noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-64

DIGITAL ELEVATION MODELS OF MOREHEAD CITY, NORTH CAROLINA:

PROCEDURES, DATA SOURCES AND ANALYSIS 

P.R. Grothe

L.A. Taylor

B.W. Eakins

K.S. Carignan

M.R. Love 

E. Lim

D.Z. Friday 

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

August 2012

Digital Elevation Models of Morehead City, North Carolina:

Procedures, Data Sources and Analysis

Prepared for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami 

Research by the NOAA National Geophysical Data Center (NGDC)

April 15, 2011

Grothe, P.G.1, L.A. Taylor2, B.W. Eakins1, K.S. Carignan1, D.Z. Friday1,E. Lim1 and M. Love1

1

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

2

NOAA, National Geophysical Data Center, Boulder, Colorado

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.gov/mgg/inundation/ 

UNITED STATES

DEPARTMENT OF COMMERCE

Rebecca Blank

Acting Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Jane Lubchenco

Under Secretary for Oceans

and Atmosphere/Administrator

National Environmental Satellite,

Data, and Information Service

Mary E. Kicza

Assistant Administrator

NOAA Technical Memorandum NESDIS NGDC-64

DIGITAL ELEVATION MODELS OF MOREHEAD CITY, NORTH CAROLINA:

PROCEDURES, DATA SOURCES AND ANALYSIS 

Pamela R. Medley1

Lisa A. Taylor2

Barry W. Eakins1

Kelly S. Carignan1

Matthew R. Love1

Elliot Lim1

Dorothy Z. Friday1

 

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

2

NOAA, National Geophysical Data Center, Boulder, Colorado 

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

August 2012
Grothe et al., 2011

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 Morehead City, North Carolina

iii

Contents

1.Introduction 1

2. Study Area 2

3. Source Elevation Data 3

3.1 Data Sources and Processing 3

3.1.1 Coastline 4

3.1.2 Bathymetry 4

3.1.3 Bathymetry-Topography 5

3.1.4 Topography 6

3.2 Establishing Common Datums 6

3.2.1 Horizontal datum transformations 6

3.2.2 Vertical datum transformations 6

3.2.3 Verifying transformations and consistency between datasets 7

4. Digital Elevation Model Development 7

4.1 Smoothing of bathymetric data 7

4.2 Smoothing of topographic data 7

4.3 Building the NAVD 88 DEM 8

4.4 Building the MHW DEM 9

4.4.1 Developing the conversion grid 9

4.4.2 Creating the MHW DEM 9

4.5 Quality Assessment of the DEMs 9

4.5.1 Horizontal accuracy 9

4.5.2 Vertical accuracy 9

4.5.3 Slope map and 3-D perspectives 9

4.5.4 Comparison of the NAVD 88 DEM with source data files 11

4.5.5 Comparison with National Geodetic Survey geodetic monuments 12

5. Summary and Conclusions 13

6. Acknowledgments 13

7.References 13

8. Data Processing Software 14

Appendix A. NOS Hydrographic Surveys 15

Appendix B. Vertical Datum Conversion Grids 18

List of Figures

Figure 1. Shaded-relief image of the Morehead City NAVD 88 DEM 1

Figure 2. Overview map of the Morehead City, NC DEM region 2

Figure 3. Data sources used in building the 1/3 arc-second Morehead City DEMs 3

Figure 4. Histogram of the differences between the NOS hydrographic soundings and the 

 Morehead City bathymetric surface. 7

Figure 5. Data density of the Morehead City NAVD 88 DEM 8

Figure 6. Slope map of the Morehead City NAVD 88 DEM 10

Figure 7. Perspective view from the southest of the Morehead City NAVD 88 DEM 10

Figure 8. Histogram of the differences between the NOS hydrographic surveys dataset and the 

Morehead City NAVD 88 DEM. 11

Figure 9. Histogram of the differences between the USGS NED 1/3 arc-second topographic 

 DEM and the Morehead City NAVD 88 DEM 11

Figure 10. Location of NGS geodetic monuments in the Morehead City region 12

Figure 11. Histogram of the differences between NGS geodetic monument elevations and the 

 Morehead City NAVD 88 DEM. 12

Figure B1. Elevation conversion values of the ‘MLLW to NAVD 88’ and “MLW to NAVD 88” 

 conversion grids derived from VDatum 18

Figure B2. Elevation conversion values of the ‘NAVD 88 to MHW’ conversion grid derived from 

 VDatum 18
Grothe et al., 2011

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List of Tables

Table 1. Specifications for the Morehead City DEMs 2

Table 2. Bathymetric datasets used in building the Morehead City NAVD 88 DEM. 4

Table 3. Bathymetric-Topographic datasets used in building the Morehead City DEMs. 5

Table 4. Topographic datasets used in building the Morehead DEMs. 6

Table 5. Data hierarchy used to assign gridding weight in MB-System. 8

Table A1. NOS hydrographic surveys available in the Morehead City, NC region. 15
Digital Elevation Models of Morehead City, North Carolina:

Procedures, Data Sources and Analysis

1. Introduction

In April 2011 the National Geophysical Data Center (NGDC), an office of the National Oceanic and Atmospheric Administration (NOAA), developed two bathymetric–topographic digital elevation models (DEMs) of 

Morehead City, North Carolina (Fig. 1). First, a 1/3 arc-second1 DEM referenced to North American Vertical Datum 

of 1988 (NAVD 88) was developed and evaluated using diverse digital datasets available for the region (grid boundary 

and sources shown in Fig. 3). Then, a 1/3 arc-second conversion grid was created to represent the relationship between 

NAVD 88 and Mean High Water (MHW) vertical datums in the Morehead City region. Finally, a 1/3 arc-second MHW 

DEM was developed by combining the NAVD 88 DEM and the vertical datum conversion grid. The MHW DEM will 

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

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

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

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

of the data sources and methodology used in developing the Morehead City DEMs.

Figure 1. Shaded-relief image of the Morehead City NAVD 88 DEM. Contour interval is 5 meters. Image is in Mercator projection.

1. The Morehead City 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 Morehead City, (34°43'23"N, 

76°43'34"W) 1/3 arc-second of latitude is equivalent to 10.27 meters; 1/3 arc-second of longitude equals 8.48 meters.
Grothe et al., 2011

2

2. Study Area

The Morehead City 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 real-time tsunami forecasts 

in an operational environment. The DEMs extend from the southern tip of Ocracoke Island, abutting NGDC’s Cape 

Hatteras DEM, to just north of Jacksonville (Table 1; Fig. 2). 

The DEM is centered on the port, Morehead City, but also encompasses the adjacent populated towns of 

Beaufort and Atlantic Beach. Low lying barrier islands dictate this regions dynamic morphology which is constantly 

changing from year to year. The coastal islands are a popular tourist destination for beach-goers. The back island 

marsh and the Pamlico Sound seperate the mainland from the barrier islands, limiting the access for each barrier 

island. Although the region has a small risk for tsunamis, storm surge from hurricanes is a hazard. These hazards make 

it imperative to have emergency plans in place to evacuate a large population off the islands quickly. 

Table 1. Specifications for the Morehead City DEMs

DEM Type Structured

Grid Area Morehead City, North Carolina

Coverage Area 76.00o to 77.27o W; 34.37o to 35.57o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984 (WGS 84)

Vertical Units Meters

Vertical Datum a) North American Veritical Datum of 1988(NAVD 88)

b) Mean High Water (MHW)

Grid Spacing 1/3 arc-second

Grid Format ESRI Arc ASCII raster grid

Figure 2. Overview map of the Morehead City, NC DEM region. Black box outlines the 1/3 arc-second DEM boundary. 

ArcMap 10.0 topographic basemap in backgroud.
Digital Elevation Models of Morehead City, North Carolina

3

3. Source Elevation Data

The best available bathymetric and topographic digital data were obtained by NGDC and shifted to common 

horizontal and vertical datums: North American Datum 1983 (NAD 83)2 and NAVD 88. 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.

3.1 Data Sources and Processing

Coastline, bathymetric, bathymetric/topographic, and topographic digital datasets (Tables 2, 3, and 4; Fig. 

3) were obtained from the following U.S. federal and state agencies: NOAA’s NGDC and Coastal Services Center 

(CSC), U.S. Geological Survey (USGS), U.S. Army Corps of Engineers (USACE), North Carolina Department of Environment and Natural Resources (NCDENR), and North Carolina Division of Emergency Management (NCDEM). 

Datasets were displayed with Earth Systems Research Institute’s (ESRI) ArcGIS to assess data quality and manually 

edit datasets.

Figure 3. Data sources used in building the 1/3 arc-second Morehead City DEMs. White areas denote no data.

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

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

insignificant for NGDC 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 waves 

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 geogprahic. At the scale of the DEM, WGS 84 and NAD 83 are identical and may be used interchangeably.
Grothe et al., 2011

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3.1.1	 Coastline

Available coastline datasets of the Morehead City region are of low-resolution and are out-of-date compared 

to the rapidly changing morphology and continual building of coastal engineering structures. To make the most effective coastline for the purpose of clipping elevation data to it, NGDC created a NAVD 88 zero contour line of the 

USGS NED 1/3 topographic DEM. The NAVD 88 zero contour line was edited to represent the morphology based on 

Google Earth satellite imagery from 2011.

 

3.1.2	 Bathymetry

Bathymetric datasets available in the Morehead City region included NOS hydrographic surveys, USACE 

hydrographic surveys, and NCDENR high-resolution multibeam swath sonar. (Table 2). 

Table 2. Bathymetric datasets used in building the Morehead City NAVD 88 DEM.

Source Year Data Type Spatial Resolution

Original Horizontal

Datum/Coordinate

System

Original Vertical

Datum URL

NGDC

1869 

to 

2003

NOS 

hydrographic 

survey 

soundings

Ranges from 5 meters 

to 2 kilometers (varies 

with scale of survey, 

depth, traffic and probability of obstructions)

Undetermined, 

United States 

Standard Datum 

1901, NAD 1913 

geographic, NAD 

1927 geographic, 

NAD 83 geographic

Local Low Water 

(LLW), Mean Low 

Water (MLW) , and 

Mean Lower Low 

Water (MLLW) 

(meters)

http://www.ngdc.

noaa.gov/mgg/bathymetry/hydro.html

NCDENR 2005

Multibeam 

swath sonar 

survey

10 feet NAD 83 geographic

National Geodetic 

Vertical Datum of 

1929 (feet)

N/A

USACE

2002 

to 

2011

Hydrographic 

channel surveys

Ranges from 2 and 30 

meters

NAD 83 North 

Carolina State 

Plane, US foot

MLW (feet) http://www.saw.

usace.army.mil/nav/

1) National Ocean Service hydrographic survey data

A total of 94 hydrographic surveys conducted between 1869 and 2003 were available in the Morehead 

City region (see Appendix A). Surveys were extracted as xyz files using GEODAS from NGDC’s online 

NOS Hydrographic database with a buffer 0.05 degrees (~5%) larger than the Morehead City DEM extent 

to support data interpolation along grid edges. The hydrographic survey data were downloaded with vertical 

datums referenced to either mean low water (MLW), mean lower low water (MLLW), or local low water 

(LLW) datum. The surveys were originially refererenced to several horizontal datums (see Table 2) but were 

transformed to NAD 83 geographic during download by GEODAS. 

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

point spacing than more recent surveys. The data were displayed in ESRI’s ArcMap and reviewed for digitizing errors against scanned original survey smooth sheets and edited as necessary. The surveys were also 

compared to various topographic and bathymetric datasets, the coastline, and Office of Coast Survey (OCS) 

Raster Nautical Charts (RNCs). Older sounding data were clipped to remove soundings that have been superseded by more recent surveys.

2) North Carolina Department of Environment and Natural Resources multibeam swath sonar survey

NCDENR provided NGDC with one multibeam swath sonar survey of Beaufort Inlet. The data were collected by Geodynamics Geologic and Oceanographic Services in June of 2005, contracted by the North Carolina Division of Coastal Managament (NCDCM). The purpose of the survey was to perform a comprehensive 

hydrographic survey of Beaufort Inlet as part of the Regional Sediment Management plan being implemented 

by the USACE Wilmington District and NCDCM. The data were provided in an ESRI Arc ASCII grid format 
Digital Elevation Models of Morehead City, North Carolina

5

with a 10 foot cell size, horizontally referenced to NAD 83 and vertically referenced to the National Geodetic 

Vertical Datum of 1929 (NGVD 29). The data is of high-quality and did not require any editing.

3) U.S. Army Corp of Engineers hydrgraphic surveys

USACE hydrographic surveys were available from the Wilmington District. Recent surveys from 2010 

and 2011 were downloaded from their navigation project web site. Several areas had not been surveyed in the 

last year and were not available on the web site. These surveys were provided to NGDC by Adam Faircloth 

from the USACE Wilmington District. Surveys ranged from 2002 to 2011 and covered the Atlantic Intracoastal Waterway, inlets and crossings, rivers, small harbors, and Morehead City and Beaufort Harbors (see 

Figure 3). The surveys had an original horizontal datum of NAD 83 North Carolina State Plane, US foot, and 

an original vertical datum of MLW. Point spacing varied by survey but typically had 5 meter spacing along 

profile and 30 meter spacing between profile lines.

3.1.3	 Bathymetry-Topography

Two bathymetric-topographic datasets were using to build the 1/3 arc-second Morehead City DEMs: 

NCDENR DEM and CSC’s 2004 USACE bathymetric-topographic lidar.

Table 3. Bathymetric-Topographic datasets used in building the Morehead City DEMs.

Source Year Data Type Spatial

Resolution

Original Horizontal

Datum/Coordinate

System

Original Vertical

Datum URL

NCDENR 2005 DEM 20 feet NAD 83 geographic

National Geodetic 

Vertical Datum of 1929 

(feet)

N/A

CSC 2004 XYZ 3 meters NAD 83 geographic NAVD 88 http://www.csc.noaa.

gov/digitalcoast/

1) North Carolina Department of Environment and Natural Resources DEM

NCDENR provided NGDC with a seamless bathymetric-topographic DEM of Bogue Inlet. The data 

were collected by Geodynamics Geologic and Oceanographic Services in June 2005, contracted by NCDCM. 

Swath-based multibeam and spatially derived singlebeam sonar were used to generate a DEM surface of the 

beach topography and seafloor bathymetry. The DEM is also part of the Regional Sediment Management 

plan being implemented by the USACE Wilmington District and NCDCM. The data were provided in an 

ESRI Arc ASCII grid format with a 20 foot cell size, horizontally referenced to NAD 83 and vertically referenced to NGVD 29. The DEM is of high-quality and did not require any editing.

2) Coastal Service Center 2004 US Army Corp of Engineers bathymetric-topographic lidar

NGDC downloaded USACE 2004 bathymetric-topographic lidar data from the CSC Digital Coast web 

site. The data were downloaded as xyz files in a horizontal datum of NAD 83 geographic and vertical datum 

of NAVD 88. NGDC split the negative values from the positive values and created shapefiles for viewing 

in ArcMap. The topographic values were not used to make the 1/3 arc-second DEM because they are superceded by more recent lidar-derived data (see section 3.1.4) and are not processed to bare earth. The negative 

values were edited as needed to remove anomolous points and to remove points that overlapped with other 

bathymetric data sets.
Grothe et al., 2011

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3.1.4	 Topography

Two topographic datasets were used to build the Morehead City DEMs: USGS NED 1/3 arc-second DEM 

and NCDEM Floodplain Mapping Program (FPMP) lidar (Tabel 4). 

Table 4. Topographic datasets used in building the Morehead DEMs.

Source Year Data Type Spatial

Resolution

Original Horizontal

Datum/Coordinate

System

Original Vertical

Datum URL

USGS 2009 Bare earth 

topographic DEM 10 meters NAD 83 geographic NAVD 88 http://ned.usgs.gov/

NCDEM 2001 Topographic lidar 5 meters NAD 83, North Carolina State Plane (feet) NAVD 88 (feet) http://www.ncfloodmaps.com/

1) US Geological Survey 1/3 arc-second National Elevation Dataset DEM

USGS NED topographic DEM provides complete 1/3 arc-second coverage of the Morehead City DEM 

coverage area. The data were downloaded from the USGS web site as raster DEMs in NAD 83 geographic 

and NAVD 88 horizontal and vertical datums. The bare-earth elevations have a vertical accuracy of +/- 7 to 

15 meters, respectively, but are most likely more accurate than that in low elevation areas. The dataset was 

derived from lidar data collected for the North Carolina Department of Transportation (NCDOT) in 2007.

2) North Carolina Department of Emergency Management, Floodplain Mapping Program Lidar

The NCDEM-FPMP was established in response to the extensive damage caused by the Hurricane Floyd 

in 1999, and these data were collected as part of its effort to modernize FEMA Flood Insurace Rate Maps 

statewide. The data, downloaded in ESRI Arc ASCII grid format, were collected at 5-meter nominal post 

spacing and referenced to NAD 83 North Carolina State Plane and NAVD 88 horizontal and vertical datums. 

Only a small section of the data along the northern barrier islands was used to fill gaps in the NED 1/3 arcsecond DEM for building the final DEM.

3.2 Establishing Common Datums

3.2.1	 Horizontal datum transformations

Datasets used to build the Morehead City DEM were downloaded referenced to NAD 83 geographic and 

NAD 83 North Carolina State Plane (feet). The relationships and transformational equations between the geographic 

horizontal datums are well established. Transformations to NAD 83 geographic were accomplished using FME software or ArcGIS’s reproject tool.

3.2.2	 Vertical datum transformations

1) Bathymetric data

Bathymetric datasets used to build the Morehead City DEMs were referenced to MLLW, MLW, LLW 

and NGVD 29 vertical datums. All hydrographic surveys were transformed to NAVD 88 by adding the xyz 

data to conversion grids created with VDatum (see section 4.3.1 for conversion grid methodology and Appendix B). VDatum does not recognize LLW as a vertical datum in North Carolina, therefore data referenced 

to the LLW vertical datum required an initial transformation to mean sea level (MSL) by adding a constant 

offset of 0.15 meters from LLW to NAVD 88 (Dr. Kurt Hess, NOAA Office of Coast Survey, Kurt.Hess@

noaa.gov, personal communication). Data in NGVD 29 vertical datum were transformed to NAVD 88 directly using VDatum software.

2) Topographic data

Topographic datasets used to build the Morehead City DEMs were referenced to NAVD 88 and NGVD 

29. Data in NGVD 29 vertical datum were transformed to NAVD 88 directly using VDatum software. No 

further transformations were required for datasets already referenced NAVD 88.
Digital Elevation Models of Morehead City, North Carolina

7

3.2.3	 Verifying transformations and consistency between datasets

After horizontal and vertical transformations were applied, dataset were checked for consistency and problems and errors were resolved before proceeding with subsequent gridding steps. All datasets were converted to xyz 

files using GDAL in preparation for gridding. 

4. Digital Elevation Model Development

4.1 Smoothing of bathymetric data

The NOS hydrographic survey data are generally sparse relative to the resolution of the 1/3 arc-second Morehead City DEMs. In order to reduce the effect of artifacts in the DEM by low-resolution NOS datasets, and to provide 

effective interpolation from the deep water into the coastal zone, a 1/3 arc-second-spacing ‘pre-surface’ bathymetric 

grid was generated using Generic Mapping Tools (GMT)3. The coastline elevation value was set to -1 meters to ensure 

a bathymetric surface below zero in areas where data are sparse or non-existent along the coast.

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

0.05 degrees (~5%) larger than the Morehead City DEM gridding region. The GMT command ‘surface’ was then used 

to apply a tight spline tension to interpolate elevations for cells without data values. The GMT grid created was converted into an ESRI Arc ASCII grid file, and clipped to the NAVD 88 zero line to eliminate data interpolation into land 

areas. The resulting surface was compared with original soundings to ensure grid accuracy (Fig. 4) and then exported 

as an xyz file for use in the final gridding process. 

Figure 4. Histogram of the differences between the NOS hydrographic soundings and the Morehead City bathymetric surface.

4.2 Smoothing of topographic data

The topographic data were generally inconsistent along the coast because of morphologic change. Also, some 

low lying islands in the sound where not included in the flight path of acquired lidar data used to build the Morehead 

City DEM. In order to created a smooth topographic surface to the coastline and create positive elevations to the 

islands with missing data, a 1/3 arc-second-spacing ‘pre-surface’ topographic grid was generated using GMT. The 

coastline was set to 0.5 meters to ensure the topographic surface had values above zero in areas where no data existed. 

See section 4.1 for more details on the methodology.

3. 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.]
Grothe et al., 2011

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4.3 Building the NAVD 88 DEM

MB-System was used to create the 1/3 arc-second Morehead City square-cell NAVD 88 DEM. The MBSystem 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 5. The 

resulting binary grid was converted to an Arc ASCII grid using the MB-System tool ‘mbm_grd2arc’ to create the final 

1/3 arc-second Morehead City NAVD 88 DEM. Figure 5 illustrates cells in the DEM that have interpolated values 

(shown in white) versus data contributing to the cell value (shown in black).

Table 5. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

NCDENR multibeam 10

NCDENR bathy/topo DEM 10

NCDEM topographic lidar 10

USACE hydrographic surveys 10

CSC USACE bathy/topo lidar 10

USGS NED DEM 10

NOS hydrographic surveys 1

Pre-surface bathymetric grid 0.1

Pre-surface topographic grid 0.1

Figure 5. Data density of the Morehead City NAVD 88 DEM; areas where source data were available are depicted in black; areas where grid 

interpolation was necessary are depicted in white.
Digital Elevation Models of Morehead City, North Carolina

9

4.4 Building the MHW DEM

The MHW DEM was created by adding a ‘NAVD 88 to MHW’ conversion grid to the NAVD 88 DEM.

4.4.1	 Developing the conversion grid

Using extents slightly larger (~5%) than the Morehead City NAVD 88 DEM, an intial 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 command ‘surface’ applied a tension spline to interpolate cell values 

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

command ‘grd2xyz’. Conversion values from NAVD 88 to MHW at each xyz point were generated using VDatum and 

the null values were removed. The median-averaged xyz file was then interpolated with the GMT command ‘surface’ 

to create the 1/3 arc-second ‘NAVD 88 to MHW’ conversion grid with the extents of the Morehead City project area 

(note that conversion grids from MLLW and MLW to NAVD 88 were created using this same methodology to convert 

vertical datums of the data to NAVD 88). 

4.4.2	 Creating the MHW DEM

Once the NAVD 88 DEM was completed and assessed for errors, the conversion grid was added to the NAVD 

88 DEM. The resulting MHW DEM was reviewed and assessed using RNCs, USGS topographic maps, and ESRI 

ArcMap 10.0 imagery.

4.5 Quality Assessment of the DEMs

4.5.1	 Horizontal accuracy

The horizontal accuracy of topographic and bathymetric features in the Morehead City DEMs is dependent 

upon the datasets used to determine corresponding DEM cell values and the cell size of the DEM. The horizontal accuracy is 10 meters for the topographic data. Bathymetric features are resolved only to within a few tens of meters in 

deep-water areas. Shallow, near-coastal regions 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.

4.5.2	 Vertical accuracy

Vertical accuracy of the Morehead City DEMs are also highly dependent upon the source datasets contributing to DEM cell values. The USGS NED topographic DEM has an estimated vertical accuracy of 7 to 15 meters but is 

most likely less for low lying areas like Morehead City. 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 bathymetric 

values between sparse, poorly located NOS soundings degrades the vertical accuracy of elevations.

4.5.3	 Slope map and 3-D perspectives

ESRI ArcCatalog was used to generate a slope grid from the Morehead City NAVD 88 DEM to allow for 

visual inspection and identification of artificial slopes along boundaries between datasets (Fig. 6). The DEM was 

transformed to UTM Zone 18 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. Three-dimensional viewing of 

the UTM-transformed DEM was accomplished using ESRI ArcScene. Analysis of preliminary grids revealed suspect 

data points, which were corrected before recompiling the DEM. Figure 7 shows a perspective view image of the 1/3 

arc-second Morehead City NAVD 88 DEM in its final version.
Grothe et al., 2011

10

Figure 6. Slope map of the Morehead City NAVD 88 DEM. Flat-lying slopes are white; dark shading denotes steep slopes. 

NAVD 88 zero line in red.

Figure 7. Perspective view from the southeast of the Morehead City NAVD 88 DEM. Ten times vertical exaggeration.
Digital Elevation Models of Morehead City, North Carolina

11

4.5.4	 Comparison of the NAVD 88 DEM with source data files

To ensure grid accuracy, the Morehead City NAVD 88 DEM was compared to source data files. All bathymetric and topographic source data were compared to the Morehead City NAVD 88 DEM using Python, GDAL, and 

Gnuplot. Histograms of the differences between individual datasets and the Morehead City NAVD 88 DEM mostly 

cluster around zero. Largest differences between source datasets and the DEM resulted from the averaging of multiple 

topographic source datasets where data coverage overlapped (i.e. Fig. 8 and Fig. 9).

Figure 8. Histogram of the differences between the NOS hydrographic surveys dataset and the Morehead City NAVD 88 DEM.

Figure 9. Histogram of the differences between the USGS NED 1/3 arc-second topographic DEM and the Morehead City NAVD 88 DEM.
Grothe et al., 2011

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4.5.5	 Comparison with National Geodetic Survey geodetic monuments

The elevations of NOAA National Geodetic Survey (NGS) geodetic monuments (Fig. 10) were extracted 

from online shapefiles of NGS Geodetic monument datasheets (http://www.ngs.noaa.gov/) which give monument positions in NAD 83 (typically sub-mm accuracy) and elevations in NAVD 88 (in meters). Monuments with conditions 

noted as ‘GOOD’ or ‘MONUMENTED’ were only included in the analysis. Monument elevations were compared 

with elevations of the Morehead City NAVD 88 DEM. Differences between the Morehead City NAVD 88 DEM and 

the NGS geodetic monument elevations range from -4 to 43 meters, with the majority of them being with +/-1 meter 

(Fig. 11). Negative values indicate that the monument elevation is less than the DEM elevation. After examination, it 

was determined that those monuments with the largest deviations do not represent ground surface as they are located 

on top of an observation tower, light house, or at the apex of other structures. 

Figure 10. Location of NGS geodetic monuments in the Morehead City region.

Figure 11. Histogram of the differences between NGS geodetic monument elevations and the Morehead City NAVD 88 DEM.
Digital Elevation Models of Morehead City, North Carolina

13

5. Summary and Conclusions

Two 1/3 arc-second bathymetric–topographic square-cell DEMs of the Morehead City region, one vertically 

referenced to NAVD 88 and the other to MHW, were developed for the purpose of modeling tsunami inundation and to 

support other coastal management activities. The best available digital data from U.S. federal, state, and local 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, FME, GMT, MB-System, QT 

Modeler, GDAL, Proj4, and Gnunplot software. VDatum was utilized throughout the development to transform data 

to common vertical datums. Futhermore, NGDC developed a conversion grid derived from VDatum that transformed 

source data to NAVD 88 and the Morehead City NAVD 88 DEM to MHW.

Recommendations to improve the Morehead City DEMs, based on NGDC’s research and analysis, include:

•	 Conduct high-resolution shallow-water bathymetric multibeam surveys to replace late 18th and early 19th 

century survey.

•	 Conduct bathymetric-topographic lidar surveys of the near-shore regions to allow for a smooth transition 

from the bathymetric data to the topographic data.

6. Acknowledgments

The developement of the Morehead City DEMs was funded by the NOAA Pacific Marine Environmental 

Laboratory. The authors thank Nazila Merati, Marie Eble, and Vasily Titov (PMEL). The authors would also like to 

thank Ken Richardson from NCDENR and Adam Faircloth from USACE, for providing NGDC with data not available online.

7. References

Love, M. R., R.J. Caldwell, K.S. Carignan, B.W. Eakins, L.A. Taylor, 2010: Digital Elevation Models of Southern 

Louisiana: Procedures, Data Sources, and Analysis.

Nautical Chart #13003 (RNC), 50th Edition, 2010. Cape Sable to Cape Hatteras. Scale 1:1,200,000. U.S. Department 

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

Nautical Chart #12200 (RNC), 49th Edition, 2007. Cape May to Cape Hatteras. Scale 1:419,706. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11555 (RNC), 40th Edition, 2006. Cape Hatteras Wimble Shoals to Ocracoke Inlet. Scale 1:80,000. 

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

Nautical Chart #11554 (RNC), 16th Edition, 2001. Pamlico River. Scale 1:40,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11552 (RNC), 50th Edition, 2010. Neuse River and Upper Part of Bay River. Scale 1:40,000. U.S. 

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

Nautical Chart #11550 (RNC), 29th Edition, 2004. Ocracoke Inlet and Part of Core Sound. Scale 1:40,000. U.S. 

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

Nautical Chart #11548 (RNC), 40th Edition, 2005. Pamlico Sound - Western Part. Scale 1:80,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11547 (RNC), 37th Edition, 2007. Morehead City Harbor. Scale 1:15,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11545 (RNC), 63rd Edition, 2008. Beaufort Inlet and Part of Core Sound. Scale 1:40,000. U.S. 
Grothe et al., 2011

14

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

Nautical Chart #11544 (RNC), 39th Edition, 2005. Portsmouth Island to Beaufort. Scale 1:80,000. U.S. Department 

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

Nautical Chart #11543 (RNC), 23rd Edition, 2005. Cape Lookout to New River. Scale 1:80,000. U.S. Department of 

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11542 (RNC), 17th Edition, 2008. New River. Scale 1:40,000. U.S. Department of Commerce, 

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #11542 (RNC), 39th Edition, 2001. Neuse River to Myrtle Grove Sound. Scale 1:40,000. U.S. 

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

8. Data Processing Software

ArcGIS v. 10.0 – developed and licensed by ESRI, Redlands, California, Hhttp://www.esri.com/H

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

Hhttp://www.safe.com/.

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

gdal.org/.

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

Geophysical Data Center, 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

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

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

MB-System v. 5.1.0 – 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/.

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

org/proj/.

Python v. 2.4.3, Python is a remarkable pwerful dynamic programming lanuage that is used in a wide variety of 

application domains. Python is free software, http://python.org/.

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

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

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

Coast Survey (OCS), and Center for Operational Oceanographic Poducts and Services (CO-OPS), http://

vdatum.noaa.gov/.
Digital Elevation Models of Morehead City, North Carolina

15

Appendix A. NOS Hydrographic Surveys

Table A1. NOS hydrographic surveys available in the Morehead City, NC region

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

F00247 1983 20,000 MLLW NAD 1927 geographic

F00447 1998 10,000 MLLW NAD 83 geographic

H01010 1869 20,000 LLW Undetermined

H01083 1870 40,000 MLW Undetermined

H01088 1870 20,000 LLW Undetermined

H01226A 1874 20,000 LLW United States Standard Datum 1901

H10222B 1874 20,000 LLW NAD 1913

H01227 1874 40,000 LLW Undetermined

H01254 1875 20,000 LLW Undetermined

H01316B 1876 20,000 LLW Undetermined

H01348 1877 20,000 LLW Undetermined

H01846 1886 10,000 MLW Undetermined

H01848 1886 10,000 MLW Undetermined

H01850 1886 10,000 MLW Undetermined

H01853 1886 10,000 MLW Undetermined

H01854 1886 10,000 MLW Undetermined

H01855 1886 10,000 MLW Undetermined

H01858 1886 10,000 MLW Undetermined

H01859 1886 10,000 MLW Undetermined

H01867 1887 20,000 MLW Undetermined

H03529 1913 20,000 LLW NAD 1913 geographic

H03530 1913 10,000 LLW NAD 1913 geographic

H03531 1913 10,000 LLW NAD 1913 geographic

H03902 1916 20,000 LLW NAD 1913 geographic

H03922 1916 20,000 LLW NAD 1913 geographic

H04149 1920 20,000 LLW NAD 1913 geographic

H04150 1920 20,000 LLW North American Datum 1913

H04151 1920 10,000 LLW NAD 1913 geographic

H04696 1926 40,000 MLW Undetermined

H04734 1927 10,000 MLW NAD 1913 geographic

H04767 1927 40,000 MLW Undetermined

H04778 1927 20,000 LLW NAD 1913 geographic

H05277 1933 10,000 MLW NAD 1927 geographic
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NOS Survey ID Year of Survey Survey Scale Original Horizontal Datum Original Vertical Datum

H05697 1935 10,000 LLW NAD 1927 geographic

H05829 1935 10,000 LLW NAD 1927 geographic

H05847 1935 10,000 LLW NAD 1927 geographic

H05856 1935 10,000 LLW NAD 1927 geographic

H05873 1935 10,000 LLW NAD 1927 geographic

H05874 1935 10,000 LLW NAD 1927 geographic

H05876 1935 10,000 LLW NAD 1927 geographic

H05903 1935 10,000 LLW NAD 1927 geographic

H05904 1935 10,000 LLW NAD 1927 geographic

H05905 1935 10,000 LLW NAD 1927 geographic

H05911 1935 10,000 LLW NAD 1927 geographic

H05918 1935 10,000 LLW NAD 1927 geographic

H05919 1935 10,000 LLW NAD 1927 geographic

H05926 1935 10,000 LLW NAD 1927 geographic

H05941 1935 10,000 LLW NAD 1927 geographic

H05946 1935 10,000 LLW NAD 1927 geographic

H05950 1935 10,000 LLW NAD 1927 geographic

H05961 1935 10,000 LLW NAD 1927 geographic

H05962 1935 20,000 LLW NAD 1927 geographic

H05963 1935 10,000 LLW NAD 1927 geographic

H05996 1935 10,000 LLW NAD 1927 geographic

H05997 1936 10,000 LLW NAD 1927 geographic

H05998 1936 10,000 LLW NAD 1927 geographic

H05999 1936 10,000 LLW NAD 1927 geographic

H06798 1943 10,000 MLW NAD 1927 geographic

H06834 1943 10,000 MLW NAD 1927 geographic

H06835 1943 20,000 LLW NAD 1927 geographic

H06836 1943 10,000 LLW NAD 1927 geographic

H07963 1952 12,500 MLW NAD 1927 geographic

H07964 1953 10,000 LLW NAD 1927 geographic

H08246* 1955 40,000 MLW NAD 1927 geographic

H08247* 1955 20,000 MLW NAD 1927 geographic
Digital Elevation Models of Morehead City, North Carolina

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

H08253* 1955 10,000 MLW NAD 1927 geographic

H08291 1956 10,000 MLW NAD 1927 geographic

H08564* 1960 5,000 MLW NAD 1927 geographic

H08766 1962 10,000 MLW NAD 1927 geographic

H09042 1969 20,000 MLW NAD 1927 geographic

H09043 1969 20,000 MLW NAD 1927 geographic

H09044 1969 80,000 MLW NAD 1927 geographic

H09060 1969 80,000 MLW NAD 1927 geographic

H09421 1974 10,000 MLW NAD 1927 geographic

H09426 1974 40,000 MLW NAD 1927 geographic

H09427 1974 40,000 MLW NAD 1927 geographic

H09431* 1974 5,000 MLW NAD 1927 geographic

H09432 1974 5,000 MLW NAD 1927 geographic

H09433* 1974 5,000 LLW NAD 1927 geographic

H09434 1974 5,000 MLW NAD 1927 geographic

H09450 1974 40,000 MLW NAD 1927 geographic

H09451 1974 40,000 MLW NAD 1927 geographic

H09464 1974 20,000 MLW NAD 1927 geographic

H09465 1974 20,000 MLW NAD 1927 geographic

H09821* 1979 20,000 LLW NAD 1927 geographic

H09863 1980 20,000 LLW NAD 1927 geographic

H10824 1998 10,000 MLLW NAD 83 geographic

H10825 1998 10,000 MLLW NAD 83 geographic

H10826 1998 10,000 MLLW NAD 83 geographic

H10827 1998 10,000 MLLW NAD 83 geographic

H10832 1998 10,000 MLLW NAD 83 geographic

H10844 1998 10,000 MLLW NAD 83 geographic

H10845 1998 10,000 MLLW NAD 83 geographic

H11285 2003 20,000 MLLW NAD 83 geographic

*indicates data were not used due to poor quality or overlapped by more recent survey
Grothe et al., 2011

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Appendix B. Vertical Datum Conversion Grids

Figure B1. Elevation conversion values of the a) ‘MLLW to NAVD 88’ conversion grid and b) “MLW to NAVD 88” derived from 

VDatum. Values equal difference between MLLW and NAVD 88. These grids were used to convert source bathymetric data to NAVD 88. 

Figure B2. Elevation conversion values of the ‘NAVD 88 to MHW’ conversion grid derived from VDatum. 

Values equal difference between NAVD and 88 and MHW. This grid was used to convert the NAVD 88 grid to MHW.
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