noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-26

DIGITAL ELEVATION MODEL OF NANTUCKET, MASSACHUSETTS:

PROCEDURES, DATA SOURCES AND ANALYSIS 

B.W. Eakins

L.A. Taylor 

K.S. Carignan

R.R. Warnken

E. Lim

P.R. Medley

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

March 2009

UNITED STATES

DEPARTMENT OF COMMERCE

Gary Locke

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Dr. Jane Lubchenco

Under Secretary for Oceans

and Atmosphere/Administrator

National Environmental Satellite,

Data, and Information Service

Mary E. Kicza

Assistant Administrator

NOAA Technical Memorandum NESDIS NGDC-26

DIGITAL ELEVATION MODEL OF NANTUCKET, MASSACHUSETTS:

PROCEDURES, DATA SOURCES AND ANALYSIS 

Barry W. Eakins1

Lisa A. Taylor2 

Kelly S. Carignan1

Robin R. Warnken2

Elliot Lim1

Pamela R. Medley1

1

Cooperative 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

March 2009
Eakins et al., 2009

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/inundation/tsunami/inundation.html

Also available from the National Technical Information Service (NTIS)

(http://www.ntis.gov)
Digital Elevation Model of Nantucket, Massachusetts

iii

Contents

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

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

3. Methodology ......................................................................................................................................2

3.1 Data Sources and Processing ......................................................................................................3

3.1.1 Shoreline................................................................................................................4

3.1.2 Bathymetry ............................................................................................................7

3.1.3 Topography..........................................................................................................18

3.2 Establishing Common Datums .................................................................................................21

3.2.1 Vertical datum transformations ...........................................................................21

3.2.2 Horizontal datum transformations.......................................................................22

3.3 Digital Elevation Model Development .....................................................................................22

3.3.1 Estimating bathymetry from satellite imagery ....................................................22

3.3.2 Verifying consistency between datasets..............................................................23

3.3.3 Smoothing of bathymetric data ...........................................................................23

3.3.4 Gridding the data with MB-System......................................................................24

3.4 Quality Assessment of the DEM...............................................................................................24

3.4.1 Horizontal accuracy.............................................................................................24

3.4.2 Vertical accuracy .................................................................................................24

3.4.3 Slope maps and 3-D perspectives........................................................................24

3.4.4	 Comparison with source data files.......................................................................26

3.4.5 Comparison with NGS geodetic monuments......................................................27

4. Summary and Conclusions...............................................................................................................28

5. Acknowledgments............................................................................................................................28

6. References........................................................................................................................................28

7. Data Processing Software ................................................................................................................29

List of Figures

Figure 1. Shaded-relief image of the Nantucket DEM...............................................................................1

Figure 2. April 19, 2007 photograph of the breach in Norton Point Beach ...............................................2

Figure 3. Source and coverage of datasets used in compiling the Nantucket DEM ..................................3

Figure 4. Digital coastline datasets at the mouth of Nantucket Harbor .....................................................5

Figure 5. Recent morphologic change at south end of Cape Cod ..............................................................6

Figure 6. Digital NOS hydrographic survey coverage in the Nantucket region ......................................10

Figure 7. Coverage of southern half of USGS multibeam swath sonar survey east of Cape Cod ...........11

Figure 8. Coverage of NOAA ENCs in the Nantucket region .................................................................12

Figure 9. Coverage of USACE hydrographic surveys in the Nantucket region.......................................13

Figure 10.	 Coverage of satellite images used to estimate bathymetry in areas of recent, significant

morphologic change in the Nantucket region....................................................................14

Figure 11. Datasets covering Katama Bay and Norton Point Beach along its south side..........................15

Figure 12. Morphologic change around Monomoy Island.........................................................................16

Figure 13. Morphologic change in Pleasant Bay .......................................................................................16

Figure 14. Datasets covering the cut in Nantucket Harbor’s east jetty ......................................................17

Figure 15. Color image of USGS NED DEM in the southern part of Nantucket Island ...........................18

Figure 16. Coastal lidar survey along the northeastern shore of Nantucket Island....................................19

Figure 17. Satellite image of breach in Norton Point Beach......................................................................20

Figure 18. NOAA tide stations and vertical datum relationships in the Nantucket region ........................21

Figure 19. Graph of pixel value vs. depth surrounding Monomoy Island .................................................22

Figure 20. Histogram of the differences between NOS hydrographic survey H11346 and 

the 1 arc-second pre-surfaced bathymetric grid.................................................................23

Figure 21. Slope map of the Nantucket DEM............................................................................................25

Figure 22. Perspective view from the southwest of the Nantucket DEM..................................................25
Eakins et al., 2009

iv

Figure 23. Histogram of the differences between the 2000 topographic lidar survey along the 

northern coast of Nantucket Island and the Nantucket DEM............................................26

Figure 24. Histogram of the differences between USGS Cape Cod multibeam swath sonar survey 

and the Nantucket DEM ....................................................................................................26

Figure 25. Histogram of the differences between NOS hydrographic survey H11078 and the 

Nantucket DEM.................................................................................................................26

Figure 26. Histogram of the differences between the USGS NED on mainland Massachusetts 

and the northwestern part of the Nantucket DEM.............................................................27

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

Nantucket DEM.................................................................................................................27

Figure 28. Location of NGS geodetic monuments and NOAA tide stations in the Nantucket 

region .................................................................................................................................28

List of Tables

Table 1.	 PMEL specifications for the Nantucket DEM ............................................................................2

Table 2. Shoreline datasets used in developing the Nantucket DEM.......................................................4

Table 3. Nautical charts available in the Nantucket region ......................................................................4

Table 4. Bathymetric datasets used in compiling the Nantucket DEM ....................................................7

Table 5. Digital NOS hydrographic surveys in the Nantucket region ......................................................8

Table 6. USGS multibeam swath sonar surveys used in compiling the Nantucket DEM......................11

Table 7. USACE hydrographic sonar surveys used in compiling the Nantucket DEM .........................13

Table 8. Topographic datasets used in compiling the Nantucket DEM..................................................18

Table 9. Relationship between MHW and other vertical datums in the Nantucket region.....................21

Table 10.	 Logarithmic equations used to convert satellite images to estimated seafloor

depths.................................................................................................................................22

Table 11. Data hierarchy used to assign gridding weight in MB-System..................................................24
Digital Elevation Model of Nantucket, Massachusetts:

Procedures, Data Sources and Analysis

1. Introduction

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

Atmospheric Administration (NOAA), developed an integrated bathymetric–topographic digital elevation model 

(DEM) of Nantucket, Massachusetts (Fig. 1) for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center

for Tsunami Research (http://nctr.pmel.noaa.gov/). The 1/3 arc-second1 coastal DEM will be used as input for the 

Method of Splitting Tsunami (MOST) model developed by PMEL to simulate tsunami generation, propagation and 

inundation. The DEM was generated from diverse digital datasets in the region (grid boundary and sources shown in 

Fig. 3) and designed to represent modern morphology. It will be used for tsunami forecasting 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 description of the data sources and methodology used in 

developing the Nantucket DEM.

Figure 1. Shaded-relief image of the Nantucket DEM. Contour interval is 25 meters.

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

square when converted to projected coordinate systems, such as UTM zones (in meters). At the latitude of Nantucket, Massachusetts (41°17′ N, 

70°6′ W) 1/3 arc-second of latitude is equivalent to 10.283 meters; 1/3 arc-second of longitude equals 7.757 meters.
Eakins et al., 2009

2

2. Study Area

The Nantucket DEM covers the coastal region surrounding the town of Nantucket, Massachusetts on 

Nantucket Island (Fig. 1). It spans Nantucket Sound, from Martha’s Vineyard on the west to Nantucket Island on the 

east, and north to Cape Cod, and extends into the Atlantic Ocean.

The coastal morphology of the Cape Cod and Nantucket Sound region can vary rapidly as alongshore currents 

move sediment along the coast. For example, in April 2007, a breach was opened along the southern shore of Katama 

Bay on Martha’s Vineyard (Fig. 2; Sigelman, 2007), dramatically altering current dynamics and sediment erosion, 

migration and deposition in the area.

3. Methodology

	 The Nantucket DEM was constructed to meet PMEL specifications (Table 1), based on input requirements for

the development of reference inundation models (RIMs) and standby inundation models (SIMs) (V. Titov, pers. comm.) 

in support of NOAA’s Tsunami Warning Centers use of SIFT to provide real-time tsunami forecasts in an operational 

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

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

respectively.2 Data processing and evaluation, and DEM assembly and assessment are described in the following 

subsections.

Table 1: PMEL specifications for the Nantucket DEM.

Grid Area Nantucket, Massachusetts

Coverage Area 70.67o to 69.49o W; 40.81o to 41.71o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984 (WGS 84)

Vertical Datum MHW

Vertical Units Meters

Cell Size 1/3 arc-second

Grid Format ESRI Arc ASCII grid

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

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

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

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

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

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

interchangeably.

Figure 2. April 19, 2007 photograph of the 

breach in Norton Point Beach. Norton Point 

Beach is a barrier sand bar along the southern 

edge of Katama Bay that connects the islands of 

Martha’s Vineyard and Chappaquiddick, though 

it occasionally opens up, as it did on April 17, 

2007, disconnecting the two islands. [Photo credit: 

Trustees of Reservations (TTOR), http://www.

mvtimes.com/news/2007/04/19/norton_point_

beach.php]
Digital Elevation Model of Nantucket, Massachusetts

3

3.1 Data Sources and Processing

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

state agencies including: NOAA’s Office of Coast Survey (OCS), Coastal Services Center (CSC), and NGDC; the

U.S. Geological Survey (USGS); the U.S. Army Corps of Engineers (USACE); and the Massachusetts Office of

Geographic and Environmental Information (MassGIS). Satellite imagery, taken by Digital Globe and extracted from 

Google Earth Pro, was used for estimating bathymetry in areas with recent, significant morphologic change. 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 shapefiles3. The shapefiles were then displayed with ArcGIS to assess data quality 

and manually edit datasets. Vertical datum transformations to MHW were accomplished using FME, based on data 

from NOAA tide stations in the region. Applied Imagery’s Quick Terrain Modeler software was used to evaluate 

processing and gridding techniques.

Figure 3. Source and coverage of datasets used in compiling the Nantucket DEM.

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

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

83 datum transformations.
Eakins et al., 2009

4

3.1.1 Shoreline

Coastline datasets of the Nantucket region (Table 2) were obtained from OCS and MassGIS, and were used 

to develop a “combined coastline” for the Nantucket DEM.

Table 2: Shoreline datasets used in developing the Nantucket DEM.

Source Year Data Type Spatial 

Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical Datum URL

NOAA OCS 2007 Nautical charts 1:10,000 to 

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

MassGIS 1992

Digitized 

1:24,000 

USGS DLG

1:25,000 NAD 83 Mass State 

Plane (meters) http://www.mass.gov/mgis/cs.htm

1) NOAA Nautical Charts

Sixteen NOAA Nautical Charts were available for the Nantucket area (Table 3), and were downloaded 

from the OCS web site (http://nauticalcharts.noaa.gov/mcd/enc/). All charts are available as georeferenced 

Raster Navigational Charts (RNCs; digital images of the charts), which were used to assess the quality of 

bathymetric datasets. Six charts were also available as NOAA Electronic Navigational Charts (ENCs)4 that 

represent chart features as individual digital objects. The ENCs are in S-57 format and include coastline data 

files referenced to MHW. Coastlines from several of the larger scale RNCs, along with identified jetties,

were manually digitized by NGDC and included in the combined coastline for Nantucket. The ENC digital 

coastlines contained many piers and other manmade structures that had to be removed when building the 

combined coastline.

Table 3: Nautical charts available in the Nantucket region.

Chart # Chart name Scale Format ENC #

13200 Georges Bank and Nantucket Shoals 1:400,000 RNC,ENC US3EC09M

13204 Georges Bank, Eastern Part 1:220,000 RNC

13218 Martha’s Vineyard to Block Sound 1:80,000 RNC, ENC US4MA23M

13228 Westport River and Approaches 1:20,000 RNC

13229 South Coast of Cape Cod 1:40,000 RNC

13230 Buzzards Bay 1:40,000 RNC, ENC US5MA25M

13232 New Bedford Harbor and Approaches 1:20,000 RNC

13233 Martha’s Vineyard 1:40,000, w/1:20,000 inset RNC

13235 Woods Hole 1:5000 RNC

13236 Cape Cod Canal and Approaches 1:20,000 RNC, ENC US5MA27M

13237 Nantucket Sound and Approaches 1:80,000 RNC, ENC US4MA43M

13238 Martha’s Vineyard Eastern Part 1:20,000, w/ 1:10,000 insets RNC

13241 Nantucket Island 1:40,000 RNC

13242 Nantucket Harbor 1:10,000 RNC, ENC US5MA40M

13244 Eastern Entrance to Nantucket Sound 1:40,000 RNC

13248 Chatham Harbor 1:20,000 RNC

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/]
Digital Elevation Model of Nantucket, Massachusetts

5

2) Massachusetts Office of Geographic and Environmental Information coastline

MassGIS modified the USGS 1:24,000 hydrography digital line graph (DLG) quadrangle files to produce

the Massachusetts coastline. MassGIS reformatted the DLG files into Arc/INFO coverages and projected

them into the Massachusetts State Plane Coordinate system, NAD 27. The coastline was then extracted 

from the files and edited. Polygon topology was also created for each quadrangle.The coverages were then

projected into the Massachusetts State Plane Coordinate system, NAD 83 (meters). This coastline contains 

many manmade features, such as piers, that had to be removed from the dataset by NGDC (e.g., Fig. 4).

Figure 4. Digital coastline datasets at the mouth of Nantucket Harbor. The combined coastline 

built by NGDC was adjusted to fit high-resolution coastal lidar data. It also captures the east and 

west jetties, which are not represented in the NOAA ENC and MassGIS coastlines. NOAA ENC 

and MassGIS coastlines also contained piers and other manmade coastal structures that had to be 

removed in developing the combined coastline.

The ENC and MassGIS coastlines were integrated into a combined coastline for the Nantucket region, 

which was then adjusted to align with the large-scale RNCs, high-resolution coastal lidar data, and satellite imagery 

in three areas (e.g., Fig. 5) where recent, significant morphologic change has occurred. The west and east jetties at

the mouth of Nantucket Harbor are submerged at high tide, with the exception of the ends nearest the beach. The east 

jetty has a cut part way out, visible in satellite imagery, that is scoured to a depth of 18 feet (Nantucket Harbor Master, 

pers. comm.). These features were manually digitized by NGDC for representation in the combined coastline and 

the Nantucket DEM (Fig. 4). The combined coastline was converted to xyz data with 10 meter point spacing, using 

NGDC’s GEODAS software, to build a pre-surfaced bathymetric grid (see Sec. 3.3.3).
Eakins et al., 2009

6

Figure 5. Recent morphologic change at south end of Cape Cod. Nauset Beach has grown rapidly southward, as observed 

in a 2008 satellite image and a 2000 coastal lidar survey and represented in the NGDC combined coastline (bold red line). 

RNC #13248 in background.
Digital Elevation Model of Nantucket, Massachusetts

7

3.1.2 Bathymetry

Bathymetric datasets used in the compilation of the Nantucket DEM included NOS hydrographic surveys, 

USGS multibeam swath sonar surveys, extracted NOAA ENC sounding data, USACE hydrographic harbor surveys, 

and Digital Globe satellite imagery in three areas with recent, significant morphologic change (Table 4; Fig. 3). NGDC

also digitized soundings at the mouth of Nantucket Harbor. Datasets were originally referenced to MHW, mean low 

water (MLW) or mean lower low water (MLLW).

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

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

 NGDC 1889 to 

2004

NOS 

hydrographic 

survey 

soundings

Ranges from 10 

meters to 1 kilometer 

(varies with scale of 

survey, depth, traffic,

and probability of 

obstructions)

NAD 27 or NAD 83 

geographic

MLW or 

MLLW 

(meters)

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

bathymetry/hydro.html

USGS 1998 to 

2004 

Multibeam 

swath sonar 4 meters NAD 83 geographic MLLW (meters)

NOAA 

ENC

2006 to 

2007

Extracted 

soundings 1:10,000 to 1:80,000 WGS 84 geographic MLLW 

(meters)

http://nauticalcharts.noaa.gov/

mcd/enc/

USACE 2004 to 

2007

Hydrographic 

surveys

Scattered soundings and 

channel profiles

NAD 27 or NAD 

83 Mass State Plane 

(feet)

MLLW

(feet)

http://www.nae.usace.army.

mil/navigation/navigation2.

asp?mystate=ma

Digital 

Globe 2008 Satellite imagery Several meters Undefined Undefined

NGDC 2008 Digitized 

depths Meters to 10s of meters WGS 84 geographic MHW (meters)

1) National Ocean Service hydrographic survey data

A total of 86 NOS hydrographic surveys conducted between 1889 and 2004 were available in digital 

form for use in developing the Nantucket DEM (Table 5; Fig. 6). The hydrographic survey data were 

originally vertically referenced to MLW or MLLW and horizontally referenced to either NAD 27 or NAD 

83 geographic datums. Three of the older surveys (H01948, H05141 and H05249) were not used in building 

the Nantucket DEM as they have been superseded by more recent surveys, with the exception of the inset for 

H01948, which had to be manually digitized by NGDC.

Data point spacing for the NOS surveys varied by collection date. In general, earlier surveys had greater 

point spacing than more recent surveys. All surveys were extracted from NGDC’s NOS Hydrographic Survey 

Database (http://www.ngdc.noaa.gov/mgg/bathymetry/hydro.html) referenced to NAD 83. The surveys were 

subsequently clipped to a polygon 0.05 degrees (~5%) larger than the Nantucket DEM area to support data 

interpolation along grid edges. 

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

ESRI ArcMap and reviewed for digitizing errors against scanned original survey smooth sheets and edited 

as necessary. The surveys were also compared to the topographic and bathymetric datasets, the combined 

coastline, and NOAA RNCs. The surveys were clipped to remove soundings that overlap more recent NOS, 

USGS and USACE bathymetric surveys.
Eakins et al., 2009

8

Table 5: Digital NOS hydrographic surveys in the Nantucket region.

Survey ID Year Scale Original Vertical Datum Original Horizontal Datum

H01948 1889 20,000 MLW Digitized in NAD 27

H05141 1931 20,000 MLW NAD 27

H05227 1932 40,000 MLW NAD 27

H05249 1932 40,000 MLW NAD 27

H05275 1932 100,000 MLW NAD 27

H05276 1932 100,000 MLW NAD 27

H05543 1934 20,000 MLW NAD 27

H05588 1934 20,000 MLW NAD 27

H05589 1934 10,000 MLW NAD 27

H06348 1938 5,000 MLW NAD 27

H06349 1938/42 10,000 MLW NAD 27

H06350 1938/42 20,000 MLW NAD 27

H06439 1939 60,000 MLW NAD 27

H06441 1939 120,000 MLW NAD 27

H06446 1939 40,000 MLW NAD 27

H06447 1939 80,000 MLW NAD 27

H06468 1942 10,000 MLW NAD 27

H06469 1939/42 10,000 MLW NAD 27

H06470 1939 10,000 MLW NAD 27

H06471 1939 10,000 MLW NAD 27

H06472 1939 10,000 MLW NAD 27

H06473 1939 10,000 MLW NAD 27

H06531 1939 10,000 MLW NAD 27

H06532 1939 20,000 MLW NAD 27

H06533 1939/42 10,000 MLW NAD 27

H06534 1939 20,000 MLW NAD 27

H06558 1940 40,000 MLW NAD 27

H06559 1940 40,000 MLW NAD 27

H06561 1940 10,000 MLW NAD 27

H06562 1940 10,000 MLW NAD 27

H06563 1940 40,000 MLW NAD 27

H06712 1940 20,000 MLW NAD 27

H06713 1940 20,000 MLW NAD 27

H06714 1940 20,000 MLW NAD 27

H06742 1942 20,000 MLW NAD 27

H08111 1953 10,000 MLW NAD 27

H08170 1954 5,000 MLW NAD 27

H08171 1956/60 20,000 MLW NAD 27

H08172 1954/56 20,000 MLW NAD 27

H08348 1955/56 10,000 MLW NAD 27

H08349 1956 10,000 MLW NAD 27

H08350 1956 40,000 MLW NAD 27

H08409 1957 25,000 MLW NAD 27

H08449 1958 10,000 MLW NAD 27

H08450 1958 20,000 MLW NAD 27

H08484 1959 10,000 MLW NAD 27

H08497 1958/59 10,000 MLW NAD 27
Digital Elevation Model of Nantucket, Massachusetts

9

Survey ID Year Scale Original Vertical Datum Original Horizontal Datum

H08599 1961 40,000 MLW NAD 27

H08600 1961 40,000 MLW NAD 27

H08601 1961 40,000 MLW NAD 27

H08602 1961 20,000 MLW NAD 27

H08603 1960/61 20,000 MLW NAD 27

H08631 1960/61 10,000 MLW NAD 27

H08760 1963 10,000 MLW NAD 27

H08761 1960/64 12,500 MLW NAD 27

H08820 1964 10,000 MLW NAD 27

H08821 1964 10,000 MLW NAD 27

H08824 1963 12,500 MLLW NAD 27

H08845 1964 10,000 MLW NAD 27

H08846 1964/65 12,500 MLW NAD 27

H08847 1965 20,000 MLW NAD 27

H08902 1966 10,000 MLW NAD 27

H08903 1966 10,000 MLW NAD 27

H08905 1966 20,000 MLW NAD 27

H09233 1971 20,000 MLW NAD 27

H09615 1976 20,000 MLW NAD 27

H09661 1976/78 10,000 MLW NAD 27

H09668 1976/77 10,000 MLW NAD 27

H09712 1977 10,000 MLW NAD 27

H09724 1977 10,000 MLW NAD 27

H09750 1978 10,000 MLW NAD 27

H10186 1985 40,000 MLLW NAD 27

H10191 1985 40,000 MLLW NAD 27

H10192 1985 40,000 MLLW NAD 27

H10198 1985 10,000 MLLW NAD 27

H10498 1993/94 10,000 MLLW NAD 83

H10504 1993/94 10,000 MLLW NAD 83

H10511 1993 10,000 MLLW NAD 83

H10520 1994 10,000 MLLW NAD 83

H10547 1994 10,000 MLLW NAD 83

H10556 1994 10,000 MLLW NAD 83

H10563 1994 10,000 MLLW NAD 83

H10817 1998 10,000 MLLW NAD 83

H11077 2001 5,000 MLLW NAD 83

H11078 2001 10,000 MLLW NAD 83

H11079 2004 20,000 MLLW NAD 83

H11318 2004 10,000 MLLW NAD 83
Eakins et al., 2009

10

Figure 6. Digital NOS hydrographic survey coverage in the Nantucket region. Some older surveys were not used as they have been 

superseded by more recent surveys. DEM boundary in purple; combined coastline in green.
Digital Elevation Model of Nantucket, Massachusetts

11

2) U.S. Geological Survey multibeam swath sonar surveys

Two multibeam swath sonar surveys conducted in Massachusetts waters (Table 6) were available 

from USGS: one along the eastern seaboard of Cape Cod, conducted on the Canadian Coast Guard vessel 

Frederick G. Creed, and another at the eastern entrance to Nantucket Sound, conducted aboard the NOAA 

ship Thomas Jefferson in cooperation with NOS. Bathymetric data from the 1998 Cape Cod5 survey (Fig. 7) 

were compiled into 4-meter grids in NAD 83 geographic coordinates and MLLW vertical datum (Poppe et 

al., 2006). Bathymetric data from NOS survey, H11079, conducted in 2004, were compiled into 3-meter grids 

in NAD 83 geographic and MLLW vertical datum (Poppe et al., 2007). NGDC downloaded the bathymetric 

grids from the USGS web site and evaluated them with ArcGIS; NOS survey H11079 (Table 5; Fig. 6) was 

also available as point data through NGDC’s NOS Hydrographic Survey Database.

Table 6: USGS multibeam swath sonar surveys used in compiling the Nantucket DEM.

Survey ID Year Original Vertical Datum Original Horizontal Datum Resolution

90815 1998 MLLW NAD 83 geographic 4m grid

H11079 2004 MLLW NAD 83 geographic 3m grid

Figure 7. Coverage of southern half of USGS multibeam swath sonar survey east of Cape Cod. DEM boundary in purple; 

combined coastline in green.

5. This data set includes bathymetry of the sea floor offshore of eastern Cape Cod, Massachusetts. The data were collected with a multibeam sea

floor mapping system during USGS survey 98015, conducted November 9 - 25, 1998. The surveys were conducted using a Simrad EM 1000 multibeam echosounder mounted aboard the Canadian Coast Guard vessel Frederick G. Creed. This multibeam system utilizes 60 electronically aimed 

receive beams spaced at intervals of 2.5 degrees that insonify a strip of sea floor up to 7.5 times the water depth (swath width of 100 to 200 m within

the survey area). The horizontal resolution of the beam on the sea floor is approximately 10% of the water depth. Vertical resolution is approximately

1 percent of the water depth. [Extracted from metadata]
Eakins et al., 2009

12

3) NOAA Electronic Navigational Chart soundings

NOAA ENC sounding data were extracted from charts #13218, 13230, 13236, 13237, and 13242. Chart 

#13242 covers Nantucket Harbor, while chart 13237 spans Nantucket Sound. Elevations of wrecks and 

exposed rocks were also extracted from these two ENCs. Soundings and submerged wreck depths are in 

WGS 84 geographic and MLLW datums, while exposed rocks and wrecks are referenced to MHW.

Figure 8. Coverage of NOAA ENCs in the Nantucket region. DEM boundary in purple; combined coastline in green.
Digital Elevation Model of Nantucket, Massachusetts

13

4) U.S. Army Corps of Engineers hydrographic surveys of harbor channels

USACE conducted three surveys at the harbor entrances in the Nantucket region: Nantucket Harbor, 

Chatham (Stage) Harbor6 and Vineyard Haven Harbor7 (Table 7; Fig. 9). All data were originally in 

Massachusetts State Plane coordinates (Island or Mainland) and either NAD 27 or NAD 83 horizontal 

datums. Depths were in feet relative to MLLW.

Table 7: USACE hydrographic sonar surveys used in compiling the Nantucket DEM.

Location Year Original Vertical 

Datum (feet)

Original Horizontal 

Datum (feet) Spatial Resolution

Nantucket Harbor 2004 MLLW NAD 27 Mass State 

Plane Island

Profiles 60 to 300 meters long, spaced 30

meters apart, with 6-meter point spacing

Vineyard Haven Harbor 2004 MLLW NAD 27 Mass State 

Plane Island Scattered soundings, 6 to 20 meters apart

Chatham (Stage) Harbor 2007 MLLW NAD 83 Mass State 

Plane Mainland

Profiles 75 to 100 meters long, spaced 10 to

20 meters apart, with 3-meter point spacing

Figure 9. Coverage of USACE hydrographic surveys in the Nantucket region. 

DEM boundary in purple; combined coastline in green.

6. A hydrographic survey of the Federal Navigation Project at Chatham (Stage) Harbor, Chatham, Massachusetts was conducted in 2007. Soundings 

were taken at prescribed intervals to determine the current condition of the channel and turning basin. Data were collected in the field utilizing Coast

Oceanographics Hypack Max v. 4.3a software. Collected information was processed and mapped utilizing Coast Oceanographics Hypack Max 4.3a 

software, Bentley Microstation V8 and Bentley InRoads SelectCAD v. 8.02. Purpose was to graphically provide bathymetric data for the channel 

and anchorage. [Extracted from metadata]

7. A hydrographic survey of the Federal Navigation Project at Vineyard Haven Harbor, Martha’s Vineyard, MA, was conducted in 2004. Soundings 

were taken at the prescribed intervals to determine the current condition of channel. Data were collected in the field utilizing Coastal Oceanographics Hypack Max v. 4.3a software. Collected information was processed and mapped utilizing Coastal Oceanographics Hypack Max 4.3a software, 

Bentley Microstation V8 and Bentley InRoads SelectCAD v. 8.02. Purpose was to graphically provide bathymetric data for the channel and anchorage. [Extracted from metadata]
Eakins et al., 2009

14

5) Estimated bathymetry from satellite imagery

The Nantucket region frequently undergoes significant and sometimes rapid coastal morphologic

change. On April 17th and 18th, 2007, a breach opened in Norton Point Beach along the south end of Katama 

Bay, on Martha’s Vineyard (Figs. 2, 10 and 11; Sigelman, 2007). Opening of the breach altered current and 

tidal dynamics in the bay and along the southern Atlantic coast, influencing sediment migration, erosion and

deposition. Publicly available digital elevation data has not been collected since this morphologic change 

occurred. Similar recent morphologic change has also occurred around Monomoy Island and in Pleasant Bay 

on Cape Cod (Figs. 10, 12 and 13). NGDC developed a methodology to convert satellite image ocean color 

in these areas to estimated bathymetric depths, which is described in Section 3.3.1.

Figure 10. Coverage of satellite images used to estimate bathymetry in areas of recent, significant morphologic change in 

the Nantucket region. DEM boundary in purple; combined coastline in green.
Digital Elevation Model of Nantucket, Massachusetts

15

Figure 11. Datasets covering Katama Bay and Norton Point Beach along its south side. A) Smooth sheet of NOS hydrographic survey 

H08820 conducted in 1964, the only bathymetric survey of the bay. Note that at this time there was an opening at the eastern end of 

Norton Point Beach. B) NOAA RNC #13238. C) Coastal topographic lidar survey conducted in 2000. D) USGS NED topographic 

DEM. E) Digital Globe satellite image taken in 2008, following opening of the breach. F) Shaded-relief image of the Nantucket DEM, 

with Katama Bay, breach and coastal depths derived from the satellite image. Combined coastline in red.
Eakins et al., 2009

16

Figure 12. Morphologic change around Monomoy Island. A) NOAA RNC #13248. B) Digital Globe satellite image taken 

in 2008. Combined coastline in red.

Figure 13. Morphologic change in Pleasant Bay. A) NOAA RNC #13248. B) Digital Globe satellite image taken in 2008. 

DEM boundary in purple; combined coastline in red.
Digital Elevation Model of Nantucket, Massachusetts

17

6) NGDC digitized depths

The east jetty at the mouth of Nantucket Harbor has a cut partway out from the coast that is not represented 

in RNC #13242 (Fig. 14A; Nantucket Harbor Master, pers. comm.), nor in the USGS NED DEM (Fig. 14B) 

or coastal topographic lidar data (Fig. 14C). This cut has scoured an 18-foot-deep depression (Nantucket 

Harbor Master, pers. comm.) that is recognizable in Digital Globe satellite imagery of the harbor (Fig. 14D). 

NGDC manually digitized soundings in and around the east jetty and scour, using MHW soundings from 

NOS surveys H08449 and H08479 as a guide (Fig. 14E). The Nantucket DEM roughly captures the east jetty 

cut and scour as identified in the satellite image (Fig. 14F).

NGDC also manually digitized soundings from the inset in NOS Survey H01948 (shoal at Bishop and 

Clerks Light) as that part of the survey did not exist in digital form. The NOS survey smooth sheet, in TIFF, 

was georeferenced in ArcMap using the NAD 27 geographic registration lines on the map.

Figure 14. Datasets covering the cut in Nantucket Harbor’s east jetty. A) NOAA RNC #13242. B) USGS NED topographic DEM. C) 

Coastal topographic lidar survey conducted in 2000. D) Digital Globe satellite image taken in 2008, with scour readily apparent. E) 

NOS and USACE soundings around the cut in the east jetty, along with NGDC digitized soundings in the cut. F) Color image of the 

Nantucket DEM, with scour depths derived from the satellite image. Combined coastline in green.
Eakins et al., 2009

18

3.1.3 Topography

Two topographic datasets in the Nantucket region were used to build the Nantucket DEM: one from USGS 

and one from CSC (Table 8; Fig. 3). NGDC also digitized values along the modified Norton Point Beach, Martha’s

Vineyard.

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

Source Year Data Type Spatial

Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Datum 

(meters)

URL

USGS 1999-

2006 NED DEM 1/3 arcsecond NAD 83 geographic NAVD88 http://ned.usgs.gov/

NOAA 

CSC

1998 and 

2000

Coastal 

topographic lidar

1 to 3 

meters NAD 83 geographic NAVD88 http://csc.noaa.gov/digitalcoast/

NGDC Digitized 

elevation points

~2.5 

meters WGS 84 geographic MHW

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

USGS’s National Elevation Dataset (NED) provides complete 1/3 arc-second coverage of the Nantucket 

region8. Data are in NAD 83 geographic coordinates and North American Vertical Datum of 1988 (NAVD88) 

vertical datum (meters), and are available for download as raster DEMs. 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 has been revised using recently collected 

lidar data. The NED DEMs, downloaded as 5 tiles, contain artificial east-west lineations on Martha’s Vineyard

and Nantucket Islands and are inconsistent with the Nantucket combined coastline (e.g., Fig. 15). They also 

contain “zero” elevation values over the open ocean, which were removed from the dataset by clipping to the 

combined coastline; zero values on “land” were adjusted to a height of 0.1 meters above MHW.

Figure 15. Color image of USGS NED DEM in the southern part of Nantucket Island. Note east-west 

lineations, and zero values over water. Combined coastline in dark red.

8. The USGS National Elevation Dataset (NED; http://ned.usgs.gov/) 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 NAVD88, except for AK, 

which is NGVD29. NED is a living dataset that is updated bimonthly to incorporate the “best available” DEM data. As more 1/3 arc-second (10 m) 

data covers the U.S., then this will also be a seamless dataset. [Extracted from USGS NED web site]
Digital Elevation Model of Nantucket, Massachusetts

19

2) Coastal Services Center lidar topography

CSC provides online access to coastal topographic lidar data from numerous federal agencies through 

its Digital Coast web site (http://csc.noaa.gov/digitalcoast/). Two joint NOAA/USGS/NASA lidar surveys9

in the Nantucket region were available through this web site: a winter 1998 survey, and a fall 2000 survey. 

These data were downloaded from the CSC web site as single files for each survey. Original data were in

ASCII XYZ format, and NAD 83 geographic and NAVD88 datums. Due to the large size of the 2000 survey, 

NGDC separated these data into 16 tiles of 1 million points each for easier visualization and editing. As the 

two surveys were flown along the coasts of Nantucket and Martha’s Vineyard Islands and the eastern coast of

Cape Cod, nominally at low tide, they were used to help define the position of the MHW combined coastline

in these areas. Both surveys contained returns from the water body surfaces (e.g., Fig. 16), which were 

manually removed in ArcMap by clipping close to the combined coastline.

Figure 16. Coastal lidar survey along the northeastern shore of Nantucket Island. Note returns from the water 

surface; vertical datum NAVD88. Combined coastline in dark red.

9. The 1996-2000 NOAA/USGS/NASA Airborne lidar Assessment of Coastal Erosion (ALACE) Project for the US Coastline data set includes data 

collected from 1996-2000 and covers the states of AL, FL, LA, MS, DE, MD, VA, CT, MA, ME, NH, NJ, NY, RI, NC, SC, GA, CA, OR, WA, 

TX, OH, and PA. Laser beach mapping uses a pulsed laser ranging system mounted onboard an aircraft to measure ground elevation and coastal 

topography. The laser emits laser beams at high frequency and is directed downward at the earth’s surface through a port opening in the bottom 

of the aircraft’s fuselage. The laser system records the time difference between emission of the laser beam and the reception of the reflected laser

signal in the aircraft. The aircraft travels over the beach at approximately 60 meters per second while surveying from the low water line to the 

landward base of the sand dunes. This data set was collected with a LIDAR (LIght Detection and Ranging) instrument designed and developed 

by the Observational Sciences Branch (OSB) of NASA at the Wallops Flight Facility in Virginia. The instrument, originally designed for mapping 

ice sheets in Greenland, is called the Airborne Topographic Mapper or ATM. The ATM II (the latest version), operates with a Spectra Physics laser 

transmitter, which provides a 7 nanoseconds long, 250 micro joules pulse at a frequency-doubled wavelength of 523 nanometers in the blue-green 

spectral region. The laser transmitter can function at pulse rates from 2 to 10 kilohertz (kHz). The laser system with a separate cooling unit weighs 

approximately 45 kilograms (kg) and requires approximately 15 amperes of power at 115 volts. The transmitted laser pulse is reflected to the surface

of the earth with the aid of a small folding mirror mounted on the back of a secondary mirror of a rotating scan mirror assembly mounted directly 

in front of the telescope. The scan mirror, which is rotated at 20 hertz, is comprised of a section of round aluminum stock, machined to a specific

off-nadir angle. A scan mirror with the off-nadir angle of 15 degrees was utilized, producing an elliptical scan pattern with a swath width equal to 

50 percent of the approximately 700-meter aircraft altitude. The reflected laser pulse is transmitted to a photo-multiplier assembly that consists of

a lens, a narrow bandpass filter, and a single photomultiplier tube. Note: The Spatial Reference section of this document may lack fully FGDCcompliant information regarding projection parameters (i.e., Central meridian, false Northing, etc.). The State Plane or UTM zone will be supplied, 

and the corresponding parameters can be found in Appendix C of: Snyder, John, 1987, Map Projections, a Working Manual (U.S. Geological Survey 

Professional Paper 1395): Washington, U.S. Government Printing Office. [Extracted from metadata]
Eakins et al., 2009

20

3) NGDC digitized elevations

Several features were poorly represented, or not represented at all, in available digital elevation datasets: 

the newly exposed parts of the Norton Point Beach that jut into Katama Bay at the breach (Figs. 11 and 17), 

and several coastal jetties and breakwaters. The 2008 Digital Globe satellite image of Katama Bay shows two 

exposed sand bars jutting into Katama Bay alongside the April 2007 breach (Fig. 17). These sand bars were 

not present in the 2000 coastal topographic lidar survey, so NGDC generated numerous scattered points on 

these sand bars, and assigned them an elevation of 1⁄2 meter above MHW.

NGDC digitized coastal jetties at Oaks Bluff, Hyannis, Wychmere and Saquatucket Harbors, and the 

mouths of the Herring and Bass Rivers. Each was assigned an elevation of 1 meter above MHW, with points 

located every 5 meters along each jetty or breakwater. NGDC also digitized the east and west jetties at the 

mouth of Nantucket Harbor. Both jetties are submerged at high tide for most of their lengths (Nantucket 

Harbor Master, pers. comm.), so NGDC assigned an elevation of 1⁄2 meter below MHW to these jetties, with 

points every 5 meters along their lengths. The coastal ends of both jetties were measured in the 2000 coastal 

topographic lidar survey, which was used to ensure their representation in the Nantucket DEM.

Figure 17. Satellite image of breach in Norton Point Beach. The breach has moved sand northwards into Katama Bay, building two 

exposed sand bars that were not present during the 2000 topographic lidar survey.
Digital Elevation Model of Nantucket, Massachusetts

21

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

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

of vertical datums including MLLW, MLW, and NAVD88. NOAA maintains eight tide stations in the Nantucket region 

(Fig. 18). Most of the mainland tide stations have been surveyed to NAVD88, though not the two island stations. All 

data were transformed to MHW to provide the maximum flooding for inundation modeling. Units were converted

from feet to meters as appropriate.

1) Bathymetric data

The NOS hydrographic surveys, the ENC extracted soundings, and the USGS multibeam sonar surveys 

were transformed from MLLW and MLW to MHW, using the differences as measured at the Nantucket Island 

NOAA tide station, #8449130 (Table 9; Fig. 18).

2) Topographic data

The USGS NED 1/3 arc-second DEMs and coastal topographic lidar data were originally referenced 

to NAVD88. Conversion to MHW, using FME software, was accomplished by averaging the difference 

between MHW and NAVD88, as measured at the mainland tide stations (Table 9; Fig. 18).

Table 9. Relationship between MHW and other vertical datums in the Nantucket region.

Vertical datum Nantucket Island Conversion value used

NAVD88 N/A -0.32

MLW -0.925 -0.93

MLLW -0.986 -0.99

Figure 18. NOAA tide stations and vertical datum relationships in the Nantucket region.
Eakins et al., 2009

22

3.2.2 Horizontal datum transformations

Datasets used in compiling the Nantucket DEM were originally referenced to WGS 84, NAD 83, and NAD 

27 geographic, and NAD 27 and NAD 83 Massachusetts State Plane Mainland (feet) and Island (feet) horizontal 

datums. The relationships and transformational equations between these horizontal datums are well established. Data 

in NAD 27 geographic or Massachusetts State Plane were converted to a horizontal datum of NAD 83 geographic 

using FME software.

3.3 Digital Elevation Model Development

3.3.1 Estimating bathymetry from satellite imagery

Ocean depths for the three areas that have undergone recent, significant morphologic change (Katama Bay,

Monomoy Island and Pleasant Bay; Fig. 10) were estimated by calculating shallow water depths from satellite images. 

In each of the areas, Digital Globe satellite images were extracted from Google Earth Pro, converted to TIFF and 

orthorectified in ArcMap using stable, identifiable features, such as road intersections and promontories. The images

were then converted to 16-bit grayscale (single value per pixel) and compared to older NOS surveys in the area. An 

approximate logarithmic relationship between pixel value and sounding was calculated for each area using MatLab

(e.g., Fig. 19), and this equation (Table 10) was applied to the grayscale image for each area to obtain estimated depths. 

The resulting Arc raster was resampled to 1/3 arc-second, clipped to the combined coastline, and converted to an ESRI 

point shapefile, which was edited to remove estimated depths from boats, piers, waves and clouds. Depths deeper

than -5 meters were set to -5 meters in the Katama Bay area, while depths deeper than -6 meters were set to -6 meters 

surrounding Monomoy Island and in Pleasant Bay.

Figure 19. Graph of pixel value vs. depth surrounding Monomoy Island. Soundings were taken from NOS survey H06472. 

Note that below about 5 meters depth, the pixel value does not correlate with depth.

Table 10. Logarithmic equations used to convert satellite images to estimated seafloor depths.

Area Equation

Katama Bay, Martha’s Vineyard y = 1.35 Ln (x-4000) -15

Monomoy Island, Cape Cod y = 1.8 Ln (x) - 20

Pleasant Bay, Cape Cod y = 1.8 Ln (x) - 20
Digital Elevation Model of Nantucket, Massachusetts

23

3.3.2 Verifying consistency between datasets

After horizontal and vertical transformations were applied, the resulting ESRI shapefiles were checked in

ArcMap for consistency between datasets. Problems and errors were identified and resolved before proceeding with

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

for gridding. Problems included:

•	 Several areas had undergone significant morphologic change that were not represented in any digital

dataset. NGDC used 2008 satellite imagery to estimate bathymetry in these areas.

•	 Data values over the ocean, bays and rivers in the NED topographic DEMs. The dataset required automated 

clipping to the combined coastline.

•	 Many coastal areas in the NED DEMs contained zero values, which were shifted to +0.1 m above MHW.

•	 Coastal topographic lidar data contained returns from the ocean surface. These data were clipped to the 

combined coastline.

•	 Digital, measured bathymetric values from NOS surveys date back over 70 years. The older NOS survey 

data were excised where more recent bathymetric data exists.

•	 One NOS survey, H01948, contained an inset detailing the shoal at Bishop and Clerks Light, which did not 

exist as digital soundings. As no other data were available to depict this feature, NGDC manually digitized 

inset soundings from this survey to ensure representation of the shoal in the Nantucket DEM.

3.3.3 Smoothing of bathymetric data

The NOS hydrographic surveys are generally sparse at the resolution of the 1/3 arc-second Nantucket DEM: 

in deep water the NOS survey data have point spacing up to 4 km apart. In order to reduce the effect of artifacts in the 

form of lines of “pimples” in the DEM due to this low resolution dataset, and to provide effective interpolation into 

the coastal zone, a 1 arc-second-spacing ‘pre-surface’ bathymetric grid was generated using GMT, an NSF-funded 

shareware software application designed to manipulate data for mapping purposes.

The NOS hydrographic point data, in xyz format, were clipped to remove overlap with the USGS and USACE 

survey data, estimated bathymetry from satellite imagery, NGDC-digitized soundings, and where NOS soundings 

crossed the modern combined coastline. The NOS data were then combined with these bathymetric data and the ENC 

sounding data into a single file, along with points extracted from the combined coastline (to provide a buffer along the

entire coastline). The coastline elevation value was set at -0.5 m to ensure that the bathymetric surface was below zero 

in areas where coastal bathymetry data are sparse or nonexistent (e.g., bays).

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

degrees (~5%) larger than the Nantucket DEM gridding region. The GMT tool “surface” was then used to apply a tight 

spline tension to interpolate elevations for cells without data values. The GMT grid created by “surface” was converted 

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

areas). The resulting surface was compared with original soundings to ensure grid accuracy (e.g., Fig. 20), converted 

to a shapefile, and then exported as an xyz file for use in the final gridding process (see Table 11).

Figure 20. Histogram of the differences between NOS hydrographic survey H11346 and the 1 arc-second pre-surfaced 

bathymetric grid.
Eakins et al., 2009

24

3.3.4 Gridding the data with MB-System

MB-System was used to create the 1/3 arc-second Nantucket DEM. MB-System is an NSF-funded shareware 

software application specifically designed to manipulate submarine multibeam sonar data, although it can utilize a

wide variety of data types, including generic xyz data. The MB-System tool “mbgrid” was used to apply a tight spline 

tension to the xyz data, and interpolate values for cells without data. The data hierarchy used in the “mbgrid” gridding 

algorithm, as relative gridding weights, is listed in Table 11. Greatest weight was given to the USACE hydrographic 

harbor surveys and coastal topographic lidar data. Least weight was given to the pre-surfaced 1 arc-second bathymetric 

grid and ENC soundings. Gridding was performed in quadrants with the resulting Arc ASCII grids seamlessly merged 

in ArcCatalog to create the final 1/3 arc-second Nantucket DEM.

Table 11. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

Coastal topographic lidar 10 to 100

USGS NED topographic DEM 1

Digitized Norton Point Beach topography 10

USACE hydrographic surveys 100

USGS multibeam swath sonar surveys 10

NOS hydrographic survey soundings 1

Bathymetry derived from satellite imagery 10

ENC soundings 0.1

Nantucket Harbor jetties 10

Pre-surfaced bathymetric grid 0.1

3.4 Quality Assessment of the DEM

3.4.1. Horizontal accuracy

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

the datasets used to determine corresponding DEM cell values. Topographic features have an estimated accuracy 

of about 10 meters: coastal topographic lidar data have an accuracy of approximately 6 meters; NED topography 

is accurate to within about 10 meters. Bathymetric features are resolved only to within a few hundred meters in 

deep-water areas. Shallow, near-coastal regions, rivers, and harbor surveys have an accuracy approaching that of sub 

aerial topographic features. Positional accuracy is limited by: the sparseness of deep-water soundings; potentially 

large positional uncertainty of pre-satellite navigated (e.g., GPS) NOS hydrographic surveys; and by the morphologic 

change that occurs in this dynamic region.

3.4.2 Vertical accuracy

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

datasets contributing to DEM cell values. Topographic areas have an estimated vertical accuracy between 0.1 to 0.3 

meters for coastal topographic lidar data, and up to 7 meters for NED topography. Bathymetric areas have an estimated 

accuracy of between 0.1 meters and 5% of water depth. Those values were derived from the wide range of input data 

sounding measurements from the early 20th century to recent, GPS-navigated sonar surveys. Gridding interpolation to 

determine values between sparse, poorly located NOS soundings degrades the vertical accuracy of elevations in deep 

water.

3.4.3 Slope maps and 3-D perspectives

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

identification of artificial slopes along boundaries between datasets (e.g., Fig. 21). The DEM was transformed to UTM

zone 19 coordinates (horizontal units in meters) in ArcCatalog for derivation of the slope grid; equivalent horizontal 

and vertical units are required for effective slope analysis. Three-dimensional viewing of the UTM-transformed DEM 

was accomplished using ESRI ArcScene (e.g., Fig. 22). Analysis of preliminary grids revealed suspect data points, 

which were corrected before recompiling the DEM. Figure 1 shows a color image of the 1/3 arc-second Nantucket 

DEM in its final version.
Digital Elevation Model of Nantucket, Massachusetts

25

Figure 21. Slope map of the Nantucket DEM. Flat-lying slopes are white; dark shading denotes 

steep slopes; combined coastline in red.

Figure 22. Perspective view from the southwest of the Nantucket DEM. 

Vertical exaggeration–times 7.
Eakins et al., 2009

26

3.4.4	 Comparison with source data files

To ensure grid accuracy, the Nantucket DEM was compared to select source data files. Files were chosen

on the basis of their contribution to the grid-cell values in their coverage areas (i.e., had the greatest weight and did 

not significantly overlap other data files with comparable weight). A histogram of the differences between a coastal

topographic lidar survey file and the Nantucket DEM is shown in Figure 23. Differences cluster around zero, with only

a handful of points, in regions of steep topography, exceeding a 1-meter discrepancy from the DEM. Figures 24 to 26 

show histograms of other datasets compared to the Nantucket DEM.

Figure 23. Histogram of the differences between the 2000 topographic lidar survey along the northern coast of Nantucket 

Island and the Nantucket DEM.

Figure 24. Histogram of the differences between USGS Cape Cod multibeam swath sonar survey and the Nantucket DEM.

Figure 25. Histogram of the differences between NOS hydrographic survey H11078 and the Nantucket DEM.
Digital Elevation Model of Nantucket, Massachusetts

27

Figure 26. Histogram of the differences between the USGS NED on mainland Massachusetts and the northwestern part of 

the Nantucket DEM.

3.4.5 Comparison with National Geodetic Survey geodetic monuments

The elevations of 388 NOAA National Geodetic Survey (NGS) geodetic monuments were extracted from 

online shapefiles of monument datasheets (http://www.ngs.noaa.gov/cgi-bin/datasheet.prl), which give monument 

positions in NAD 83 (typically sub-mm accuracy) and elevations in NAVD88 (in meters). Elevations were shifted to 

MHW vertical datum (see Table 9) for comparison with the Nantucket DEM (see Fig. 28 for monument locations). 

Differences between the Nantucket DEM and the NGS geodetic monument elevations range from -21 to 11 meters, 

with the majority of them being within a few meters; negative values indicate that the DEM is less than the monument 

elevation (Fig. 27). Inspection of datasheets for those monuments with significant discrepancy from the DEM show

that they are caused by poor accuracy in monument location (+/- 6 arc-seconds; ~180 meters), monuments located on 

manmade structures such as bridges, piers or lighthouses (not the ground surface), or by monuments that are lost.

Figure 27. Histogram of the differences between NGS geodetic monument elevations and the Nantucket DEM.
Eakins et al., 2009

28

Figure 28. Location of NGS geodetic monuments and NOAA tide stations in the Nantucket region. 

NGS monument elevations were used to evaluate the DEM.

4. Summary and Conclusions

An integrated bathymetric–topographic DEM of the Nantucket, Massachusetts region, with cell size of 1/3 

arc-second, was developed for the PMEL NOAA Center for Tsunami Research. The best available digital data from 

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

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

FME, GMT, MB-System and Quick Terrain Modeler software. 

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

•	 Conduct hydrographic surveys in near-shore areas, especially in harbors, bays and estuaries, and where 

recent, significant morphologic change has occurred (i.e., Katama Bay, surrounding Monomoy Island,

and Pleasant Bay).

•	 Complete topographic lidar surveying of coastal areas in entire region.

5. Acknowledgments

The creation of the Nantucket DEM was funded by the NOAA Center for Tsunami Research at PMEL. The 

authors thank Nazila Merati and Vasily Titov (PMEL), Edward O’Donnell and Dan Bradley (USACE, New England 

district office), and the Nantucket Harbor Master.

6. References

Nautical Chart #13229 (RNC), 3rd Edition, 2004. South Coast of Cape Cod. Scale 1:40,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #13237 (ENC), 3rd Edition, 2007. Nantucket Sound and Approaches. Scale 1:80,000. U.S. Department 

of Commerce, NOAA, National Ocean Service, Coast Survey.
Digital Elevation Model of Nantucket, Massachusetts

29

Nautical Chart #13238 (RNC), 3rd Edition, 2007. Martha’s Vineyard Eastern Part. Scale 1:20,000. U.S. Department 

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

Nautical Chart #13242 (ENC), 3rd Edition, 2001. Nantucket Harbor. Scale 1: 10,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Poppe, L.J., V.F. Paskevich, B. Butman, S.D. Ackerman, W.W. Danforth, D.S. Foster, and D.S. 

Blackwood, 2006. Geological Interpretation of Bathymetric and Backscatter Imagery

of the Sea Floor Off Eastern Cape Cod, Massachusetts. U.S. Geological Survey Open-File Report 2005-1048. 

http://woodshole.er.usgs.gov/pubs/of2005-1048/

Poppe, L.J., S.D. Ackerman, D.S. Foster, D.S. Blackwood, S.J. Williams, M.S. Moser, H.F. Stewart, and K. A. 

Glomb, 2007. Sea-Floor Character and Sedimentary Processes of Great Round Shoal Channel, Offshore 

Massachusetts. U.S. Geological Survey Open-File Report 2007-1138. http://woodshole.er.usgs.gov/pubs/

of2007-1138/

Sigelman, N., 2007. Storm breaches Norton Point Beach. The Martha’s Vineyard Times, April 19, 2007. http://www.

mvtimes.com/news/2007/04/19/norton_point_beach.php

7. Data Processing Software

ArcGIS v. 9.2, developed and licensed by ESRI, Redlands, Massachusetts, http://www.esri.com/

FME 2008 GB – Feature Manipulation Engine, developed and licensed by Safe Software, Vancouver, BC, Canada, 

http://www.safe.com/

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

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

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

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

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

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

Quick Terrain Modeler v. 6.0.1, lidar processing software developed by John Hopkins University’s Applied Physics 

Laboratory and maintained and licensed by Applied Imagery, http://www.appliedimagery.com/