DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-1

DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA:

PROCEDURES, DATA SOURCES AND ANALYSIS 

L.A.Taylor

B.W. Eakins

R.R. Warnken 

K.S. Carignan

G.F. Sharman

D.C. Schoolcraft

P.W. Sloss

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

January 2008


DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

UNITED STATES

DEPARTMENT OF COMMERCE

Carlos M. Gutierrez

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

VADM Conrad C. Lautenbacher, Jr.

Under Secretary for Oceans

and Atmosphere/Administrator

National Environmental Satellite,

Data, and Information Service

Mary E. Kicza

Assistant Administrator

NOAA Technical Memorandum NESDIS NGDC-1

DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA:

PROCEDURES, DATA SOURCES AND ANALYSIS

Lisa A. Taylor1

Barry W. Eakins2

Robin R. Warnken1

Kelly S. Carignan2

George F. Sharman1

David C. Schoolcraft1

Peter W. Sloss1

1

NOAA, National Geophysical Data Center, Boulder, Colorado

2

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

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

January 2008
ii

Taylor et al., 2008

NOTICE

Menton of a commercal company or product does not consttute an endorsement by the 

NOAA Natonal Envronmental Satellte, Data, and Informaton Servce. Use of nformaton 

from ths publcaton concernng propretary products or the test of such products for publcty

or advertsng purposes s not authorzed. 

Corresponding author contact:

Lsa A. Taylor

NOAA, Natonal Geophyscal Data Center

Marne Geology and Geophyscs Dvson

35 Broadway, E/GC 3

Boulder, Colorado 80305

Phone: 303-497-6767

Fax: 303-497-653

E-mal: Lsa.A.Taylor@noaa.gov

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

Also avalable from the Natonal Techncal Informaton Servce (NTIS)

(http://www.nts.gov)
iii

DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

Contents

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

2. Study Area..........................................................................................................................................1

3. Methodology ......................................................................................................................................3

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

3.1.1. Shoreline................................................................................................................5

3.1.2. Bathymetry ............................................................................................................6

3.1.3. Topography..........................................................................................................13

3.1.4. Topography/Bathymetry......................................................................................14

3.2. Establishing Common Datums .................................................................................................15

3.2.1. Vertical transformations ......................................................................................15

3.2.2. Horizontal transformations..................................................................................15

3.3. Digital Elevation Model Development .....................................................................................15

3.3.1. Verifying consistency between datasets..............................................................15

3.3.2. Smoothing of sparse datasets ..............................................................................16

3.3.3. Gridding the data with MB-System.....................................................................16

3.4. Quality Assessment of the Grids...............................................................................................17

3.4.1. Horizontal accuracy.............................................................................................17

3.4.2. Vertical accuracy .................................................................................................17

3.4.3. Comparison with South Carolina tidal bench marks...........................................17

3.4.4. Slope map and 3-D perspectives.........................................................................18

4. Summary and Conclusions...............................................................................................................19

5. Acknowledgments............................................................................................................................19

6. References........................................................................................................................................19

List of Figures

Figure 1. Shaded-relief image of the Myrtle Beach, South Carolina area .................................................2

Figure 2. Coverage of data sources used to compile the Myrtle Beach, South Carolina grids..................4

Figure 3. NGA and NHD coastlines in the area of North Inlet ..................................................................5

Figure 4. NOS hydrographic survey data coverage in gridding area .........................................................7

Figure 5. Murrells Inlet jetty ......................................................................................................................9

Figure 6. Horse Ford Channel ..................................................................................................................10

Figure 7. Spatial coverage of USGS interferometric sonar bathymetry data...........................................11

Figure 8. Atlantic Intracoastal Waterway data coverage..........................................................................12

Figure 9. Slope map of the 1/3 arc-second grid .......................................................................................18

List of Tables

Table 1. PMEL specifications for the Myrtle Beach, South Carolina grids..............................................3

Table 2. Bathymetric data sources used in gridding .................................................................................6

Table 3. NOS hydrographic survey data included in compilation of the Myrtle Beach grids..................8

Table 4. Topographic data sources used in gridding...............................................................................13

Table 5. Beach profile data sources included in grid compilation..........................................................14

Table 6. Data hierarchy used to assign gridding weight in MB-System.................................................16

Table 7. Comparison of tidal bench mark elevations with the 1 arc-second Myrtle Beach grid............17
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Taylor et al., 2008

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DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

Digital Elevation Models of Myrtle Beach, South Carolina:

Procedures, Data Sources and Analysis

1. Introduction

In May 2006, the National Geophysical Data Center (NGDC), an office of the National Oceanic and 

Atmospheric Administration (NOAA), developed two spatially coincident bathymetric/topographic digital elevation 

models (DEMs) of Myrtle Beach, South Carolina for the Pacific Marine Environmental Laboratory (PMEL) NOAA 

Center for Tsunami Research (http://nctr.pmel.noaa.gov/). A 1/3 arc-second (~10 meter) elevation grid was generated 

from numerous, diverse digital datasets in the region (grid boundary shown in Figs. 1 and 2). This grid was then 

resampled to generate a 1 arc-second (~30 meter) grid. The grids will be used as input for the Method of Splitting 

Tsunami (MOST) Model (http://www.pmel.noaa.gov/pubs/PDF/tito1927/tito1927.pdf) developed by PMEL to 

simulate tsunami generation, propagation and inundation. An intermediate 9 arc-second bathymetric grid of the East 

Coast previously developed by NGDC, will also be used as input to the MOST Model. This report provides a summary 

of the data sources and methodology used in developing the grids for Myrtle Beach, South Carolina. 

2. Study Area

 The study area covers the coastal community of Myrtle Beach, South Carolina in an area known as the 

Grand Strand. It is situated within Long Bay between the Atlantic Intracoastal Waterway to the west and the Atlantic 

Ocean to the East (Fig. 1). According to the 2000 Census, the city of Myrtle Beach is at the heart of the 13th fastest 

growing metropolitan area in the U.S. with a population of 22,759. Known for its wide sandy beaches and numerous 

golf courses, the Myrtle Beach area is one of the major coastal resorts and tourist destinations along the South Atlantic 

seaboard, attracting over 14 million visitors a year (http://www.cityofmyrtlebeach.com/).

About half of South Carolina is part of the South Atlantic Coastal Plain which is fringed by the Sea Islands 

and separated from the mainland by salt marshes, lagoons, and sounds. The Coastal Plain surface is almost level, rising 

gradually inland. The gently, curving, elliptical shoreline follows a northeast to southwest direction. Swamps and 

marshes are widespread and are made up of characteristic dark peat and muck, while other soils are generally sandy 

and light. Below the surface are unconsolidated sedimentary beds of sand and clay.


Taylor et al., 2008

Figure 1. Shaded-relief image, derived from the 1 arc-second grid, of the Myrtle Beach, South Carolina area. Red triangles 

locate tidal bench marks listed in Table 7. Contour interval: 5 meters, referenced to MHW.
DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

3. Methodology

The digital elevation grids were developed to meet PMEL required specifications (Table 1), based on input 

requirements for the MOST inundation model. The best available data were obtained by NGDC and used to produce 

the grids. Data processing, grid assembly, and quality assessment are described in the following subsections.

Table 1: PMEL specifications for the Myrtle Beach, South Carolina grids.

Grid Area Myrtle Beach, South Carolina

Coverage Area 78.4o W to 79.2o; 33.25o N to 33.95o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System (WGS84)

Vertical Datum Mean High Water

Vertical Units Meters

Grid Spacings 1 arc-second and 1/3 arc-seconds

Grid Format ASCII raster grid

3.1 Data Sources and Processing

Shoreline, bathymetric, topographic, and shoreline-crossing data (Fig. 2) were obtained from numerous 

federal and state government agencies, universities and private companies, including the U.S. Geological Survey 

(USGS), the NOAA National Ocean Service (NOS), the U.S. Army Corps of Engineers (USACE), Coastal Carolina 

University (CCU), Coastal Science and Engineering, Inc. (CSE), and Horry County, South Carolina. Datasets were 

converted into ESRI shape files and transformed to Mean High Water (MHW) and WGS84 geographic coordinates. 

As no V-Datum (http://chartmaker.ncd.noaa.gov/csdl/vdatum.htm) model was available for the gridding area, vertical 

datum transformations were applied based on tidal model information supplied by PMEL.
Taylor et al., 2008

Figure 2. Coverage of data sources used to compile the 1 arc-second and 1/3 arc-second Myrtle 

Beach, South Carolina grids.
DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

3.1.1 Shoreline

Digital shorelines were obtained from the USGS and the National Geospatial-Intelligence Agency (NGA).

The NGA Global Imagery-Derived Shoreline corresponds closely with Horry County topographic LiDAR along the 

open ocean-land boundary and with satellite imagery viewable with Google Earth (e.g., Fig. 3). The USGS National 

Hydrography Dataset (NHD) shoreline provides a more accurate representation of inland water bodies (rivers, inlets 

and the Intracoastal Waterway). As the open ocean beach face is clearly resolvable in the LiDAR data, and shorelinecrossing beach profiles exist for much of the coastline, neither digital shoreline was used in the gridding process.

1) National Hydrography Dataset

The USGS collaborated with the U.S. Environmental Protection Agency (EPA) to produce the National 

Hydrography Dataset (http://nhd.usgs.gov/). The NHD is a comprehensive set of seamless digital spatial data 

based upon the content of USGS Digital Line Graph (DLG) 1:100,000 scale hydrography data integrated 

with reach-related information from the EPA Reach File Version 3 (RF3). It contains information about 

surface water features such as lakes, ponds, streams, rivers, springs and wells. 

2) NGA Global Imagery-Derived Shoreline 

The NGA Global Imagery-Derived Shoreline is an unclassified vector dataset generated by Earth Satellite 

Corporation (EarthSat) of Rockville, Maryland for NGA, under contract to Boeing in 2004. The shoreline is 

referenced to MHW and constructed from consistently orthorectified Landsat TM satellite imagery (GeoCover 

Ortho), acquired between 1998-2002 for NASA under the Global Land Mapping Program (GLMP). NDVI 

and SWIR models were used to define the landward extent of inundation (i.e., MHW). Independently verified 

positional accuracy for the source product (GeoCover Ortho) is consistently better than 50m root mean 

square (RMS) error. 

Figure 3. NGA and NHD coastlines in the area of North Inlet (left) and a Google Earth view of 

same area (right). The NHD coastline defines inland waterways, while the NGA coastline defines the 

MHW mark along the open ocean-land boundary.
Taylor et al., 2008

3.1.2 Bathymetry

Bathymetric datasets used in the compilation of the grids included 20 NOS hydrographic surveys, USGS 

gridded interferometric sonar data, USACE Atlantic Intracoastal Waterway surveys, and digitized depths taken from 

NOS Coast Pilot, Volume 4 and NOAA Nautical Chart #11534.

Table 2. Bathymetric data sources used in gridding.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

NOS 1925 to 

1972

Hydrographic 

survey soundings

Ranges from 10 meters to 1

kilometer (varies with scale 

of survey, depth, traffic and 

probability of obstructions)

NAD27 

(undocumented for 

H04521)

MLW

http://www.ngdc.

noaa.gov/mgg/

bathymetry/hydro.

html

USGS 1999 to 

2002

Interferometric 

sonar grid 100 meters grid spacing

WGS84, UTM Zone 

17 MLLW

http://pubs.usgs.gov/

of/2004/1013/index.

html

USACE 2005 to 

2006

Hydrographic 

surveys within 

the Intracoastal 

Waterway

Two parallel survey lines ~ 

20 meters apart with ~ 0.4 

meter point spacing

NAD83, South 

Carolina State Planes, 

US foot 

MLW http://www.sac.usace.

army.mil/

1) NOS hydrographic survey data

A total of 20 NOS hydrographic surveys conducted between 1925 and 1972 were included in the grid 

compilation (Fig. 4). The survey data were originally vertically referenced to Mean Low Water (MLW) and 

horizontally referenced to NAD27, with one survey referenced to an undocumented original horizontal datum 

(Table 3). Three surveys conducted in 1924 (H04450, H04450I2, and H04450I3) were not included in the 

compilation, as more recent survey data covered the same areas. Data point spacing for the surveys ranged 

from about 10 meters in shallow water to 1 kilometer in deep water. All surveys were extracted from NGDC’s 

online database (http://www.ngdc.noaa.gov/mgg/bathymetry/hydro.html) in NAD83 and MLW datums.
DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

Figure 4. NOS hydrographic survey data coverage in gridding area, which provides 

complete coverage of marine areas, though sparse in deep water.
Taylor et al., 2008

Table 3. NOS hydrographic surveys included in the compilation of the Myrtle Beach, South Carolina grids.

Survey ID Year Region Survey Scale Vertical Datum Horizontal Datum

H04521 1925 Open Ocean 20,000 MLW UNDOCUMENTED

H05656 1934 Inland Waterway 10,000 MLW NAD27

H05815 1935 Inland Waterway 10,000 MLW NAD27

H05839 1935 Inland Waterway 10,000 MLW NAD27

H05840 1935 Inland Waterway 10,000 MLW NAD27

H05841 1935 Inland Waterway 10,000 MLW NAD27

H05842 1935 Inland Waterway 10,000 MLW NAD27

H05843 1935 Inland Waterway 10,000 MLW NAD27

H06539 1940 Open Ocean 80,000 MLW NAD27

H08797 1964 Open Ocean 40,000 MLW NAD27

H08838 1964/65 Open Ocean 20,000 MLW NAD27

H09096 1970 Open Ocean 20,000 MLW NAD27

H09102 1965 Open Ocean 20,000 MLW NAD27

H09117 1970 Open Ocean 40,000 MLW NAD27

H09195 1971 Open Ocean 10,000 MLW NAD27

H09229 1971/72 Open Ocean 20,000 MLW NAD27

H09230 1971/72 Open Ocean 40,000 MLW NAD27

H09260 1971/72 Open Ocean 40,000 MLW NAD27

H09289 1972 Open Ocean 20,000 MLW NAD27

H09290 1972 Open Ocean 20,000 MLW NAD27

Due to geomorphologic and anthropogenic changes since the NOS hydrographic surveys were conducted, 

and resulting inconsistencies between bathymetric and recent topographic elevation values, some editing of 

the NOS data was necessary. Original NOS hydrographic smooth sheets, NOAA navigation charts (11532, 

11534, and 11535) and Google Earth satellite imagery were referenced before making changes to the data, 

which are documented below. 

• North Inlet: Hydrographic Surveys H04521 and H08838. Soundings from these surveys, 

conducted in 1925 and 1964/65 respectively, were edited to represent the presence of a single major 

entry into the North Inlet (79.16° W, 33.33° N). Soundings that defined multiple entries into the inlet 

were deleted using NOAA navigation charts and images from Google Earth as references. 

• Winyah Bay: Hydrographic Survey H05815. Soundings from this survey, conducted in 1935 

were deleted along the northeast portion of Winyah Bay and the connecting waterway to North Inlet 

to represent current shorelines. NOAA navigation charts and Google Earth satellite imagery were 

used for reference. 

• Pawleys Inlet: Hydrographic Survey H08838. Twelve soundings from this 1964/65 survey of 

Pawleys Inlet (79.14° W, 33.39° N) were deleted because they did not match USGS NED topographic 

data values.

• Pee Dee and Waccamaw Rivers: Hydrographic Surveys H05841, H05842, and H05843. These 

surveys, all conducted in 1935, cover the Pee Dee River and Waccamaw River north of Winyah 

Bay. NOAA navigation charts and Google Earth satellite imagery were used to identify changes in 

meandering rivers, sizes and shapes of islands and silt deposits since the surveys were conducted. 

Horry County topographic LiDAR and USGS NED data were deleted in rivers and tributaries where 

NOS soundings from these surveys exist. 
DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

• Murrells Inlet: Hydrographic Surveys H09102 and H09289. NOS hydrographic soundings in 

the area of Murrells Inlet (79.03° W, 33.53° N) were collected in 1965 and 1972 before construction 

of the Murrells Inlet jetty between 1977 and 1980. The only topography data available for this area 

was USGS NED data, which did not include the jetty; Google Earth satellite imagery clearly shows 

the jetty (Fig. 5). NGDC digitized the jetty with ESRI ArcMap, assigning a 1-meter elevation above 

MHW, and using Google Earth and NOAA Nautical Chart #11534 for reference.

Figure 5. Murrells Inlet jetty. The digitized jetty is shown on the left with surrounding NOS soundings and NED 

topographic data; neither dataset includes the jetty. The Google Earth satellite image on the right, in conjunction 

with NOAA Nautical Chart #11534, was used to digitize the jetty as a 1-meter elevation feature for gridding.

• Little River Inlet: Hydrographic Surveys H09195 and H09229. NOS soundings from these 

two surveys, conducted in 1971 and 1972, were clipped to more recent NED topographic data 

north of the Little River Inlet (78.52° W, 33.86° N), as neither survey reflects the current coastline 

configuration.

• Hydrographic Survey H09096. Sixteen soundings were deleted by clipping to NED topographic 

data at the open-ocean shoreline.
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Taylor et al., 2008

• Horse Ford, Little River Inlet: Hydrographic Survey H05656. Eleven soundings from the 

1934 H05656 NOS survey (Fig. 6) were shifted horizontally up to 15 meters to accommodate 

geomorphologic changes in the Horse Ford Channel in the Little River Inlet (78.56° W, 33.86° N). 

Forty two soundings from Little River were also removed to reflect changes in the same channel 

(78.6° W, 33.86° N). Horry County topographic LiDAR tiles #735745 and #730745 and GoogleEarth 

satellite imagery were referenced to locate the modern river channel. In addition, soundings that 

lay within the Intracoastal Waterway were excised to reflect modern dredging of the channel (see 

Intracoastal Waterway discussion in Section 3.1.1.3 below).

Figure 6. Horse Ford Channel in the Little River Inlet. Figure on left shows original sounding locations in red 

and modified sounding locations in yellow. The same area is depicted on the right (red box) in a Google Earth 

satellite image.
11

DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

2) USGS Interferometric Sonar Bathymetry Data

The USGS interferometric sonar data used in the gridding compilation were collected between 1999 and 

2003 on multiple USGS cruises. The data extends from the Little River to Winyah Bay and from about 200 

meters to 10 km offshore (Fig. 7). Depths were originally referenced to Mean Lower Low Water (MLLW) 

and UTM Zone 17, in meters. Swath width averaged 70 meters with trackline spacing of 300 meters. The data 

were collected using a SEA Ltd. Submetrix 2000 Series interferometric sonar system (234 kHz), mounted 

below a Seatronics TSS DMS2-05 motion reference unit (MRU). Data were acquired at a 0.133 second 

ping rate and logged at a 2K sample rate using the SEA Ltd. RTS2000 acquisition software. Bathymetric 

swath width varied as a function of depth, but averaged roughly 10 times water depth within the depth range 

between 6 to 14 meters (Baldwin, et al., 2004). The USGS used a GRASS interpolation routine (spline with 

tension) to generate a continuous surface with a grid cell size of 100 meters, which was provided to NGDC. 

This grid was subsequently resurfaced by NGDC to generate depth points every 10 meters, which were then 

used in the gridding process.

 

Figure 7. Spatial coverage and shaded relief image of USGS 

interferometric sonar bathymetry data.
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Taylor et al., 2008

3) USACE surveys in the Intracoastal Waterway

The Navigation Section of the Charleston District, USACE provided four hydrographic surveys 

covering much of the Atlantic Intracoastal Waterway in Horry County, South Carolina. The data were 

collected between December 2005 and January 2006 and cover most of the area from just northeast of 

the Highway 17 bridge to Highway 501. Surveys consist of two parallel track lines spaced ~20 meters 

apart, with soundings every 0.4 meters. As coverage of the waterway was incomplete, NGDC digitized 

the remaining channel segments in ESRI ArcMap using soundings of 12 feet below MLW (the minimum 

dredged depth in the waterway). Figure 8 shows the segments of the waterway surveyed by the USACE 

and the segments digitized by NGDC at dredge depth (see NOAA Nautical Chart #11534 data and Coast 

Pilot, Volume 4 for more information on the waterway). 

Figure 8. Atlantic Intracoastal Waterway data coverage. Blue indicates segments of the 

channel surveyed by the USACE, green those segments digitized by NGDC and assigned 

a depth of 12 feet below MLW.
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DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

3.1.3 Topography

Topographic datasets used in the compilation of the grids include high-resolution topographic LiDAR data 

collected and processed by Sanborn Inc. for Horry County, South Carolina, and the USGS National Elevation Dataset 

1 arc-second gridded data for the northeast and southwest corners of the gridding area.

Table 4. Topographic data sources used in gridding.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

Horry 

County -

Sanborn

2005 LiDAR 1 to 7 meters NAD83 NAVD88

http://www.

horrycounty.org/;

http://www.

sanborn.com/

technologies/lidar.

htm

USGS various National Elevation 

Data Set (NED) 1 arc-second WGS84 NAVD88 (MSL)

http://seamless.

usgs.gov/

1) Horry County, South Carolina topographic LiDAR data

Topographic LiDAR data collected and processed by Sanborn, Inc. was obtained from Horry County, 

South Carolina. The data were collected between February 17 and 27, 2005 at a ground spacing of 1.2

meters referenced to NAVD88 and NAD83, South Carolina State Planes. This dataset is proprietary and 

was released for compilation of these grids only: it cannot not be made available to the general public at 

the time of this writing. 

One new NGS station was set and two existing NGS stations were used as Airborne GPS base 

stations for this project. A ground control network was surveyed using GPS to tie the newly set station to 

existing NGS control monuments. The data were collected at an altitude of 1,200 meters above ground 

level, with an airspeed of 140 knots, a scan frequency of 32 Hertz, a scan width half angle of 20 degrees, 

and pulse rate of 50 kHz. Data were processed to produce a bare-earth filtered data set with variable 

spacing between points of 1 to 7 meters. The maximum RMS value is 0.139 meters and the maximum 

standard deviation is 0.103 meters. The average RMS is 0.097 meters. A total of 744 LiDAR files, “tiles”, 

were used in the gridding process, each covering a roughly 1500-meter square area (see Fig. 1 for spatial 

coverage of entire dataset).

The LiDAR dataset includes elevations over water bodies, reflecting the surface of those bodies. ESRI 

ArcMap was used to interactively clip elevation values less than zero that are seaward of the coastline. 

Elevation values were also deleted along the Intracoastal Waterway, and where NOS hydrographic data 

was available north of Winyah Bay in the Pee Dee and Waccamaw rivers and adjoining tributaries, and 

in the Little River Inlet. 

2) USGS National Elevation Dataset

USGS National Elevation Dataset (NED) 1 arc-second DEM data was used for small areas in the 

northeast and southwest corners of the gridding area (Fig. 1). The bare-earth elevations were originally 

referenced to NAD83 and NAVD88 and 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 quad maps and aerial photos based on surveys conducted 

in the 1970s and 1980s. This dataset was also interactively clipped where NOS survey data exists, along 

the Atlantic Intracoastal Waterway, and along the coastline.
14

Taylor et al., 2008

3.1.4 Topography/Bathymetry

High-resolution “beach-profiles”, elevation measurements that cross the subaerial-submarine coastal transition 

zone, were collected by two groups: Coastal Carolina University and Coastal Science and Engineering, Inc. These 

measurements provide excellent control on the open-ocean coastal relief boundary, though the transect lines are widely 

spaced, and were not conducted at river inlets.

Table 5. Beach profile data sources included in grid compilation.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

Coastal 

Carolina 

University

2006 Beach profile

Profile spacing ranges from 

200 to 1000 meters; point 

spacing along profile~ 0.3 

meters

NAD83, South 

Carolina State Plane3900

NAVD88 http://camelot-

2.coastal.edu/

Coastal 

Science & 

Engineering, 

Inc

2005 Beach profile

Profile spacing ranges from 

80 to 780 meters; point 

spacing along profile 1 to 32 

meters

NAD83, South 

Carolina State Plane3900

NGVD29

http://www.

coastalscience.

com/

1) Beach profiles, Coastal Carolina University

Beach profiles were obtained from Dr. Scott Harris of the Coastal Carolina University (CCU). CCU, 

with funding from the South Carolina Office of Ocean and Coastal Resource Management (SC OCRM), 

USGS, and the South Carolina Sea Grant Consortium, collects beach profiles of the South Carolina 

coast every year using the same benchmarks (see Fig. 1 for spatial coverage). The data were originally 

referenced to NAVD88 and South Carolina State Plane, with elevation units of feet. Elevations were 

taken ~0.3 meters along each profile, though profiles are spaced between 200 and 1000 meters apart. 

Profiles are generally between 1000 and 1300 meters in length, and span roughly the 5 to -8 meter 

elevation range. 

2) Beach profiles, Coastal Science and Engineering, Inc.

Beach profiles surveyed in May of 2005 were obtained from Coastal Science and Engineering, 

Inc. (CSE). The data were originally referenced to NGVD29 and South Carolina State Plane. Subaerial 

portions of the profiles were measured using RTK-GPS with a Trimble 5700. Hydrographic portions of 

the profiles were obtained via a shallow-draft survey boat equipped with RTK-GPS linked to a Sonar 

Lite (Ohmex Ltd.) precision echo sounder sampling at 1 to 10 Hz. After removing spikes from the 

hydrographic data, it was reduced using a 7-point running average filter. The hydrographic data were 

then coupled with the onshore data to create individual profiles. Elevation points vary between 1 and 

32 meters apart along individual profiles, with the profiles themselves spaced 80 to 780 meters apart. 

Profiles are generally 100 to 125 meters long, covering an elevation range from approximately 2 to -2 

meters. The area covered by the CSE beach profiles is completely overlapped by CCU beach profiles, 

though the CSE profiles are more closely spaced within this area (see Fig. 1). This dataset is proprietary 

and was released for compilation of these grids only: it cannot not be made available to the general 

public at the time of this writing. 
15

DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

Datasets used in the compilation of the grids were originally referenced to a number of vertical datums 

including Mean Low Water (MLW), Mean Lower Low Water (MLLW), North American Vertical Datum of 1988 

(NAVD88), and National Geodetic Vertical Datum of 1929 (NGVD29). All datasets were transformed to a Mean High 

Water (MHW) datum to provide the worst case scenario for inundation modeling. 

1) Bathymetric data

As no VDatum (http://nauticalcharts.noaa.gov/csdl/vdatum.htm) model was available for the gridding 

area, transformation of bathymetric data to MHW was achieved by using data and information supplied by 

PMEL. Three ASCII grids, with values representing the differences between MHW and depths referenced to 

MSL, NAVD88 and MLLW, were generated by PMEL by dividing coastline polygons from the USGS/EPA 

National Hydrographic Dataset (http://nhd.usgs.gov) into zones, and linearly interpolating between nearby 

tide gauge station values. The difference between adjacent zones was constrained to less than 0.1 meter. The 

polygons were then converted to a 3-arc second resolution grid and filtered slightly to reduce stair-stepping 

effects between zones. The entire open ocean area was represented by one zone, the value for which was 

computed using the mean values of the coastal tide gauge stations. These interpolated surfaces were applied to 

the bathymetric datasets using FME software to convert data points to MHW; FME is an integrated collection 

of spatial extract, transform, and load tools for data transformation and data translation (http://www.safe.com). 

Soundings referenced to MLW were adjusted to MLLW by adding 0.047 metersderived by computing the 

average difference between MLW and MLLW over the gridding areaprior to transformation to MHW.

2) Topographic data

LiDAR and NED topographic data were converted to MHW using FME software by adding a constant 

value of 0.546 meters, which was derived by computing the average difference between NAVD88 and MHW 

over the gridding area.

3.2.2 Horizontal datum transformations

Datasets used to compile the grids were originally horizontally referenced to NAD83, NAD27, South Carolina 

State Plane and WGS84. NOS hydrographic survey data were converted to WGS84 using GEODAS (NADCON); the 

relationships and transformational equations between these horizontal datums are well established. All other data, 

except that referenced to NAD83 (the difference between NAD83 and WGS84 is negligible; Wilson, 1995) were 

converted to a horizontal datum of WGS84 using FME software. 

3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

After horizontal and vertical transformations were applied, ESRI shape files were generated for each data 

file, and value consistency between datasets was checked in ESRI ArcMap. Problems and errors were identified and 

resolved before proceeding with subsequent gridding steps; the quality-assessed ESRI shape files were subsequently 

converted to xyz files in preparation for gridding. Problems included:

• The LiDAR topographic data had numerous “wells”, anomalous elevations that were 5 to 25 meters deeper 

than surrounding points. Typically, these points were surrounded by open spaces without elevation points, so 

were readily identifiable as erroneous elevations: they were deleted from the shape files prior to creation of 

the xyz files.

• The LiDAR data within the Waccamaw River basin exhibits unusual, and artificial, east-west lineaments 

(prominent in Fig. 1, where blue bands of east-west “below MHW” areas are visible). This problem appears 

to have occurred during data collection or initial processing and could not be rectified by NGDC.

• The southwest NED dataset contains artificial plateaus resulting from meter-high steps in the dataset from 

one elevation to the next (see Fig. 9). This problem could not be corrected.
16

Taylor et al., 2008

•	 The sparseness of the NED data (30-meter spacing) and the fact that each point reflects an average of a 30-

by-30 meter area on the ground, resulted in significant offset with other datasets, especially the CCU beach 

profiles. One feature that could not be avoided was a step-down on the back side of the beach profile data, 

as that dataset’s landward-most elevations are on the order of 2 meters, while the NED data in the same area 

was approximately 1 meter. The beach profile data was considered to be more accurate than the NED data 

and was preferentially utilized in grid development (see Table 6).

3.3.2 Smoothing of sparse datasets

Several datasets are sparse at the resolution of the 1/3 arc-second (10 meter) grid. Three of these (USGS 

interferometric sonar bathymetry, and the CCU and CSE beach profiles) are recent and considered to be of high quality. 

Nevertheless, the distance between beach profiles (80 to 1000 meters) and the distance between USGS interferometric 

sonar depths is significantly larger than the required 10-meter spacing of the 1/3 arc-second grid. As a result, each of 

these datasets was separately pre-surfaced to 10-meter spacing to fill in gaps between the beach profiles, and within 

the interferometric sonar data. The beach profiles were pre-surfaced using a Delauney triangulation method in GMT 

(http://gmt.soest.hawaii.edu/), while the interferometric sonar data was pre-surfaced using a tight spline interpolation 

method, also in GMT. Each of the resulting surfaces was closely cropped to the spatial coverage of the original data to 

ensure that extrapolation outside the coverage window was minimized.

In deep water, the NOS survey data had point spacings up to 1 kilometer apart. In order to reduce the effect 

of artifacts in the form of lines of “pimples” in the grid due to this low resolution dataset, a 1 arc-second-spacing 

surface was generated from the 13 “open-ocean surveys” using ESRI ArcCatalog (Table 3). This surface was closely 

clipped to the spatial extent of these 13 surveys, and then exported as an xyz file. The original soundings were checked 

against the processed values to ensure grid value accuracy. The 7 NOS inland-waterway surveys (H05656, H0518, 

H-5839, H05840, H05841, H05842 and H05843; Table 3) had soundings tens of meters apart, which also necessitated 

pre-surfacing to ensure that grid cells within the inland waterways had values representative of the river systems, and 

not that of the surrounding topographic data. These surfaces were created using the ESRI ArcCatalog “IDW” tool, 

which interpolates grid cells within 3 to 5 cells of existing data. The resulting grid for each inland waterway survey 

was exported as xyz point data for input to the 1/3 arc-second Myrtle Beach grid.

3.3.3 Gridding the data with MB-System

All processed xyz files were gridded using MB-System (http://www.ldeo.columbia.edu/res/pi/MB-System/). 

MB-System is an NSF-funded share-ware software application specifically designed to manipulate submarine 

multibeam sonar data, though it can utilize a wide variety of data types, including generic xyz data. The MB-System 

tool ‘mbgrid’ was used to create the 1/3 arc-second Myrtle Beach grid—a modeled surface draping the point data—of 

weighted sounding and topographic point data, using a tight spline tension to interpolate cells without data values. 

The data hierarchy used in the ‘mbgrid’ gridding algorithm as relative gridding weights is listed in Table 6. Greatest 

weight was given to the pre-surfaced beach profile grids, which define the subaerial to submarine transition, and to the 

Intracoastal Waterway to ensure its representation in the grids. Least weight was given to the pre-surfaced, deep-water 

NOS grid. The 1/3 arc-second grid was resampled using the ESRI ArcCatalog ‘Raster Resample’ tool to create the 1

arc-second grid.

Table 6. Data hierarchy used to assign gridding weight in MB-System.

Dataset Relative Gridding Weight

USACE Intracoastal Waterway surveys 100

NGDC-digitized Intracoastal Waterway dredged depths 100

Beach profiles, Coastal Science and Engineering, Inc. 100

Beach profiles, Coastal Carolina University 50

Horry County Topographic LiDAR 50

USGS NED topography 10

USGS interferometric sonar bathymetry data: pre-surfaced 5

NOS hydrographic surveys: gridded inland waterways 1

NOS hydrographic surveys: gridded open ocean 0.1
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DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

3.4 Quality Assessment of the Grids.

3.4.1. Horizontal accuracy

The digital elevation grids have an estimated horizontal accuracy of no better than 10 meters for topographic 

features; the LiDAR data has an accuracy of ~2 meters for individual postings, while the NED data is accurate to 

within about 15 meters. Bathymetric features are resolved only to within a few hundred meters in deep water areas; 

shallow, near-coastal regions have an accuracy approaching the subaerial topographic features. Positional accuracy is 

limited by: transformation of multiple datasets from various datums; sparseness of deep-water and inland-waterway 

soundings; potentially large positional accuracy of pre-satellite navigated (GPS) hydrographic surveys; and natural 

and artificial morphologic change that has occurred since the hydrographic surveys were conducted.

3.4.2 Vertical accuracy

The grids have an estimated vertical accuracy of 0.1 to 1 meters for topographic areas and 0.1 meters to 5% 

of water depth for bathymetric areas (~1.5 meters in the southeast corner of the grids). Topographic values are largely 

derived from Horry County LiDAR surveys, which have an estimated vertical accuracy of 0.1 to 0.15 meters. The 

1 arc-second NED data have a vertical accuracy of ~1 meter. Bathymetric values were derived from the wide range 

of input data sounding measurements from the early 20th century to recent, GPS-navigated interferometric sonar 

surveys and beach profiles. Gridding interpolation to determine values between sparse, poorly-located NOS soundings 

degrades the vertical accuracy of elevations in deep-water. Also suspect are the accuracy of values within inlandwaterways, as substantial morphologic change has occurred in some areas since the NOS hydrographic surveys of the 

1930s to 1970s (e.g., dredging and jetty building).

3.4.3 Comparison with South Carolina tidal bench marks

Twenty tidal bench marks lying within the Myrtle Beach study area were compared with values taken at the 

same locales from the 1 arc-second (~30 meter) grid (see Fig. 1 and Table 7 for station locations). Each bench mark has 

a geographic position recorded to within 1 arc-second, with an accuracy of +/-6 arc-seconds (http://tidesandcurrents.

noaa.gov/). Most bench marks are level with the ground surface; the rest are within a few centimeters. The National 

Geodetic Survey (NGS) data sheets for the tidal stations also document bench mark elevation above MHW, in meters, 

allowing for direct comparison with grid values at those locations. Most grid values compare favorably with the known 

bench mark elevations. Bench marks with three of the largest discrepancies (stations 8661299, 8661582, and 8661991) 

fall within the coverage of the NED topographic dataset in the southwest corner of the grid. This particular dataset 

has known deficiencies, as documented above. Three others are in the vicinity of the Little River Inlet: two (8660166 

and 8660265) are located on the boundaries between datasets and likely result from gridding interpolation between 

the datasets; the other (8660098) is embedded within high-resolution topographic LiDAR dataits discrepancy is of 

unknown origin.

Table 7. Comparison of tidal bench mark elevations, in meters, with the 1 arc-second Myrtle Beach grid.

Number Year Longitude Latitude Bench Mark Grid Value Difference

8660098 1975 078° 34’45”W 33° 52’07”N 5.025 6.62464 1.59964

8660147 1975 078° 34’41”W 33° 51’39”N 7.416 6.82289 -0.59310

8660166 1986 078° 39’00”W 33° 51’23”N 2.362 -1.38160 -3.74360

8660265 1976 078° 37’49”W 33° 49’59”N 1.531 -0.96400 -2.49500

8660642 1982 078° 48’40”W 33° 45’51”N 10.849 10.54685 -0.30214

8660854 1982 078° 55’06”W 33° 42’40”N 5.864 5.26787 -0.59612

8660983 1982 079° 00’24”W 33° 41’21”N 5.095 5.40844 0.31344

8661070 1979 078° 55’15”W 33° 39’23”N 3.876 2.88868 -0.98731

8661139 1982 079° 05’47”W 33° 39’02”N 3.703 3.15566 -0.54733

8661299 1981 079° 09’11”W 33° 36’28”N 3.473 0.67200 -2.80099

8661419 1975 079° 00’32”W 33° 35’01”N 3.837 4.17221 0.33521

8661529 1982 079° 01’50”W 33° 33’35”N 1.519 0.72462 -0.79437

8661559 1975 079° 02’30”W 33° 33’04”N 1.980 0.91384 -1.06615

8661582 1982 079° 01’22”W 33° 32’40”N 1.465 -0.44029 -1.90529

8661684 1986 079° 04’09”W 33° 30’35”N 1.176 0.23777 -0.93822

8661989 1982 079° 07’30”W 33° 26’13”N 1.693 2.00841 0.31541
18

Taylor et al., 2008

8661991 1975 079° 10’45”W 33° 26’16”N 3.767 1.18093 -2.58606

8662071 1975 079° 07’56”W 33° 24’44”N 1.735 1.14338 -0.59161

8662245 1982 079° 11’43”W 33° 21’02”N 0.811 0.12637 -0.68462

8662299 1976 079° 11’40”W 33° 20’06”N 0.842 0.14878 -0.69321

Standard Deviation: 1.25010

3.4.4 Slope map and 3-D perspectives

ESRI ArcCatalog was used to generate a slope map from the 1/3 arc-second grid to allow for visual inspection 

of the grid, and identification of artificial slopes along boundaries between datasets (Fig. 9); the grid was transformed 

to UTM Zone 17 coordinates (horizontal units in meters) in ArcCatalog for derivation of the slope grid. Analysis 

of preliminary slope maps revealed suspect data points, which were corrected before regridding the data. Known 

limitations in the grids are also apparent from the slope map, specifically, small steps at the edges of the surfaced USGS 

interferometric sonar data, where that recent survey abuts the older NOS hydrographic survey data, and along the 

landward side of the pre-surfaced beach profile data, where it misfits the NED topographic data. Also, artificial steps, 

appearing as contour lines in the southwest corner of Figure 9, result from the low quality of the NED topographic data 

in that region. Of particular interest is the presence of numerous man-made features visible in the region of the grid 

covered by the high-resolution topographic LiDAR data. Three-dimensional viewing of the UTM-transformed grid 

was accomplished using ESRI ArcScene. This interactive, rotational viewing tool was especially valuable for locating 

LiDAR “wells” present within the grid, which were subsequently deleted from the original data prior to regridding.

Figure 9. Slope map of the 1/3 arc-second grid with NGA coastline in red. Flat-lying 

slopes are white; dark shading denotes steep slopes. Note the artificial “contour lines” 

present in the southwest corner of the image, resulting from the NED topographic data in 

this region.
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DIGITAL ELEVATION MODELS OF MYRTLE BEACH, SOUTH CAROLINA

4. Summary and Conclusions

Two topographic/bathymetric digital elevation models of the Myrtle Beach, South Carolina area were 

developed for the Pacific Marine Environmental Laboratory (PMEL), NOAA Center for Tsunami Inundation Mapping 

Efforts (TIME). The two grids cover the same geographic area and have cell spacings of 1 arc-second and 1/3 arcsecond. The best available data from U.S. federal and state agencies, universities and private companies were obtained 

for grid compilation. The data were quality checked, processed and gridded using ESRI ArcGIS, FME, GMT and 

MB-System software. 

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

• Incorporate coastal bathymetric/topographic LiDAR data when available.

• Improvement of topography in the regions currently covered by NED 1 arc-second data (in the northeast and 

southwest parts of the grids).

• NOS mapping of inland waterways where significant morphologic change has occurred since the original 

surveys utilized in this study were conducted, especially in Little River Inlet and Murrells Inlet.

• Investigation into suspected data-collection or initial-processing problems with the topographic LiDAR data. 

The east-west lineaments apparent in Fig. 1 (Waccamaw River basin) are artificial in origin.

5. Acknowledgments

The creation of these grids was funded by the NOAA, Pacific Marine Environmental Laboratory. The authors 

thank Chris Chamberlin, Angie Venturato, and Hal Mofjeld (PMEL), Jane Denny (USGS), Doug Holmes (Navigation 

Section, Charleston district, USACE), Michael Scott Harris and Braden Reynolds (Coastal Carolina University), Doug 

Dusini (Coastal Science & Engineering, Inc.), Tim Oliver (Horry County, South Carolina), and John Von Rosenberg 

(NGA).

6. References

Baldwin, W. E., R. A. Morton, J. F. Denny, S. V. Dadisman, W. C. Schwab, P. T. Gayes, and N. W. Driscoll. 2004. 

Maps Showing the Stratigraphic Framework of South Carolina’s Long Bay from Little River to Winyah Bay. USGS 

Open File Report 2004-1013. URL: http://pubs.usgs.gov/of/2004/1013/index.html.

Coast Pilot, Volume 4, 37th Edition. 2005. Atlantic Coast: Cape Henry, VA to Key West, FL. U.S. Department of 

Commerce, NOAA, National Ocean Service.

Nautical Chart #11532, 9th Edition. 1975. Winyah Bay, South Carolina. U.S. Department of Commerce, NOAA, 

National Ocean Service, Coast Survey.

Nautical Chart #11534, 29th Edition. 1997. Intracoastal Waterway, Myrtle Grove Sound and Cape Fear River to 

Casino Creek, North Carolina−South Carolina. U.S. Department of Commerce, NOAA, National Ocean Service, 

Coast Survey.

Nautical Chart #11535, 11th Edition. 1992. Little River Inlet to Winyah Bay, North Carolina−South Carolina. U.S. 

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

Wilson, W. Stanley, 1995. Use of the “NAD/WGS84” Datum Tag on Mapping Products. Federal Register, Vol. 60, No. 

157, August 15, 1995. U.S. Department of Commerce, NOAA, National Ocean Service, National Geodetic Survey.