noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-60

DIGITAL ELEVATION MODELS OF PORT TOWNSEND, WASHINGTON:

PROCEDURES, DATA SOURCES AND ANALYSIS 

E. Lim 

L.A. Taylor

B.W. Eakins

K.S. Carignan

R.J. Caldwell 

P.R. Grothe 

D.Z. Friday 

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-60

DIGITAL ELEVATION MODELS OF PORT TOWNSEND, WASHINGTON:

PROCEDURES, DATA SOURCES AND ANALYSIS 

Elliot Lim1

Lisa A. Taylor2

Barry W. Eakins1

Kelly S. Carignan1

R. Jason Caldwell1 

Pamela R. Grothe1 

Dorothy Z. Friday1

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

2NOAA, National Geophysical Data Center, Boulder, Colorado 

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

August 2012
Lim 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 Port Townsend, Washington

iii

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 7

3.1.3 Topography 15

3.1.4 Bathymetry-topography 21

3.2 Establishing Common Datums 23

3.2.1 Vertical datum transformations 23

3.2.2 Horizontal datum transformations 23

3.3 Digital Elevation Model Development 24

3.3.1 Verifying consistency between datasets 24

3.3.2 Smoothing of bathymetric data 24

3.3.3 Building the NAVD 88 DEM 26

3.3.4 Building the MHW DEM 26

3.4 Quality Assessment of the DEMs 29

3.4.1 Horizontal accuracy 29

3.4.2 Vertical accuracy 29

3.4.3 Slope map and 3-D perspectives 29

3.4.4 Comparison with National Geodetic Survey geodetic monuments 32

3.4.5 NAVD 88 DEM comparison with source data files 33

4..Summary and Conclusions 35

5..Acknowledgments 35

6..References 35

7..Data Processing Software 35

List of Figures

Figure 1. Shaded-relief image of the Port Townsend NAVD 88 1/3 arc-second DEM 1

Figure 2. Google Earth imagery of the Port Townsend DEM region. 2

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

Figure 4. Digital shoreline datasets used in developing a combined shoreline of the Port Townsend

region 5

Figure 5. Combined shoreline of Port Townsend Harbor shown with ESRI World 2D Imagery 6

Figure 6. Source and coverage of bathymetric datasets used in compiling the Port Townsend DEMs 7

Figure 7. Digital NOS hydrographic survey coverage in the Port Townsend region 8

Figure 8. Color image of the CHS multibeam bathymetry 13

Figure 9. Example of gross anomalies in the CHS multibeam bathymetry 13

Figure 10. NGDC digitized points in Port Townsend Harbor. 14

Figure 11. Source and coverage of topographic datasets used in compiling the Port Townsend DEMs 15

Figure 12. Coverage of the NED 1/3 arc-second topographic DEM 16

Figure 13. Image of the NED 1/9 arc-second topographic DEM over ESRI’s World 2D Imagery 17

Figure 14. Data gaps and anomalies over Whidbey Island in the NED 1/9 arc-second DEM 18

Figure 15. Color topographic-relief image of the CDED topographic DEM 19

Figure 16. USGS topographic quadrangle indicating levees along the banks of Stanwood, WA 20

Figure 17. Coverage of PSLC bathymetric-topographic lidar 21

Figure 18. Coverage of JALBTCX bathymetric-topographic lidar 22

Figure 19. Histogram of the differences between NOS hydrographic survey H11317 and the 1/3 arcsecond pre-surfaced bathymetric grid 25
Lim et al., 2012

iv

Figure 20. Histogram of the differences between CHS multibeam swath sonar bathymetry and the and

the 1/3 arc-second pre-surfaced bathymetric grid 25

Figure 21. Image of the NAVD 88 to MHW conversion grid used to create the MHW DEM. 27

Figure 22. Histogram of the differences between the conversion grid and xyz difference files using 

NOS hydrographic survey data. 28

Figure 23. Slope map of the Port Townsend NAVD 88 DEM 30

Figure 24. Perspective view from the west of the 1/3 arc-second Port Townsend NAVD 88 DEM 31

Figure 25. Data contribution plot of the Port Townsend NAVD 88 DEM 31

Figure 26. Location of NGS geodetic monuments in the Port Townsend region 32

Figure 27. Histogram of the differences between NGS geodetic monument elevations and the Port

Townsend NAVD 88 DEM 33

Figure 28. Histogram of the differences between select PSLC topographic data points and the Port

Townsend NAVD 88 DEM 34

Figure 29. Histogram of the differences between select NED 1/9 data points and the Port Townsend

NAVD 88 DEM 34

Figure 30. Histogram of the differences between CDED topographic data points and the Port 

Townsend NAVD 88 DEM 34

List of Tables

Table 1. Specifications for the Port Townsend DEMs 3

Table 2. Shoreline datasets used in developing the Port Townsend DEMs 5

Table 3. Bathymetric datasets used in compiling the Port Townsend DEMs. 7

Table 4. Digital NOS hydrographic surveys in the Port Townsend region 9

Table 5. Topographic datasets used in compiling the NAVD 88 Port Townsend DEMs 15

Table 6. Bathymetric-topographic datasets used in compiling the NAVD 88 Port Townsend DEMs 21

Table 7. Relationships between vertical datums at the Port Townsend tide station 23

Table 8. Relationship between NAVD 88 and other vertical datums at the Port Angeles tide station 23

Table 9. Data hierarchy used to assign gridding weight in MB-System. 26
Digital Elevation Models of Port Townsend, Washington:

Procedures, Data Sources and Analysis

1. Introduction

The National Geophysical Data Center (NGDC), an office of the National Oceanic and Atmospheric Administration (NOAA), has developed two bathymetric–topographic digital elevation models (DEMs) of Port Townsend,

Washington (Fig. 1). A 1/3 arc-second1 DEM referenced to North American Vertical Datum of 1988 (NAVD 88) was

carefully developed and evaluated. An NAVD 88 to mean high water (MHW) 1/3 arc-second conversion grid was

then created to represent the relationship between NAVD 88 and MHW in the Port Townsend region. A 1/3 arc-second

MHW DEM, combining the NAVD 88 DEM and the conversion grid, 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 (http://nctr.pmel.noaa.gov/) to simulate tsunami generation, propagation and inundation. The NAVD

88 DEM was generated from diverse digital datasets in the region (grid boundary and sources shown in Fig. 3). The

MHW DEM will be used for tsunami inundation 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.

This report provides a summary of the data sources and methodology used in developing both Port Townsend DEMs.

1. The Port Townsend DEMs are built upon a grid of cells that are square in geographic coordinates (latitude and longitude), however, the cells

are not square when converted to projected coordinate systems such as UTM zones (in meters). At the latitude of Port Townsend, Washington,

(48°6'59"N, 122°46'31"W) 1/3 arc-second of latitude is equivalent to 10.30 meters; 1/3 arc-second of longitude equals 6.90 meters.

Figure 1. Shaded-relief image 

of the Port Townsend NAVD 

88 1/3 arc-second DEM. 

Contour interval is 500 meters 

for topography and 50 meters 

for bathymetry. Image is in 

Mercator projection.
Lim et al., 2012

2

2. Study Area

The Port Townsend, WA DEM spans an area of approximately 10,000 square kilometers and covers the Juan

De Fuca Strait, Puget Sound, San Juan Islands, and Vancouver Island. Port Townsend is located 40 miles northwest of

Seattle, on the tip of the Olympic Peninsula. The region offers a diverse variety of landscapes, which include mudflats,

rocky shores, mountains, tidal-flats, and floodplains (Fig. 2). Areas such as Puget Sound are estuaries composed

of a series of underwater basins and sills, which are the result of the Cascadia subduction zone and the Wisconsin

Glaciation (http://www.tititudorancea.com/z/puget_sound.htm).

The last major subduction zone earthquake in this region was a magnitude 8.7 that occurred in the Cascadia

subduction zone in 1700. The length of the fault rupture was approximately 1000 kilometers with an average slip of 20

meters. The geological record reveals that earthquakes with a moment magnitude of 8 or higher occur every 500 years

on average and are often accompanied by tsunamis. Previous earthquakes are estimated to have occurred in 1310 AD,

810 AD, 400 AD, 170 BC, and 600 BC (http://www.cosmosmagazine.com/node/3437/full).

Figure 2. Google Earth imagery of the Port Townsend DEM region. Blue box denotes DEM boundary. Yellow boundary denotes the 

international boundary between the U.S. and Canada.
Digital Elevation Models of Port Townsend, Washington

3

3. Methodology

The Port Townsend NAVD 88 and MHW DEMs were 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 realtime 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 NAVD 88. The resulting NAVD 88 DEM was then transformed to MHW using a conversion grid for

modeling of maximum flooding (see section 3.3.4). Data were gathered in an area slightly larger (~5%) than the DEM

extents. This data “buffer” ensures that gridding occurs 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. PMEL specifications for the Port Townsend, WA DEMs.

Grid Area Port Townsend, Washington

Coverage Area 123.62o to 122.20o W; 47.91o to 48.79o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984 (WGS 84)

Vertical Datums A. North American Vertical Datum of 1988 (NAVD 88)

B. Mean High Water

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.
Lim et al., 2012

4

3.1 Data Sources and Processing

Shoreline, bathymetric, topographic, and bathymetric-topographic digital datasets (Fig. 3) were obtained

from several U.S. federal, state, and international agencies including: NGDC; NOAA’s Coastal Services Center

(CSC) and Office of Coast Survey (OCS); USACE’s Joint Airborne Lidar Bathymetry Technical Center of Expertise

(JALBTCX); the Canadian Hydrographic Service (CHS), the Canadian Digital Elevation Data (CDED); Washington

State Department of Ecology (WSDE); Puget Sound Lidar Consortium (PSLC); and the U.S. Geological Survey

(USGS). Safe Software’s FME data translation tool package was used to shift datasets to NAD 83 geographic horizontal datum and to convert them into ESRI ArcGIS shapefiles or ASCII xyz files3

. The shapefiles were then displayed

with ArcGIS and the xyz files were displayed with Applied Imagery’s Quick Terrain Modeler (QT Modeler) to assess data quality and manually edit datasets. Vertical datum transformations to NAVD 88 were accomplished using

NOAA’s Vertical Datum (VDatum) transformation tool. ESRI’s online World 2D imagery was used to analyze and

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

Figure 3. Source and coverage of datasets used in compiling the Port Townsend NAVD 88 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.
Digital Elevation Models of Port Townsend, Washington

5

3.1.1 Shoreline

Shoreline datasets of the Port Townsend region were obtained from WSDE, CSC, and OCS (Table 2; Fig. 4).

Shorelines from NOAA’s OCS were obtained as Electronic Navigational Charts (ENCs)4 and were evaluated but were

not used because of inconsistencies in coverage and lower spatial resolutions. The two remaining datasets were edited

and merged to develop a “combined shoreline” of the Port Townsend region.

Table 2. Shoreline datasets used in developing the Port Townsend NAVD 88 DEMs.

Source Year Data Type Spatial 

Resolution

Original Horizontal Datum/

Coordinate System

Original Vertical 

Datum URL

WSDE 2001 vector

shoreline 1:24,000 NAD 83 HARN State PlaneWA South Mean high water

http://www.ecy.wa.gov/services/

gis/data/data.htm

CSC 1995 vector

shoreline 1:24,000 NAD 83 geographic Mean sea level http://www.csc.noaa.gov/

Figure 4. Digital shoreline datasets used in developing a combined shoreline of the Port Townsend region. Water areas shown in white. Land 

areas shown in beige.

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/]
Lim et al., 2012

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1) Washington State Department of Ecology

The WSDE shoreline dataset includes Puget Sound, Hood Canal, the Strait of Juan de Fuca, and the

Pacific coastline. In support of the Southwest Washington Coastal Erosion Study, WSDE edited, digitized,

and updated the shoreline using USGS 7.5’ quadrangles to define the mean high tide line between 2000-2010.

2) NOAA’s Coastal Services Center

NOAA’s CSC compiled a vectorshoreline using multi-temporal collection of NOAAcoastalsurvey maps

(T-sheets) ranging from 1901-1995. Where T-sheets were unavailable, NOAA’s Extracted Vector Shoreline

(EVS) was used to compile seamless shoreline coverage throughout the region.

The WSDE and CSC shoreline datasets were merged using ArcCatalog and used to create a “combined

shoreline” of the Port Townsend region. The combined shoreline was modified to include large offshore rocks, breakwaters, harbors, and small inlets as shown on the larger-scale RNCs, USGS quadrangles and ESRI’s World 2D imagery (e.g., Fig 5). ESRI’s World 2D imagery was used to identify piers and docks, which were removed from the

combined shoreline, and to reflect the most current coastal morphology. The combined shoreline was clipped to 0.05

degrees larger than the DEM boundary to allow for interpolation along the edges of the DEMs.

Figure 5. Combined shoreline of Port Townsend Harbor (green line) shown with ESRI World 2D Imagery. Piers are not included in the shoreline 

as water can flow beneath them, but large rocks and breakwaters were included as they are solid structures.
Digital Elevation Models of Port Townsend, Washington

7

3.1.2 Bathymetry

Bathymetric datasets available for use in the compilation of the Port Townsend DEMs include 133 NOS hydrographic surveys; CHS multibeam bathymetry, and NGDC digitized bathymetry. (Table 3; Fig. 6).

Table 3. Bathymetric datasets used in compiling the Port Townsend DEMs.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

NGDC

1887

to

2006

NOS

hydrographic

survey

soundings

Ranges from 5 meters to

1.2 kilometers (varies with

scale of survey, depth,

traffic and probability of

obstructions)

NAD 83 geographic, 

NAD 83 UTM Zone

10 North

MLW and

MLLW

(meters)

http://www.ngdc.noaa.gov/

mgg/bathymetry/hydro.html

CHS 2010 Gridded data ~10 meters WGS 84 geographic

Assumed

MSL

(meters)

http://www.dfo-mpo.gc.ca/

index-eng.htm

NGDC 2010 digitized

points n/a WGS 84 geographic NAVD 88

Figure 6. Source and coverage of bathymetric datasets used in compiling the Port Townsend DEMs. 
Lim et al., 2012

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1) National Ocean Service hydrographic survey data

A total of 133 NOS hydrographic surveys conducted between 1887 and 2006 were available for use in

developing the Port Townsend DEMs. Surveys were extracted as xyz files using GEODAS5 from NGDC’s online NOS hydrographic database with a buffer 0.05 degrees (~5%) larger than the Port Townsend DEM area

to support data interpolation along grid edges. The downloaded digital hydrographic survey data were vertically referenced to mean low water (MLW) or mean lower low water (MLLW) and horizontally referenced

to NAD 83 geographic and NAD 83 UTM Zone 10 North (Table 4; Fig. 7). The most recent surveys were

provided to NGDC by NOS, retrieved from the NGDC multibeam database. These surveys were provided as

bathymetric attributed grids (BAG). The horizontal and vertical datums of the BAG data are NAD 83 UTM

Zone 10 North and in MLLW respectively (Table 4).

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

point spacing than large scale surveys. All NOS survey data were converted to NAVD 88 using the VDatum

transformation tool (see Sec. 3.2.1). The data were then converted to shapefiles using FME software and

displayed in ESRI ArcMap and reviewed for digitizing errors and edited as necessary. The surveys were also

compared to other bathymetric datasets, the combined shoreline, NOAA RNCs, and USGS 7.5’ quadrangles.

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

multibeam data.

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

superseded by more recent surveys. 

5. 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 Port Townsend, Washington

9

Table 4. Digital NOS hydrographic surveys in the Port Townsend region.

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

F00399 1994 5,000 Mean Lower Low Water North American Datum of 1983

H01815+ 1887 20,000 Mean Low Water North American Datum of 1983

H01887 1888 10,000 Mean Lower Low Water North American Datum of 1983

H02049 1890 20,000 Mean Low Water North American Datum of 1983

H02079 1889 20,000 Mean Low Water North American Datum of 1983

H02080 1891 20,000 Mean Low Water North American Datum of 1983

H02211 1894 40,000 Mean Lower Low Water North American Datum of 1983

H02215 1894 10,000 Mean Lower Low Water North American Datum of 1983

H02216 1894 10,000 Mean Lower Low Water North American Datum of 1983

H05158 1931 20,000 Mean Lower Low Water North American Datum of 1983

H05159 1931 40,000 Mean Lower Low Water North American Datum of 1983

H05160 1931 10,000 Mean Lower Low Water North American Datum of 1983

H05659 1935 20,000 Mean Lower Low Water North American Datum of 1983

H05709 1935 20,000 Mean Lower Low Water North American Datum of 1983

H06193 1937 5,000 Mean Lower Low Water North American Datum of 1983

H06474 1939 10,000 Mean Lower Low Water North American Datum of 1983

H06475 1939 10,000 Mean Lower Low Water North American Datum of 1983

H06476 1939 10,000 Mean Lower Low Water North American Datum of 1983

H06477 1939 5,000 Mean Lower Low Water North American Datum of 1983

H06577 1940 10,000 Mean Lower Low Water North American Datum of 1983

H06607 1940 10,000 Mean Lower Low Water North American Datum of 1983

H06612 1941 20,000 Mean Lower Low Water North American Datum of 1983

H06613 1941 20,000 Mean Lower Low Water North American Datum of 1983

H06614 1941 10,000 Mean Lower Low Water North American Datum of 1983

H06615 1941 10,000 Mean Lower Low Water North American Datum of 1983

H06616 1941 10,000 Mean Lower Low Water North American Datum of 1983

H06617 1941 10,000 Mean Lower Low Water North American Datum of 1983

H06618 1940 5,000 Mean Lower Low Water North American Datum of 1983

H06645 1940 10,000 Mean Lower Low Water North American Datum of 1983

H06649 1940 10,000 Mean Lower Low Water North American Datum of 1983

H06650 1940 10,000 Mean Lower Low Water North American Datum of 1983

H06651 1941 10,000 Mean Lower Low Water North American Datum of 1983

H06652 1941 20,000 Mean Lower Low Water North American Datum of 1983

H06653 1943 40,000 Mean Lower Low Water North American Datum of 1983

H06654 1940 10,000 Mean Lower Low Water North American Datum of 1983

H06738 1942 20,000 Mean Lower Low Water North American Datum of 1983

H06746 1943 10,000 Mean Lower Low Water North American Datum of 1983

H06747 1941 20,000 Mean Lower Low Water North American Datum of 1983

H06748 1941 10,000 Mean Lower Low Water North American Datum of 1983

H06750 1941 10,000 Mean Lower Low Water North American Datum of 1983
Lim et al., 2012

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

H06757 1942 5,000 Mean Lower Low Water North American Datum of 1983

H06816 1943 10,000 Mean Lower Low Water North American Datum of 1983

H06817 1943 10,000 Mean Lower Low Water North American Datum of 1983

H06818 1943 20,000 Mean Lower Low Water North American Datum of 1983

H07045 1945 20,000 Mean Lower Low Water North American Datum of 1983

H07080 1947 10,000 Mean Lower Low Water North American Datum of 1983

H07099 1947 10,000 Mean Lower Low Water North American Datum of 1983

H07168 1946 10,000 Mean Lower Low Water North American Datum of 1983

H07613 1948 10,000 Mean Lower Low Water North American Datum of 1983

H07809 1951 10,000 Mean Lower Low Water North American Datum of 1983

H07929 1944 10,000 Mean Lower Low Water North American Datum of 1983

H07962 1953 10,000 Mean Lower Low Water North American Datum of 1983

H08084 1955 10,000 Mean Lower Low Water North American Datum of 1983

H08085 1953 10,000 Mean Lower Low Water North American Datum of 1983

H08086 1955 10,000 Mean Lower Low Water North American Datum of 1983

H08087 1953 5,000 Mean Lower Low Water North American Datum of 1983

H08115 1954 10,000 Mean Lower Low Water North American Datum of 1983

H08116 1954 10,000 Mean Lower Low Water North American Datum of 1983

H08117 1957 5,000 Mean Lower Low Water North American Datum of 1983

H08173 1954 5,000 Mean Lower Low Water North American Datum of 1983

H08174 1955 10,000 Mean Lower Low Water North American Datum of 1983

H08317 1956 10,000 Mean Lower Low Water North American Datum of 1983

H08319 1956 10,000 Mean Lower Low Water North American Datum of 1983

H08320 1956 10,000 Mean Lower Low Water North American Datum of 1983

H08321 1956 10,000 Mean Lower Low Water North American Datum of 1983

H08322 1956 10,000 Mean Lower Low Water North American Datum of 1983

H08323 1956 10,000 Mean Lower Low Water North American Datum of 1983

H08324 1957 10,000 Mean Lower Low Water North American Datum of 1983

H08331 1955 10,000 Mean Lower Low Water North American Datum of 1983

H08331I 1955 10,000 Mean Lower Low Water North American Datum of 1983

H08333 1955 10,000 Mean Lower Low Water North American Datum of 1983

H08398 1957 10,000 Mean Lower Low Water North American Datum of 1983

H08399 1957 10,000 Mean Lower Low Water North American Datum of 1983

H08400 1957 10,000 Mean Lower Low Water North American Datum of 1983

H08459 1958 10,000 Mean Lower Low Water North American Datum of 1983

H08478 1959 30,000 Mean Lower Low Water North American Datum of 1983

H08481 1959 10,000 Mean Lower Low Water North American Datum of 1983

H08518 1960 10,000 Mean Lower Low Water North American Datum of 1983

H08519 1960 10,000 Mean Lower Low Water North American Datum of 1983

H08520 1960 10,000 Mean Lower Low Water North American Datum of 1983

H08542 1962 10,000 Mean Lower Low Water North American Datum of 1983
Digital Elevation Models of Port Townsend, Washington

11

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

H08543 1960 10,000 Mean Lower Low Water North American Datum of 1983

H08544 1960 10,000 Mean Lower Low Water North American Datum of 1983

H08609 1961 10,000 Mean Lower Low Water North American Datum of 1983

H08699 1962 10,000 Mean Lower Low Water North American Datum of 1983

H08700 1962 10,000 Mean Lower Low Water North American Datum of 1983

H08701 1962 10,000 Mean Lower Low Water North American Datum of 1983

H08753 1963 10,000 Mean Lower Low Water North American Datum of 1983

H08754 1966 10,000 Mean Lower Low Water North American Datum of 1983

H08894 1966 10,000 Mean Lower Low Water North American Datum of 1983

H08895 1966 10,000 Mean Lower Low Water North American Datum of 1983

H08910 1966 10,000 Mean Lower Low Water North American Datum of 1983

H08911 1966 10,000 Mean Lower Low Water North American Datum of 1983

H08912 1966 10,000 Mean Lower Low Water North American Datum of 1983

H08914 1967 10,000 Mean Lower Low Water North American Datum of 1983

H08915 1966 10,000 Mean Lower Low Water North American Datum of 1983

H08916 1969 10,000 Mean Lower Low Water North American Datum of 1983

H08927 1967 20,000 Mean Lower Low Water North American Datum of 1983

H08928 1967 10,000 Mean Lower Low Water North American Datum of 1983

H08929 1967 10,000 Mean Lower Low Water North American Datum of 1983

H08930 1967 10,000 Mean Lower Low Water North American Datum of 1983

H08933 1967 10,000 Mean Lower Low Water North American Datum of 1983

H09038 1969 10,000 Mean Lower Low Water North American Datum of 1983

H09414 1974 40,000 Mean Lower Low Water North American Datum of 1983

H09417 1974 40,000 Mean Lower Low Water North American Datum of 1983

H09743 1978 10,000 Mean Lower Low Water North American Datum of 1983

H10072 1982 10,000 Mean Lower Low Water North American Datum of 1983

H10534 1994 10,000 Mean Lower Low Water North American Datum of 1983

H10535 1994 10,000 Mean Lower Low Water North American Datum of 1983

H10555 1994 10,000 Mean Lower Low Water North American Datum of 1983

H10564 1995 10,000 Mean Lower Low Water North American Datum of 1983

H10583 1995 10,000 Mean Lower Low Water North American Datum of 1983

H10587 1995 5,000 Mean Lower Low Water North American Datum of 1983

H10608 1995 10,000 Mean Lower Low Water North American Datum of 1983

H10621 1996 10,000 Mean Lower Low Water North American Datum of 1983

H10626 1995 10,000 Mean Lower Low Water North American Datum of 1983

H10662 1995 5,000 Mean Lower Low Water North American Datum of 1983

H10755 1997 10,000 Mean Lower Low Water North American Datum of 1983

H10766 1998 10,000 Mean Lower Low Water North American Datum of 1983

H10783 1998 10,000 Mean Lower Low Water North American Datum of 1983

H10792 2000 10,000 Mean Lower Low Water North American Datum of 1983

H10828 1999 n/a Mean Lower Low Water North American Datum of 1983
Lim et al., 2012

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

H10887 1999 10,000 Mean Lower Low Water North American Datum of 1983

H10911 1999 10,000 Mean Lower Low Water North American Datum of 1983

H10939 1999 20,000 Mean Lower Low Water North American Datum of 1983

H11038 2001 10,000 Mean Lower Low Water North American Datum of 1983

H11039 2002 20,000 Mean Lower Low Water North American Datum of 1983

H11040 2002 5,000 Mean Lower Low Water North American Datum of 1983

H11188 2002 10,000 Mean Lower Low Water North American Datum of 1983

H11268 2003 10,000 Mean Lower Low Water North American Datum of 1983

H11269 2004 10,000 Mean Lower Low Water North American Datum of 1983

H11316 2004 20,000 Mean Lower Low Water North American Datum of 1983

H11317 2004 10,000 Mean Lower Low Water North American Datum of 1983

H11371* 2005 20,000 Mean Lower Low Water NAD 83 UTM Zone 10 North

H11375* 2005 10,000 Mean Lower Low Water NAD 83 UTM Zone 10 North

H11419* 2005 10,000 Mean Lower Low Water NAD 83 UTM Zone 10 North

H11420* 2005 10,000 Mean Lower Low Water NAD 83 UTM Zone 10 North

H11552* n/a 10,000 Mean Lower Low Water NAD 83 UTM Zone 10 North

H11553* n/a 10,000 Mean Lower Low Water NAD 83 UTM Zone 10 North

H11556* 2006 5,000 Mean Lower Low Water NAD 83 UTM Zone 10 North

H11631* n/a n/a Mean Lower Low Water NAD 83 UTM Zone 10 North

H11632* n/a n/a Mean Lower Low Water NAD 83 UTM Zone 10 North

H11748* n/a n/a Mean Lower Low Water NAD 83 UTM Zone 10 North

H11750* n/a n/a Mean Lower Low Water NAD 83 UTM Zone 10 North

H11751* n/a n/a Mean Lower Low Water NAD 83 UTM Zone 10 North

* denotes BAG survey + denotes excluded survey
Digital Elevation Models of Port Townsend, Washington

13

2) Canadian Hydrographic Service Multibeam Bathymetry

Canadian multibeam bathymetry was provided by Rob Hare of CHS. A Memorandum Of Understanding (MOU) was signed by NOAA and CHS giving NOAA clearance to integrate CHS bathymetry into the

Port Townsend DEMs in support of improved tsunami forecasting and warning. Surveys are in WGS 84

geographic, which NGDC gridded at a cell size of 1 arc-second (Fig. 8) and are in an assumed MSL vertical

datum. Depth anomalies were manually removed in Fledermaus (e.g., Fig. 9).

Figure 8. Color image of the CHS multibeam bathymetry provided by the Canadian Hydrographic Service. DEM boundary in white and 

combined coastline in green.

Figure 9. Examples of gross anomalies in the CHS multibeam bathymetry. Spikes were manually removed using Fledermaus.
Lim et al., 2012

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3) NGDC-digitized depths

NGDC digitized bathymetric values in areas where bathymetric data were sparse and inconsistent with

corresponding RNCs and satellite imagery. Figure 10 illustrates how digitized depths were used to represent

the seabed of Port Townsend Harbor. Elevation values were assigned to points based on RNCs, satellite imagery, and peripheral datasets and were referenced to the NAVD 88 vertical datum.

Figure 10. NGDC digitized points in Port Townsend Harbor.
Digital Elevation Models of Port Townsend, Washington

15

3.1.3 Topography

The topographic datasets used to build the Port Townsend DEMs include: USGS NED 1/3 and 1/9 arc-second

DEMs, PSLC, and CDED (Table 5; Fig. 11). NGDC also digitized elevation points to represent breakwaters, jetties,

levees, and harbors that were not resolved in the topographic datasets.

Table 5. Topographic datasets used in compiling the Port Townsend DEMs.

Source Year Data Type Spatial Resolution Original Horizontal Datum/ Coordinate System

Original Vertical 

Datum URL

USGS

NED

1999-

2007 DEMs 1/3 and 1/9 arcseconds NAD 83 geographic NAVD 88 http://ned.usgs.gov

PSLC 2009 Lidar ~1 meter NAD 83 WA State Plane

North NAVD 88 http://pugetsoundlidar.

ess.washington.edu/

CDED 2007 DEM ~8 meters NAD 83 geographic MSL http://www.geobase.ca/

data/cded/index.html

NGDC 2010 digitized

points n/a WGS 84 geographic NAVD 88

Figure 11. Source and coverage of topographic datasets used in compiling the Port Townsend DEMs.
Lim et al., 2012

16

1) United States Geological Survey National Elevation Dataset 1/3 arc-second topographic DEM

The USGS National Elevation Dataset (NED) provides complete 1/3 arc-second coverage of the Port

Townsend region6. The dataset is available for download as raster DEMs in NAD 83 geographic horizontal

datum and NAVD 88 vertical datum (meters). The 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 quadrangle maps and aerial photographs

based on topographic surveys and topographic lidar. The NED DEM included “zero” elevation values over

the open ocean, which were removed from the dataset by clipping to the “combined shoreline”. 

NGDC used the NED 1/3 arc-second DEM in those regions where the NED 1/3 is derived from the

higher resolution NED 1/9 data. In addition, the NED 1/3 was used where no NED 1/9, CDED, or lidar data

existed (Fig. 12).

Figure 12. Coverage of the NED 1/3 arc-second topographic DEM used in building the Port Townsend DEMs. This dataset was used only where 

higher resolution topography was unavailable. 

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 Georgia. 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 NAVD 88, 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]
Digital Elevation Models of Port Townsend, Washington

17

2) United States Geological Survey National Elevation Dataset 1/9 arc-second topographic DEM

The USGS provides limited high-resolution NED 1/9 arc-second DEMs, derived from 3 meter point

spacing lidar data. Data are in NAD 83 geographic coordinates and NAVD 88 vertical datum (meters), and

are available for download as raster DEMs. The horizontal accuracy is 3 meters and the vertical accuracy, depending on the source data, is less than 22 centimeters. The NED 1/9 DEM included “zero” elevation values

over the open ocean, which were removed by clipping to the “combined shoreline”.

In areas where recent lidar surveys have been conducted, the lidar superseded the older NED 1/9 arcsecond DEM, primarily in the San Juan Islands (Fig. 11). In addition, the NED 1/9 contained sections of

anomalous elevation values that were manually removed from the dataset (e.g., Fig. 13).

Figure 13. Image of the NED 1/9 arc-second topographic DEM draped over ESRI’s World 2D Imagery. NGDC deleted anomalous elevation 

values of “0” shown in black. 
Lim et al., 2012

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3) Puget Sound Lidar Consortium Lidar

PSLC is an informal group of local agency staff and federal research scientists devoted to developing

public domain high-resolution lidar products for the Puget Sound region. Although PSLC provides extensive

lidar coverage around the boundaries of the Port Townsend DEMs, the NED 1/9 arc-second topographic

DEM integrates PSLC lidar collected prior to 2005. Lidar surveys conducted in 2009 for the San Juan Islands

and the town of Everett were used in the Port Townsend DEMs.

Although the NED 1/9 topographic DEM integrates PSLC lidar prior to 2005, it contained anomalous

values and data gaps over Whidbey Island and Snohomish County (e.g., Fig. 14). In these areas, PSLC lidar

from 2001 and 2003 respectively, were used in the Port Townsend DEMs. Data are in NAD 83 Washington

State Plane North projected coordinates and NAVD88 vertical datum (meters), and have a vertical accuracy

of ~1 meter.

Figure 14. A. Data gaps and anomalies over Whidbey Island in the NED 1/9 arc-second DEM. 

B. PSLC lidar of the same region.
Digital Elevation Models of Port Townsend, Washington

19

4) Canadian Digital Elevation Dataset

CDED consist of an ordered array of ground elevations at regularly spaced intervals at scales of 1:50,000

and 1:250,000. For the region surrounding Vancouver Island, the grid spacing is 0.75 arc-seconds at the

1:50,000 scale (Fig. 15). Data are in NAD 83 geographic coordinates, referenced to MSL (meters), and are

available for download as raster DEMs. The extracted elevations have a vertical accuracy of +/- 5 to 10 meters depending on the source data resolution. See the CDED web site for specific source information (http://

www.geobase.ca/geobase/en/data/cded/index.html).

Figure 15. Color shaded-relief image of the CDED topographic DEM. DEM extent in white and combined shoreline in red.
Lim et al., 2012

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5) NGDC digitized elevations

After inspecting the topographic datasets available for the Port Townsend DEMs, many breakwaters,

jetties, and levees were either not represented or were unresolved in the topographic datasets. NGDC digitized these features with point spacings of 10 meters using USGS topographic quadrangles, RNCs, and

ESRI’s World 2D imagery as references.

In Stanwood, WA, NGDC digitized levees with point spacings of 10 meters and fixed elevation values

of 3.8 meters at NAVD 88 vertical datum, as indicated in the NED 1/9 topographic DEM. The levees prevent

this lowland area from flooding at high tide (Fig. 16).

Figure 16. A. USGS topographic quadrangle indicating levees along the banks of Stanwood, WA. 

B. Color image of the Port Townsend MHW DEM. Many levees were not represented in the topographic datasets and were digitized by NGDC. 

Elevation values below zero are colored blue even though the levees prevent these areas from flooding at high tide. 
Digital Elevation Models of Port Townsend, Washington

21

3.1.4 Bathymetry-topography

Bathymetric-topographic lidar obtained from PSLC and JALBTCX provide bare-earth coverage of the coasts

of Possesion Sound and Skagit Bay respectively, (Table 6). The surveys were conducted in 2001 and 2004 and have

a vertical accuracy of approximately +/- 1 meter. Horizontal and vertical datums for the PSLC dataset are WGS 84

Washington State Plane North projected coordinates and MLLW respectively. Horizontal and vertical datums for the

JALBTCX datset are NAD 83 Washington State Plane North projected coordinates and NAVD 88 respectively. The

PSLC dataset was converted to NAVD 88 using the VDatum transformation tool (see Sec. 3.2.1).

Table 6. Bathymetric-topographic datasets used in compiling the Port Townsend DEMs.

Source Year Data Type Spatial Resolution Original Vertical 

Datum

Original Horizontal 

Datum/Coordinate 

System

URL

PSLC 2001 lidar ~1 meter MLLW (meters) WGS 84 WA State

Plane North

http://pugetsoundlidar.ess.

washington.edu/

JALBTCX 2004 lidar ~1 meter NAVD88 (feet) NAD 83 WA State

Plane North

http://rocky2.ess.washington.

edu/data/raster/bluegreen/

1) PSLC bathymetric-topographic lidar

Bathymetric-topographic lidar collected in 2001 was obtained from PSLC and covers the coast of Possesion Sound (Fig. 17). Data are in WGS 84 Washington State Plane North projected coordinates and MLLW

vertical datum (meters). The vertical accuracy is ~1 meter. NGDC converted the data from MLLW to NAVD

88 using the VDatum transformation tool (see Sec. 3.2.1).

Figure 17. Coverage of PSLC bathymetric-topographic lidar.
Lim et al., 2012

22

2) JALBTCX bathymetric-topographic lidar

Bathymetric-topographic lidar collected in 2004 was obtained from JALBTCX and covers areas of

Skagit Bay (Fig. 18). Data are in NAD 83 Washington State Plane North projected coordinates and NAVD 88

vertical datum (feet). The vertical accuracy is ~1 meter. Lidar is particularly useful in lowland areas such as

Skagit Bay because of the wide tidal ranges that expose and cover the intertidal zone.

Figure 18. Coverage of JALBTCX bathymetric-topographic lidar. Lidar is particularly useful in lowland areas such as Skagit Bay because of the 

wide tidal ranges that expose and cover the intertidal zone.
Digital Elevation Models of Port Townsend, Washington

23

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation and evaluation of the Port Townsend NAVD 88 DEM were originally

referenced to a number of vertical datums including MLLW, MLW, MSL, and NAVD 88. All datasets except for the

Canadian topographic data were transformed to NAVD 88 using the VDatum transformation tool (http://vdatum.noaa.

gov/). The tidal relationships at the Port Townsend and Port Angeles tide stations (http://tidesandcurrents.noaa.gov/)

are provided in Table 7 and 8 respectively. Locations of the tide stations are illustrated in Figure 26.

1) Bathymetric data

The NOS hydrographic surveys and CHS multibeam were transformed from MLW, MLLW, and MSL to

NAVD 88 using VDatum.

2) Topographic data

The NED 1/3 arc-second DEM, NED 1/9 arc-second DEM, and PSLC lidar were originally referenced

to NAVD 88 and required no vertical transformation. The CDED DEM is a Canadian dataset originally referenced to MSL. NGDC used the relationship between MSL and NAVD 88 at the Port Angeles, WA tide station

(Table 8). Conversion to NAVD 88, using FME software, was accomplished by subtracting a constant offset

value of 1.166 meters

Table 7. Relationships between vertical datums at the Port Townsend tide station (# 9444900).

Vertical datum Value (m)

MHHW 2.596

MHW 2.389

MSL 1.522

MLW 0.760

MLLW 0.00

Table 8. Relationship between NAVD 88 and other vertical datums at the Port Angeles tide station (# 9444090).

Vertical datum Value (m) Difference to NAVD 88 (m)

MHHW 2.153 2.024

MHW 1.987 1.858

MSL 1.295 1.166

MLW 0.586 .457

NAVD 88 0.129 0.00

MLLW 0.00 -0.129

3.2.2 Horizontal datum transformations

Datasets used to compile the Port Townsend NAVD 88 DEM were originally referenced to WGS 84 geographic, NAD 83 geographic, NAD 27 geographic, NAD 83 UTM Zone 10 North, NAD 83 Washington State Plane

North, and WGS 84 Washington State Plane North. The relationships and transformational equations between the

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

FME software.
Lim et al., 2012

24

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 and Quick Terrain 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 topographic datasets. The lower resolution datasets were clipped to high resolution

datasets.

•	 Data values over the ocean in the NED DEMs, CDED, and the PSLC lidar topographic datasets. These

datasets required automated clipping to the combined coastline or were edited manually.

•	 Digital, measured bathymetric values from NOS surveys date back over 123 years. More recent data, differed

from older NOS data by as much as 50 meters vertically. The older NOS survey data were excised where

more recent bathymetric data exists.

•	 The breakwaters, jetties, and levees in the Port Townsend DEMs are not well-represented in available

elevation data. Limited lidar data in the region and written descriptions of the features were used to estimate

elevations.

3.3.2 Smoothing of bathymetric data

Older NOS hydrographic survey data are generally sparse at the resolution of the Port Townsend DEMs

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 these low resolution datasets, and to provide effective interpolation into the coastal zone,

a 1/3 arc-second-cell size ‘pre-surface’ bathymetric grid in NAVD 88 vertical datum was generated using GMT7, an

NSF-funded software application designed to manipulate data for mapping purposes (http://gmt.soest.hawaii.edu/).

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

surveys. All of the bathymetric data were combined with points extracted from the adjusted MHW coastline—to provide a buffer along the entire coastline. The coastline elevation values were assigned values congruent to the MHWNAVD 88 conversion grid to prevent artificial flooding in the Port Townsend MHW DEM (see Sec. 3.4.4).

The point data were then median-averaged using the GMT tool ‘blockmedian’ to create a 1/3 arc-second

grid 0.05 degrees (~5%) larger than the Port Townsend DEM region. The GMT tool ‘surface’ was then used to apply

a tight spline tension to interpolate elevations for cells without data values. The GMT grid created by ‘surface’ was

converted into an ESRI Arc ASCII grid file, and clipped to the combined coastline (to eliminate data interpolation

into land areas). The resulting surface was compared with original soundings to ensure grid accuracy. Figures 19 and

20 show histograms of the NOS and CHS multibeam compared to the 1/3 arc-second pre-surfaced bathymetric grid.

Differences cluster around zero with the three datasets.

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

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

either edited or not used. The gridded bathymetric surface was then converted to an xyz file for use in building the

NAVD 88 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 Models of Port Townsend, Washington

25

Figure 19. Histogram of the differences between NOS hydrographic survey H11317 and the 1/3 arc-second pre-surfaced bathymetric grid. 

Figure 20. Histogram of the differences between the CHS multibeam swath sonar bathymetry and the 1/3 arc-second pre-surfaced bathymetric 

grid. 
Lim et al., 2012

26

3.3.3 Building the NAVD 88 DEM

MB-System was used to create the 1/3 arc-second Port Townsend NAVD 88 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 9. Greatest weight

was given to the NED 1/9 arc-second DEM, CHS multibeam, JALBTCX bathymetric-topographic lidar, PSLC bathymetric-topographic lidar, and PSLC topographic lidar. Least weight was given to the pre-surfaced bathymetric grid.

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

Dataset Relative Gridding Weight

NGDC Bathymetric Surface 0.01

NGDC digitized elevations 1

NOS hydrographic surveys 10

USGS NED 1/3 Topographic DEM 10

CDED topographic DEM 10

PSLC topographic lidar 100

PSLC bathymetric-topographic lidar 100

JALBTCX 100

CHS Multibeam 100

USGS NED 1/9 Topographic DEM 100

3.3.4 Building the MHW DEM

The MHW DEM was created by adding an NAVD 88-to-MHW conversion grid to the NAVD 88 DEM.

1) Developing the conversion grid

Using extents slightly larger (~ 5 percent) than the DEM, an initial xyz file was created that contained the

coordinates of the four bounding vertices and midpoint of the larger extents. The elevation value at each of

the points was set to zero. The GMT tool ‘surface’ applied a tension spline to interpolate cell values making

a zero-value 3 arc-second grid. This zero-value grid was then converted to an intermediate xyz file using the

GMT tool ‘grd2xyz’.

Conversion values from NAVD 88 to MHW at each xyz point were generated using VDatum. Null values were removed and a converted xyz file was created by clipping the data to the combined shoreline using

FME. The converted xyz file was then interpolated with the GMT tool ‘surface’ to create the 1/3 arc-second

‘NAVD 88 to MHW’ conversion grid with the extents slightly larger (~ 5 percent) than the NAVD 88 DEM

(Fig. 21).
Digital Elevation Models of Port Townsend, Washington

27

Figure 21. Image of the NAVD 88 to MHW conversion grid used to generate the MHW DEM. Coastline in black. Brown box denotes DEM 

boundary. Values in meters. 

2) Assessing the accuracy of the conversion grid

The NAVD 88-to-MHW conversion grid was assessed using the NOS survey data. For testing of this

methodology, the NOS hydrographic survey data were transformed from MLLW to NAVD 88 using VDatum.

Shapefiles of the resultant xyz files were created and null values removed using FME. The shapefiles were

then merged to create a single shapefile of all NOS surveys with a vertical datum of NAVD 88. A second

shapefile of NOS data were created with a vertical datum of MHW using the same method. Elevation differences between the MHW and NAVD 88 shapefiles were computed after performing a spatial join in ArcGIS.

To verify the conversion grid methodology, the difference shapefile created using ArcGIS was converted

to xyz format using FME. The ‘NAVD 88-to-MHW’ grid was then compared to the difference xyz file. Figure

22 indicates an agreement to approximately +/- 0.00640 meters with a mean difference of 0.00008 meters.

Errors in the source datasets require rebuilding only the NAVD 88 DEM.
Lim et al., 2012

28

Figure 22. Histogram of the differences between the conversion grid and xyz difference files using NOS hydrographic survey data.

3) Creating the MHW DEM

Once the NAVD 88 DEM was complete and assessed for errors, the NAVD 88 to MHW conversion grid

was added to it using ArcCatalog. The resulting MHW DEM was reviewed and assessed using RNCs, USGS

topographic maps, and ESRI World 2D imagery. Problems encountered were determined to reside in source

datasets, which were corrected before building a new NAVD 88 DEM.
Digital Elevation Models of Port Townsend, Washington

29

3.4 Quality Assessment of the DEMs

3.4.1 Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Port Townsend DEMs is dependent

upon DEM cell size and source datasets. Topographic features in the DEM have an estimated horizontal accuracy of

10 meters: gridded PSLC lidar data have an accuracy of approximately 1 meter, NED 1/3 topographic DEM is accurate to approximately 10 meters, NED 1/9 topographic DEM is accurate to approximately 3 meters, and the CDED

topographic DEM is accurate to approximately 8 meters. Bathymetric-topographic features obtained from PSLC and

JALBTCX have an estimated horizontal accuracy of approximately 1 meter. Bathymetric features are resolved only

to within a few tens of meters in deep-water areas. Shallow, near-coastal regions, rivers, and harbor surveys have an

accuracy approaching that of sub-aerial topographic features. Positional accuracy is limited by the sparseness of deepwater 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 Port Townsend DEMs is also dependent upon the source datasets

contributing to DEM cell values. Topographic data have an estimated vertical accuracy of approximately 0.1 meters

for PSLC lidar data, approximately 1 meter for the NED 1/9 arc-second DEM, and 7-15 meters for the NED 1/3 arcsecond DEM. 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 Port Townsend NAVD 88 DEM to allow for

visual inspection and identification of artificial slopes along boundaries between datasets (Fig. 23). The DEM was

transformed to NAD 83 UTM Zone 10 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. Figure 1 shows a color image of the 1/3 arc-second Port Townsend NAVD 88 DEM in its final version. Figure

24 shows a perspective rendering of the final NAVD 88 DEM. Figure 25 shows a data contribution plot of the Port

Townsend DEMs.
Lim et al., 2012

30

Figure 23. Slope map of the Port Townsend NAVD 88 DEM. Flat-lying slopes are shown in white; dark shading denotes steep slopes; combined 

shoreline indicated in red. 
Digital Elevation Models of Port Townsend, Washington

31

Figure 24. Perspective view from the west of the 1/3 arc-second Port Townsend NAVD 88 DEM. Vertical exaggeration–times 2.

Figure 25. Data contribution plot of the Port Townsend NAVD 88 DEM. Black depicts DEM cells constrained by source data; white depicts cells 

with elevation values derived from interpolation. Due to the scale of the image, sparse soundings may not be visible in the graphic. Shoreline is 

shown in red.
Lim et al., 2012

32

3.4.4 Comparison with National Geodetic Survey geodetic monuments

The elevations of 1856 U.S. geodetic monuments were extracted from the NOAA NGS web site (http://www.

ngs.noaa.gov/) in shapefile format (see Fig. 26 for monument locations). Shapefile attributes give positions in NAD

83 geographic (typically sub-mm accuracy) and elevations in NAVD 88 (in meters). Elevations were compared to the

Port Townsend NAVD 88 DEM (Fig. 27). Differences between the DEM and the monument elevations range from

-109.94 to 345.48 meters. The majority are within several meters. Large differences in elevations occurred where

monuments are located on road cuts, on the top of buildings, or have conversion errors evident on the NGS data sheet

(e.g., feet instead of meters).

Figure 26. Location of NGS geodetic monuments, shown as yellow circles, in the Port Townsend region. Triangles represent NOAA tide stations 

mentioned in the report.
Digital Elevation Models of Port Townsend, Washington

33

Figure 27. Histogram of the differences between NGS geodetic monument elevations and the Port Townsend NAVD 88 DEM.

3.4.5 NAVD 88 DEM comparison with source data files

To ensure grid accuracy, select bathymetric and topographic source data files were compared to the Port

Townsend NAVD 88 DEM using Fledermaus, FME and ArcMap.

A histogram of the differences between data points from the PSLC topographic lidar data DEM and the Port

Townsend NAVD 88 DEM is shown in Figure 28. Differences cluster around zero with the majority are within ± 0.5

meters.

A random selection of NED 1/9 topographic points were compared to the Port Townsend NAVD 88 DEM

(Fig. 29; a random selection was used to represent the overall survey as there were too many points to statistically

compare with current processing methods). The histogram shows the differences in elevations are clustered around

zero and the majority are within ± 1 meter. 

Comparison of all the CDED topographic points and the Port Townsend NAVD 88 DEM are shown in Figure

30. The histogram shows the differences in elevations are clustered around zero with the elevation differences ranging

from -0.0064 to +25.27. The largest differences occur along the edge of steep slopes.
Lim et al., 2012

34

Figure 28. Histogram of the differences between select PSLC topographic data points and the Port Townsend NAVD 88 DEM.

Figure 29. Histogram of the differences between select NED 1/9 data points and the Port Townsend NAVD 88 DEM.

Figure 30. Histogram of the differences between CDED topographic data points and the Port Townsend NAVD 88 DEM.
Digital Elevation Models of Port Townsend, Washington

35

4. Summary and Conclusions

Two integrated bathymetric–topographic digital elevation models of the Port Townsend, Washington region

with cell sizes of 1/3 arc-second, were developed for the Pacific Marine Environmental Laboratory (PMEL) NOAA

Center for Tsunami Research. The best available digital data from U.S. federal, 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, and VDatum software.

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

•	 Conduct high-resolution bathymetric surveys along the Canada/United States border.

•	 Conduct bathymetric-topographic lidar surveys in lowland areas along estuaries.

•	 Extend VDatum coverage into Canada.

5. Acknowledgments

The creation of the Port Townsend DEM was funded by the NOAA Pacific Marine Environmental Laboratory. The authors thank Nazila Merati, Marie Eble, and Vasily Titov (PMEL). Special thanks to Rob Hare (CHS) who

provided Canadian multibeam bathymetry.

6. References

Nautical Chart #18400, 48th Edition, 2008. Strait of Georgia and Strait of Juan De Fuca. Scale 1:200,000. U.S.

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

Nautical Chart #18421, 49th Edition, 2008. Strait of Georgia and Strait of Juan De Fuca. Scale 1:80,000. 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, Washington, Hhttp://www.esri.com/H

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,

Hhttp://www.safe.com/.

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

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

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

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

GMT v. 4.3.4 – Generic Mapping Tools, freeware developed and maintained by Paul Wessel and Walter Smith, funded

by the National Science Foundation, Hhttp://gmt.soest.hawaii.edu/H

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

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

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, Hhttp://www.appliedimagery.com/H

VDatum Transformation Tool, Washington - Washington Juan De Fuca, v. 01 – 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/.
Lim et al., 2012

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VDatum Transformation Tool, Washington - Washington Puget Sound, v. 01 – 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/.