operational environment. 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 MHW. Data were gathered in an area slightly larger than the DEM extents. This

data “buffer” ensured that gridding occurred across rather than along the DEM boundaries to prevent edge effects.

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

 Table 1: Specifications for the Kachemak Bay DEM. 

Grid Area Kachemak Bay, Alaska

Coverage Area 152.10o to 150.89o W; 59.32o to 59.82o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984 (WGS 84)

Vertical Datums Mean High Water (MHW)

Vertical Units Meters

Cell Size 1/3 arc-second

Grid Format ESRI Arc 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 DEMs. 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. These DEMs are 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 DEMs, WGS 84 and NAD 83 geographic are identical and

may be used interchangeably.
Friday et al., 2012

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3.1 Data Sources and Processing

Shoreline, bathymetric, and topographic digital datasets were obtained from federal, state, and local agencies

and institutions (Fig. 4) including: NGDC; NOAA’s National Ocean Service (NOS), National Geodetic Survey (NGS),

Office of Coast Survey (OCS), and Coastal Services Center (CSC); the Kachemak Bay National Estuarine Research

Reserve (KBRR); the Kenai Peninsula Borough; the Alaska Department of Fish and Game (ADFG); the U.S. Army

Corps of Engineers (USACE); the U.S. Fish and Wildlife Service (USFWS); the U.S. Geological Survey (USGS); the

National Geospatial Intelligence Agency (NGA); and the National Aeronautics and Space Administration (NASA).

Datasets were shifted to NAD 83 geographic horizontal datum using ESRI’s ArcGIS, Proj.4, and NOAA’s Vertical Datum (VDatum) transformation tool. Data were visually displayed with ArcGIS and Applied Imagery’s Quick 

Terrain Modeler (QT Modeler), to assess quality and manually edit datasets. Vertical datum transformations were accomplished using a constant offset derived from the Homer tide station (see Sec. 3.2.1).

Figure 4. Source and coverage of datasets used in compiling the Kachemak Bay DEM.
Digital Elevation Models of Kachemak Bay, Alaska

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3.1.1 Coastline

Alaska shoreline data were obtained from USFWS (Table 2). The vector coastline was developed by USFWS

in 2006 and combines the best available data from several other sources and organizations. In the Kachemak Bay area,

the coastline closely matched the available topographic data, bathymetric data, and aerial imagery.

Table 2. Shoreline dataset used in developing the Kachemak Bay DEM.

Source Year Data Type Spatial Resolution Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum

USFWS 2006 Compiled Vector

Coastline Various WGS 84 Geographic Undefined

1) United States Fish and Wildlife Service coastline

USFWS compiled a seamless digital vector coastline of the State of Alaska using their own data, along

with data from six other sources, including: the USGS National Hydrography Dataset, NOAA’s Electronic

Navigational Charts (ENCs), National Geographic Topographic Software (derived from USGS topographic

quadrangle maps), the U.S. Army Corps of Engineers, and the Alaska Department of Natural Resources. The

USFWS coastline dataset was provided to NGDC in 2006 by Bret Christensen of the USFWS.

To further define the coastline for the Kachemak Bay DEM, NGDC clipped the USFWS coastline to

the Kachemak Bay DEM extent and reviewed it by comparing it with SRTM topographic digital elevation

models, lidar datasets, NOS hydrographic soundings, USACE hydrographic soundings, NOAA Raster Nautical Charts (RNCs), and ESRI’s World 2D online imagery (Fig. 5). Small adjustments to the coastline were

made as necessary. The comparison helped to ensure that features such as jetties and rocks were accurately

reflected in the coastline. The final edited coastline was converted to xyz data with a ten meter point spacing, using NGDC’s GEODAS software, for use in building a ‘pre-surface’ bathymetric grid (see Sec. 3.3.2).

Figure 5. Overview of final coastline in blue. ESRI’s online World 2D Imagery in background. 
Friday et al., 2012

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3.1.2 Bathymetry

Three bathymetric datasets were used to build the Kachemak Bay DEM (Table 3). These included twelve

recent high-resolution NOS bathymetric surveys, six early NOS hydrographic surveys; and one hydrographic survey

from the USACE Alaska District. 

Table 4. Bathymetric datasets used in compiling the Kachemak Bay DEM.

Source Year Data Type Spatial Resolution

Original 

Horizontal Datum/

Coordinate System

Original 

Vertical 

Datum

URL

Early

NOS

1915 to

1983

Hydrographic

survey

xyz data

Ranges from less than 10 m

to 600 m (varies with scale of

survey, recency of survey, and

distance from shore)

Early Alaska Datums; 

NAD 27 geographic MLLW

HUhttp://www.ngdc.noaa.

gov/mgg/bathymetry/

hydro.htmlUH

Recent

NOS

2008-

2009

Hydrographic

survey

bag data

4 to 8 meters

Universal Transverse

Mercator (UTM)

NAD 83, Zone 5

MLLW http://www.hydropalooza.noaa.gov/

USACE 2008

Hydrographic

survey

xyz data

1 to 30 meters

State Plane

Coordinate System

NAD 27,

Alaska Zone 4

MLLW http://www.poa.usace.

army.mil/en/hydro/

1) Early National Ocean Service hydrographic survey data

A total of 17 NOS hydrographic surveys conducted between 1915 and 1981 were available for use in

building the Kachemak Bay DEM (Table 4; Fig. 6), though most of the surveys were superseded by newer

data. Only six were used in the final DEM. The surveys were extracted from NGDC’s online NOS hydrographic database using GEODAS3

. The hydrographic survey data were downloaded vertically referenced to

mean lower low water (MLLW) and horizontally referenced to NAD 83 geographic.

The data point spacing of the surveys varies by scale. In general, small scale surveys have greater point

spacing than large scale surveys. The data were converted to shapefiles, and were edited and clipped to the

DEM buffer area in ESRI’s ArcGIS. The surveys were compared to the original survey smooth sheets, other

bathymetric datasets, the Kachemak Bay coastline, topographic data, and NOS raster nautical charts (RNCs).

Some surveys were manually shifted to fit the coastline. Older surveys were clipped or removed to eliminate

data that were overlapped by more recent bathymetric data.

3. GEODAS 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 hydrographic survey data from NAD 27 to NAD 83. NADCON is the U.S. Federal

Standard for NAD 27 to NAD 83 datum transformations.
Digital Elevation Models of Kachemak Bay, Alaska

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Table 5. Early digital NOS hydrographic surveys available for use in developing the Kachemak Bay DEM.

NOS 

Survey ID

Year of 

Survey Area of Survey Survey Scale

Original 

Horizontal Datum

Original 

Vertical Datum

H03802* 1915 Pt. Gore to Port Graham 1:60,000 Undetermined MLLW

H05082* 1930 Nuka Passage 1:20,000 Early Alaska Datums MLLW

H09569* 1980 Bear Cove and Vicinity 1:10,000 Early Alaska Datums MLLW

H09840* 1979 Cape Starichkof 1:20,000 Early Alaska Datums MLLW

H09876 1980 Bear Island to Millers Landing 1:20,000 Early Alaska Datums MLLW

H09877 1981 Vukon Island to Glacier Spit 1:20,000 Early Alaska Datums MLLW

H09878* 1980 Port Graham 1:10,000 Early Alaska Datums MLLW

H09879 1980 Koyuktolik Bay to Point Pogbibshi 1:20,000 Early Alaska Datums MLLW

H09884* 1980 Halibut Cove and Peterson Bay 1:10,000 Early Alaska Datums MLLW

H09893* 1980 Sadie Cove to Tutka Bay 1:10,000 Early Alaska Datums MLLW

H09900* 1980 Homer Spit 1:5,000 Early Alaska Datums MLLW

H09941* 1981 Kasitsha Bay to Edred Passage 1:10,000 Early Alaska Datums MLLW

H09958 1981 Entrance to Kachemak Bay 1:20,000 NAD 27 geographic MLLW

H09967 1981 Diamond Creek to Laida Spit 1:20,000 NAD 27 geographic MLLW

H10091 1983 Bluff Point to Flat Island 1:40,000 Early Alaska Datums MLLW

H10421* 1981 Barabaloa Point to Point Pogibshi 1:20,000 NAD 27 geographic MLLW

H10967* 1981 Seldovia Bay 1:5,000 NAD 27 geographic MLLW

* Denotes surveys that have been superseded by more recent data.

Figure 6. Early NOS hydrographic survey coverage in the Kachemak Bay region. Red box shows DEM extents. 
Friday et al., 2012

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2) Recent National Ocean Service hydrographic survey data

Twelve recent NOS surveys were available for use in the Kachemak Bay DEM (Table 5; Fig. 7). The

surveys were collected as part of the Hydropalooza project— a large-scale NOAA project to map

the Kachemak Bay seafloor and coastline. The recent NOS data are high-resolution sonar depictions of the

bathymetry in Kachemak Bay. 

Some of the surveys were not available for download from NGDC’s online NOS hydrographic survey

database and were obtained by hard drive from NOS. All of the recent NOS surveys required no editing, and

were converted into xyz files for use in final gridding.

Table 6. Recent digital NOS hydrographic surveys used in developing the Kachemak Bay DEM.

NOS 

Survey ID

Year of 

Survey Area of Survey Survey Scale

Original 

Horizontal Datum

Original 

Vertical Datum

H11933 2008 Kachemak Bay 1:10,000 NAD 83 MLLW

H11934 2008 Vicinity of Glacier Spit 1:10,000 NAD 83 MLLW

H11936* 2008 Not Specified 1:10,000 NAD 83 MLLW

H11938 2008 Kachemak Bay 1:10,000 NAD 83 MLLW

H12084* 2009 Not Specified 1:10,000 NAD 83 MLLW

H12085 2009 Not Specified 1:10,000 NAD 83 MLLW

H12086 2009 Not Specified 1:10,000 NAD 83 MLLW

H12087 2009 Southern Portion of Cook Inlet 1:10,000 NAD 83 MLLW

H12088 2008 Not Specified 1:10,000 NAD 83 MLLW

H12089 2008 Not Specified 1:10,000 NAD 83 MLLW

H12090 2008 Not Specified 1:10,000 NAD 83 MLLW

H12114 2008 Not Specified 1:10,000 NAD 83 MLLW

* Denotes Surveys that were not available from the NGDC online NOS hydrographic survey database for Kachemak Bay DEM.

Figure 7. Recent NOS hydrographic survey coverage in the Kachemak Bay region. Red box shows DEM extent.
Digital Elevation Models of Kachemak Bay, Alaska

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3) U.S. Army Corps of Engineers hydrographic surveys

Four hydrographic survey datasets were available from the USACE Alaska district website (Fig. 8), but

only one of the USACE surveys was used in gridding the final Kachemak Bay DEM. The surveys provided

depth information near the communities of Homer and Seldovia, and were horizontally referenced to Alaska

State Plane NAD 27 Zone 4. Surveys that overlapped high resolution NOS surveys or lidar data were not

used, leaving only the ‘Homer Spit 1’(see Fig. 8) survey for use in the final DEM.

Figure 8. Spatial coverage of USACE hydrographic survey data in Kachemak Bay. 
Friday et al., 2012

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3.1.3 Topography

Three topographic datasets were used in building the Kachemak Bay DEM (Table 6; Fig. 9). The datasets

included NOAA coastal lidar data, Kenai Peninsula lidar data (from a variety of organizations), and NGS/NASA

SRTM data. ASTER data and USGS NED data were also assessed, but were not used in the final DEM because of

data quality issues.

Table 7. Topographic datasets used in compiling the Kachemak Bay DEM.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Datum URL

NOAA 2008 Lidar 5-7 meters NAD 83 geographic NAVD 88 http://www.csc.noaa.gov/digitalcoast/data/index.html

Multiple 2010 Lidar 4 feet State Plane Alaska

Zone 4 NAD 83 feet NAVD 88 n/a

NASA 1999 SRTM 1 arc-second WGS 84 NAVD 88

(EGM 96 Geoid)

http://edcsns17.cr.usgs.gov/

EarthExplorer/

Figure 9. Source and coverage of topographic datasets used in compiling the Kachemak Bay DEMs.
Digital Elevation Models of Kachemak Bay, Alaska

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1) NOAA Integrated Ocean and Coastal Mapping Kenai Peninsula Lidar

In the summer of 2008, NOAA funded and obtained a coastal lidar dataset of the area surrounding

Homer, Alaska and Kachemak Bay. The data provide coverage of the coastline along the northern half of

Kachemak Bay, from Anchor Point towards the head of the bay (See Fig. 9). The data were processed to

bare earth and provided to NGDC as NAD 83 ascii raster files via NOAA’s CSC website. The data were

transformed into xyz format for use in gridding the final DEM.

2) Kenai Peninsula Lidar

In 2008, the Kenai Watershed Forum led an effort to acquire detailed lidar data of a large area of the

Kenai peninsula. The resulting Kenai peninsula lidar dataset provides coverage of the land area north of

Kachemak Bay (See Fig. 9). The data were collected by Aero-metric, Inc., under contract with the USGS. 

Funding and processing were provided by the Kenai Watershed Forum, the Kenai Peninsula Borough, the

Alaska State Department of Natural Resources, the University of Alaska at Fairbanks Geophysical Institute,

the U.S. Environmental Protection Agency, Resource Data, Inc., and USFWS, among others. 

The data were provided to NGDC by Steve Baird at the Kachemak Bay Research Reserve and Chris

Clough at the Kenai Peninsula Borough GIS department. The data were provided to NGDC as a high resolution (4 foot cell size) ESRI raster grid in Alaska State Plane Zone 4 NAD 83 feet. The grid was reprojected

to WGS 84 geographic and sampled to 5 meter resolution in ArcMap, converted to xyz points, transformed

to meters, and used for gridding the final DEM.

3) Shuttle Radar Topography Mission InSAR data

The SRTM was a joint international project run by the National Geospatial Intelligence Agency (NGA)

and NASA4

. In 2000, the SRTM project obtained global elevation data at 1 arc-second resolution using an

interferometric synthetic aperture radar (InSAR) system. One SRTM raster grid tile was downloaded from

the SRTM website for use in the Kachemak Bay DEM; the data were used to provide coverage along the

southern half of Kachemak Bay, where lidar data was unavailable (See Fig. 9). The SRTM dataset provided

the best coverage option when compared to ASTER and NED datasets, which did not provide reliable information near the coastline.

4. The SRTM data setsresult from a collaborative effort by the NationalAeronautics and SpaceAdministration (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;

http://srtm.usgs.gov/]
Friday et al., 2012

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3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation of the Kachemak Bay DEM were originally referenced to a number of vertical datums including MLW, MLLW, EGM 96 Geoid, and NAVD 88. All datasets were transformed to MHW using a

constant offset transformation based on the Homer tide station values (Fig.10). The locations and tidal relationships of

Kachemak Bay area tide stations (http://tidesandcurrents.noaa.gov/) are provided in Table 7. Only the Homer tide station values were used in the transformation process because of the central location of the station within the DEM extent. Also, it was the only tide station that provided information about the relationship between MHW and NAVD 88.

1) Bathymetric data

The early and recent NOS hydrographic surveys and USACE hydrographic surveys were transformed

from MLLW and MLW to NAVD 88 using the difference values from the Homer tide station (Table 7).

2) Topographic data

All lidar datasets were transformed from NAVD 88 to MHW using the difference value from the Homer tide station. SRTM data were transformed from the EGM 1996 Geoid to the NAVD 88 Geoid 2009

using a geoid height transformation grid acquired from NGA (http://earth-info.nga.mil/GandG/wgs84/

gravitymod/egm96/egm96.html) and NOAA’s Vertical Transformation tool VDatum. The SRTM data were

then transformed to MHW using the difference value from the Homer tide station..

Table 8. Tide stations in the Kachemak Bay area and relationships between vertical datums in meters.

 Tide 

Station Name DifferenceNAVD 88 to MHW

Difference

MLLW to MHW

Difference

MLW to MHW Latitude Longitude

9455437 Port Graham n/a 4.941 4.427 59.35 -151.83

9455500 Seldovia n/a 5.252 4.771 59.44 -151.72

9455557 Homer 3.834 5.328 4.825 59.60 -151.42

9455595 Bear Cove n/a 5.404 4.893 59.73 -151.02

9455606 Anchor Point n/a 5.366 4.834 59.77 -151.87

Average 5.2582 4.75

Figure 11. Locations of NOAA tide stations in the Kachemak Bay area.
Digital Elevation Models of Kachemak Bay, Alaska

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3.2.2 Horizontal datum transformations

Datasets used in the compilation of the Kachemak Bay DEM were originally referenced to WGS 84 geographic, NAD 83 geographic, Universal Transverse Mercator (UTM) NAD 83 Zone 5, Alaska State Plane Zone 4

NAD 27, and Alaska State Plane Zone 4 NAD 83. The relationships and transformational equations between these

geographic horizontal datums are well established. Transformations to NAD 83 geographic were accomplished using

Proj.4, ArcGIS, and VDatum software.

3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

After horizontal and vertical transformations were applied to the source datasets, the resulting transformed

data were viewed in ArcMap and QT Modeler for consistency. Any problems and errors were identified and resolved

before proceeding with subsequent gridding steps. Once evaluated, compared, and corrected, the data were converted

into final xyz files in preparation for the DEM gridding process. Problems included:

•	 Obvious small errors and anomalous points within datasets.

•	 Inconsistent overlapping NOS datasets. Earlier data were eliminated in these areas.

•	 Some topographic lidar data contained water-return values over the ocean, which needed to be clipped from

the datasets using the Kachemak Bay coastline.

•	 Inconsistencies between lidar data and SRTM data. The SRTM data are older and lower quality data. A

small buffer was used between the two datasets to allow for interpolation between them. This prevented the

appearance of a “step” or “wall” in the final DEM where the datasets meet near the head of Kachemak Bay.

•	 Inconsistencies between the USACE hydrographic survey and early NOS surveys. Early NOS surveys were

clipped out where they overlapped the more recent USACE survey.

3.3.2 Smoothing of bathymetric data

The early NOS hydrographic survey data are generally sparse at the resolution of the Kachemak Bay DEM,

especially in the deep water areas near the mouth of Kachemak Bay. To reduce the effect of artifacts due to this, a 1/3

arc-second ‘pre-surface’ bathymetric grid in MHW vertical datum was generated using GMT5, an NSF-funded software application designed to manipulate data for mapping purposes (HUhttp://gmt.soest.hawaii.edu/UH). 

To create the bathymetric surface, all bathymetric datasets were converted into xyz points, and were combined with points extracted from the Kachemak Bay coastline—to provide a breakline along the entire coastline. The

coastline elevation values were set to -0.2 meters, to ensure the bathymetric surface approached zero relative to MHW

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

The point data were then median-averaged using the GMT tool ‘blockmedian’. The GMT tool ‘surface’ was

used to apply a tight spline tension to interpolate elevations for cells without data values, and to create 1/3 arc-second

grid, 0.05 degrees (~5%) larger than the Kachemak Bay DEM region. The GMT grid created by ‘surface’ was clipped

to the Kachemak Bay coastline, and the resulting surface was compared with original bathymetric soundings to ensure

grid accuracy.

Examples of the comparisons are shown in Figures 11 and 12, which show histograms of the early NOS data

and recent NOS data, respectively, compared to the 1/3 arc-second pre-surfaced bathymetric grid. Differences cluster

around zero with a range of -11 to +9 meters when compared to the bathymetric surface. Points with the largest differences are located either in deep water, or in areas where dense data contain multiple elevation values per cell, which

were averaged to create the bathymetric surface value. The final bathymetric surface was converted into xyz files for

use in building the Kachemak Bay DEM (See Sec. 3.3.3).

9. 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.]
Friday et al., 2012

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Figure 12. Histogram of the differences between the early NOS bathymetric data and the 1/3 arc-second pre-surfaced bathymetric grid. 

Figure 13. Histogram of the differences between the recent NOS bathymetric data and the 1/3 arc-second pre-surfaced bathymetric grid. 
Digital Elevation Models of Kachemak Bay, Alaska

15

3.3.3 Building the DEM

MB-System6 was used to create the 1/3 arc-second Kachemak 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 8. The greatest weights were

assigned to the lidar datasets, the pre-surfaced bathymetric grid, and the recent high-resolution NOS surveys. The

least weight was given to the SRTM data, the USACE hydrographic soundings, and early NOS surveys.

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

Dataset Relative Gridding Weight

NOAA/NGS Kenai Lidar 100

USGS Lidar DEM 100

Recent NOS Surveys 10

Pre-Surfaced Bathymetric Grid 10

Early NOS Surveys 1

USACE Hydrographic Surveys 1

SRTM Topographic DEM 1

3.4 Quality Assessment of the DEM 

3.4.1 Horizontal accuracy

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

upon the DEM cell size and the accuracy of source datasets. Topographic features have an estimated horizontal accuracy of 10-30 meters: lidar data have an accuracy of less than five meters, but DEM cell size is 10 meters; SRTM data are

accurate to approximately 30 meters. Bathymetric features are resolved to only within a few tens of meters in deepwater areas. Recent NOS surveys and shallow, near-coastal regions 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 Kachemak Bay DEM is also dependent upon the source datasets

contributing to DEM cell values. Topographic data have an estimated vertical accuracy less than 1 meter for bareearth lidar data and up to 20 meters for non bare-earth SRTM DEMs. Bathymetric values have an estimated accuracy

between 0.1 meters and 5% of water depth. The bathymetric depth values used in building the DEM were derived from

a wide range of sounding measurements, from the early twentieth century NOS surveys to recent, high-resolution,

GPS-navigated bathymetric surveys. Gridding interpolation— used to determine bathymetric values between sparse,

poorly located NOS soundings— may degrade 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 Kachemak Bay DEM to allow for visual inspection and identification of artificial slopes along boundaries between datasets (Fig. 13). The DEM was transformed to

NAD 83 UTM Alaska 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. Figure 14 shows a data contribution plot of the Kachemak Bay DEM (the data contribution plot does not include information about the buffer area).

Analysis of preliminary grids using QT Modeler and Fledermaus revealed suspect data points, which were corrected

before recompiling the DEM. Figure 15 shows a color perspective image of the 1/3 arc-second Kachemak Bay DEM

in its final version. 

6. 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.]
Friday et al., 2012

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Figure 14. Slope map of the Kachemak Bay DEM. Flat-lying slopes are shown in white; dark shading indicates steep slopes; coastline shown in 

red. 

Figure 15. Data contribution plot of the Kachemak Bay DEM. Black represents DEM cells that are constrained by source data; 

white represents cells with elevation values that were derived from interpolation; coastline shown in red. 
Digital Elevation Models of Kachemak Bay, Alaska

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Figure 16. Perspective view from the west of the 1/3 arc-second Kachemak Bay DEM. Vertical exaggeration— times 2. 
Friday et al., 2012

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3.4.4 Comparison with National Geodetic Survey geodetic monuments

The elevations of 43 geodetic monuments were extracted from the NOAA NGS web site (http://www.ngs.

noaa.gov/) in shapefile format (see Fig. 16 for monument locations). The associated shapefile attributes provided

monument postions in NAD 83 geographic, and elevations in NAVD 88. These elevation data were transformed to

MHW and compared to the Kachemak Bay DEM (Fig. 17). Differences between the DEM and the monument elevations range from negative three to nine meters, with a mean of 0.14 meters. Differences of one to two meters occur

where the NGS monuments are found in ditches near the road, or on poles that extend slightly from the ground. Four

monuments differed from the DEM by more than four meters, and all were all located on docks, buildings, or walls.

Figure 17. Location of NGS geodetic monuments in the Kachemak Bay region. 

Figure 18. Histogram of the differences between NGS geodetic monument elevations and the Kachemak Bay DEM. 
Digital Elevation Models of Kachemak Bay, Alaska

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3.4.5 Comparison with older DEMs

In 2004, PMEL created several DEMs of the Kachemak Bay area for tsunami research purposes. These

DEMs are available from NGDC at http://www.ngdc.noaa.gov/dem/ and were used in a 2005 State of Alaska Department of Natural Resources Division of Geological and Geophysical Resources study. The final report from this

study can be found at http://www.dggs.alaska.gov/webpubs/dggs/ri/text/ri2005_002.PDF. The 2004 DEMs provided

eight arc-second and three arc-second bathymetric coverage of large sections of Kachemak Bay, and one arc-second

bathymetric-topographic DEM coverage of the communities of Homer and Seldovia.

In order to compare the 2010 Kachemak Bay DEM with the 2004 PMEL DEMs, NGDC resampled the 2010

Kachemak Bay DEM to lower resolutions and differenced the 2010 DEM with the 2004 DEMs. The DEMs differed

greatly in some areas, and subtle improvements in quality were evident. Examples from the one arc-second DEM

comparisons can be seen in Figure 18. 

The comparison on the left shows the end of the Homer spit. PartAshows a portion of the 2004 Homer DEM,

while Part B shows a portion of the 2010 Kachemak Bay DEM, and Part C shows the difference grid. Notable changes

along the Homerspit occurred at the mouth of the harbor (red area in Part C), where the newer DEM (Part B) does a better job of depicting the channel due to newly obtained USACE data. The comparison on the right is of Seldovia Harbor.

Part D shows a portion of the 2004 Homer DEM, while Part E shows a portion of the 2010 Kachemak Bay DEM, and

Part F shows the difference grid. Notable changes in Seldovia harbor occur where the horizontal locations of the jetties and breakwaters are more accurately represented in the 2010 DEM, and also along the coast, where the data flows

more smoothly toward the zero elevation. These improvements in the DEM are a result of high-quality recent NOS

surveysthat were obtained in 2008 and 2009 as part of the Hydropalooza project (http://www.hydropalooza.noaa.gov/).

Figure 19. Comparisons of 2010 Kachemak Bay DEM and 2004 PMEL Homer DEMs. Coastline and land area shown in black.
Friday et al., 2012

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3.4.6 DEM comparison with source data files

To ensure grid accuracy, the Kachemak Bay DEM was compared to several source datasets. Examples are

shown in figures 19 through 21. 

The Kenai lidar dataset (Fig. 19) varied from the Kachemak Bay DEM by a median of zero meters, ranging

from a minimum difference of -9.56 meters to a maximum difference of 40.59 meters. These minimum and maximum differences occurred where the Kenai data set overlapped the NOAA/NGS lidar dataset, which showed small

differences from the Kenai lidar dataset in some areas. The entire NOAA/NGS lidar dataset (Fig. 20) overlapped the

Kenai lidar dataset, and so its differences from the DEM are distributed with a median of -0.004 meters, a minimum

difference of -29.65 meters and a maximum difference of 32.16 meters.

The USACE hydrographic survey data (Fig. 21) varied from the Kachemak Bay DEM by a median of -0.005

meters, with a minimum difference of -17.62 meters and a maximum difference of 4.56 meters. The largest differences in the USACE data occurred where the data bordered early NOS datasets. Early NOS datasets and recent NOS

datasets (not shown) also indicated consistency with the final grid.

Figure 20. Histogram of the differences between a portion of the USGS Kenai lidar dataset and the Kachemak Bay DEM.
Digital Elevation Models of Kachemak Bay, Alaska

21

Figure 21. Histogram of the differences between the NOAA coastal lidar data and the Kachemak Bay DEM. 

Figure 22. Histogram of the differences between the USACE hydrographic survey data points and the Kachemak Bay DEM. 
Friday et al., 2012

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4. Summary and Conclusions

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

cell size of 1/3 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, 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,

Fledermaus, GMT, MB-System, QT Modeler, and VDatum software.

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

•	 Conduct lidar surveys along the southern coast of Kachemak Bay.

•	 Conduct high-resolution bathymetric surveys in deep water areas near the mouth of Kachemak Bay.

•	 Extend VDatum tidal conversion coverage to include Alaska.

5. Acknowledgments

The creation of the Kachemak Bay DEM was funded by the NOAA Pacific Marine Environmental Laboratory. The authors thank Nazila Merati, Marie Eble, and Vasily Titov (PMEL); Steve Baird (Kachemak Bay National

Estuarine Research Reserve); and Chris Clough (Kenai Peninsula Borough GIS Division).

6. References

Nautical Chart # 16645 (RNC), 18th Edition, 2002. Gore Point to Anchor Point. Scale 1:82,662. U.S. Department of

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16646 (RNC), 13th Edition, 2007. Ports of Southeastern Cook Inlet. Scales 1:30,000; 1:20,000;

1:15,000; and 1:7,500. U.S. Department of Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #16647 (RNC), 3rd Edition, 2001. Cape Elizabeth to Anchor Point. Scale 1:100,000. U.S. Department

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

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/.

Fledermaus v. 6.7.0 and 7.0.0 – 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. 4.3.4 – 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

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. 7.0.2 – 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, Version 2.3.2— developed and maintained by NOAA’s National Geodetic Survey

(NGS), Office of Coast Survey (OCS), and Center for Operational Oceanographic Products and Services

(CO-OPS), http://vdatum.noaa.gov/welcome.html. 
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