noaa / NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

National Environmental Satellite,

Data, and Information Service

NOAA Technical Memorandum NESDIS NGDC-37

DIGITAL ELEVATION MODEL OF OCEAN CITY, MARYLAND:

PROCEDURES, DATA SOURCES AND ANALYSIS 

P.R. Grothe

L.A. Taylor

B.W. Eakins

R.R. Warnken 

K.S. Carignan

E. Lim

R.J. Caldwell

D.Z. Friday

National Geophysical Data Center

Marine Geology and Geophysics Division

Boulder, Colorado

July 2010 

UNITED STATES

DEPARTMENT OF COMMERCE

Gary Locke

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Dr. Jane Lubchenco

Under Secretary for Oceans

and Atmosphere/Administrator

National Environmental Satellite,

Data, and Information Service

Mary E. Kicza

Assistant Administrator

NOAA Technical Memorandum NESDIS NGDC-37

DIGITAL ELEVATION MODEL OF OCEAN CITY, MARYLAND:

PROCEDURES, DATA SOURCES AND ANALYSIS 

Pamela R. Grothe1

Lisa A. Taylor2

Barry W. Eakins1

Robin R. Warnken2

Kelly S. Carignan1 

Elliot Lim1

R. Jason Caldwell1

Dorothy Z. Friday1

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

2NOAA, National Geophysical Data Center, Boulder, Colorado 

National Geophysical Data Center

Marine Geology and Geophysics Division 

Boulder, Colorado

July 2010 
Grothe et al., 2010

ii

NOTICE

Mention	of	a	commercial	company	or	product	does	not	constitute	an	endorsement	by	the	

NOAA National Environmental Satellite, Data, and Information Service.	Use	of	information	

from	this	publication	concerning	proprietary	products	or	the	test	of	such	products	for	publicity

or	advertising	purposes	is	not	authorized.	

Corresponding project contact:

Lisa A. Taylor

NOAA National Geophysical Data Center

Marine Geology and Geophysics Division

325 Broadway, E/GC 3

Boulder, Colorado 80305

Phone: 303-497-6767

Fax: 303-497-6513

E-mail: Lisa.A.Taylor@noaa.gov

http://www.ngdc.noaa.gov/mgg/inundation/tsunami/inundation.html

Also available from the National Technical Information Service (NTIS)

(http://www.ntis.gov)
Digital Elevation Model of Ocean City, Maryland

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Contents

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

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

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

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

3.1.1	 Coastline................................................................................................................5

3.1.2	 Bathymetry ............................................................................................................8

3.1.3	 Topography..........................................................................................................15

3.2	 Establishing Common Datums .................................................................................................23

3.2.1	 Vertical datum transformations ...........................................................................23

3.2.2	 Horizontal datum transformations.......................................................................25

3.3	 Digital Elevation Model Development .....................................................................................25

3.3.1	 Verifying consistency between datasets..............................................................25

3.3.2	 Smoothing of bathymetric data ...........................................................................26

3.3.3	 Integrating topographic datasets..........................................................................27

3.3.4	 Gridding the data with MB-System......................................................................28

3.4	 Quality Assessment of the DEM...............................................................................................29

3.4.1	 Horizontal accuracy.............................................................................................29

3.4.2	 Vertical accuracy .................................................................................................29

3.4.3	 Slope maps and 3-D perspectives........................................................................30

3.4.4	 Comparison with source data files.......................................................................32

3.4.5	 Comparison with NGS geodetic monuments......................................................35

4.	 Summary and Conclusions...............................................................................................................36

5.	 Acknowledgments............................................................................................................................36

6.	 References........................................................................................................................................36

7.	 Data Processing Software ................................................................................................................37

List of Figures

Figure 1. 	 Shaded-relief image of the Ocean City DEM.............................................................................1

Figure 2.	 Google Earth satellite image of the jetties at Ocean City Inlet ..................................................2

Figure 3.	 Source and coverage of datasets used in building the Ocean City DEM....................................4

Figure 4.	 Digital coastline datasets used in building the Ocean City DEM...............................................5

Figure 5.	 USGS Ocean City Inlet jetty shapefile used as a trace for the final coastline ............................6

Figure 6.	 Final coastline based on 2008 USGS topographic lidar survey..................................................7

Figure 7.	 The final coastline including fingers, inlets, and channels visible in satellite imagery

but not represented in the ENC coastlines...........................................................................7

Figure 8.	 Spatial coverage of bathymetric datasets used in building the Ocean City DEM ......................8

Figure 9.	 Digital NOS hydrographic survey coverage in the Ocean City region.....................................12

Figure 10.	 Digital USACE hydrographic survey coverage in the Ocean City region................................13

Figure 11.	 New boat harbor shown in satellite imagery but not included on the 2007

RNC #12211 or other bathymetric data .............................................................................14

Figure 12.	 Spatial coverage of topographic datasets used in building the Ocean City DEM ....................16

Figure 13.	 Spatial coverage of the NED 1/9 arc-second DEM available versus used in the

final Ocean City DEM .......................................................................................................17

Figure 14.	 Vertical stripes in marshy areas in the NED 1/9 arc-second DEM...........................................18

Figure 15.	 USGS 2008 bare-earth lidar data on Assateague Island ...........................................................19

Figure 16.	 Problems in the CSC 2005 bare-earth lidar illustrated in a gridded image ..............................20

Figure 17.	 Spatial coverage of the 2005 non-bare-earth topographic dataset used in building

	 the Ocean City DEM with the resulting shapefile after rooftops were

	 deleted to approximate bare-earth .....................................................................................21

Figure 18.	 NGDC-digitized breakwaters and jetties..................................................................................22

Figure 19.	 Spatial coverage of the VDatum transformation tool................................................................23
Grothe et al., 2010

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Figure 20.	 Location of NOAA tide stations near Ocean City ....................................................................24

Figure 21.	 Histogram of the differences between all NOS hydrographic surveys and the

	 1 arc-second pre-surface bathymetric grid ........................................................................26

Figure 22.	 Histogram of the differences between all USACE hydrographic surveys and the

	 1 arc-second pre-surfaced bathymetric grid ......................................................................26

Figure 23.	 Problems in the final grid due to a missing strip of lidar data filled in with the NED

	 1/3 arc-second DEM..........................................................................................................27

Figure 24.	 Ocean City DEM data distribution plot..........................................................................................................28

Figure 25.	 Slope map of the Ocean City DEM ..........................................................................................30

Figure 26.	 Perspective view from the southeast of the Ocean City DEM..................................................31

Figure 27.	 Histogram of the differences between all the NOS suveys and the Ocean City

	 DEM ..................................................................................................................................32

Figure 28.	 Histogram of the differences between all the USACE surveys and the Ocean

	 City DEM...........................................................................................................................32

Figure 29.	 Histogram of the differences between all the CSC 2002 bare-earth lidar data

	 and the Ocean City DEM...................................................................................................33

Figure 30.	 Histogram of the differences between select CSC 2005 bare-earth lidar data

	 and the Ocean City DEM............................................................................................................................33

Figure 31.	 Histogram of the differences between all CSC 2005 non-bare-earth topographic-bathymetric

	 lidar data and the Ocean City DEM...................................................................................33

Figure 32.	 Histogram of the differences between select USGS 2008 bare-earth lidar data and

	 the Ocean City DEM .........................................................................................................34

Figure 33.	 Histogram of the differences between select NED 1/3 arc-second topographic

	 DEM data points and the Ocean City DEM ......................................................................34

Figure 34.	 Histogram of the differences between select NED 1/9 arc-second topographic

	 DEM data points and the Ocean City DEM ......................................................................34

Figure 35.	 Location of NGS geodetic monuments.....................................................................................35

Figure 36.	 Histogram of the differences between NGS geodetic monument elevations and

	 the Ocean City DEM .........................................................................................................35

List of Tables

Table 1.	 PMEL specifications for the Ocean City DEM...........................................................................3

Table 2.	 Coastline datasets used in building the Ocean City DEM..........................................................5

Table 3.	 Electronic navigational charts available in the Ocean City region.............................................6

Table 4.	 Bathymetric datasets used in building the Ocean City DEM .....................................................8

Table 5.	 Digital NOS hydrographic surveys available in the Ocean City region .....................................9

Table 6.	 USACE hydrographic surveys used in building the Ocean City DEM ....................................13

Table 7.	 Topographic datasets used in building the Ocean City DEM...................................................15

Table 8.	 Relationship between mean high water and NAVD 88 vertical datums at tide

stations located within the Ocean City DEM extent..........................................................24

Table 9.	 Data hierarchy used to assign gridding weight in MB-System..................................................28
Digital Elevation Model of Ocean City, Maryland:

Procedures, Data Sources and Analysis

1. Introduction

	 In October 2009, the National Geophysical Data Center (NGDC), an office of the National Oceanic and

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

(DEM) of Ocean City, Maryland (Fig. 1) for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center

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

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

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

Fig. 3) and will be used for tsunami inundation modeling, as part of the tsunami forecast system Short-term Inundation

Forecasting for Tsunamis (SIFT) currently being developed by PMEL for the NOAA Tsunami Warning Centers. This

report provides a summary of the data sources and methodology used in developing the Ocean City DEM.

Figure 1. Shaded-relief image of the Ocean City DEM. Contour interval is 10 meters.

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

square when converted to projected coordinate systems, such as UTM zones (in meters). At the latitude of Ocean City, Maryland (38°19.4235′ N,

75°6.3111′ W) 1/3 arc-second of latitude is equivalent to 10.28 meters; 1/3 arc-second of longitude equals 8.10 meters.
Grothe et al., 2010

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2. Study Area

The Ocean City DEM includes the dynamic barrier islands, Fenwick and Assateague Islands (see Fig. 1),

located off the Delaware, Maryland and Virginia coasts. Ocean City is a popular vacation destination located in an area

with 30 million people. Approximately eight million people visit this area annually (http://www.mgs.md.gov/coastal/

osr/ocsand1.html). 

	 	 Barrier islands are unstable environments with constant erosion, deposition and migration of sediment from

wave action. Ocean City’s urbanization increases the instability of these islands, creating hazards to the economy and

property. Along with general sediment transportation issues, rising sea level is another concern for coastal managers

and property owners. To help stabilize the barrier islands, Ocean City uses beach nourishment and dune stabilization

as the primary methods to protect their beaches and communities (http://www.mgs.md.gov/coastal/osr/ocsand1.html).

	 	 In 1933, a hurricane opened up the current Ocean City Inlet. The U.S. Army Corps of Engineers built two

stone jetties to stabilize the inlet for it to remain open as a navigation channel (Fig. 2). The jetties maintain the opening

of the inlet but have affected the longshore southerly drift of sand. This has trapped sand to the north at Ocean City

Beach, but starved Assateague Island, south of the inlet, shifting the island more than 500 meters towards the shore

(http://pubs.usgs.gov/circ/c1075/conflicts.html).

Figure 2. Google Earth satellite image of the jetties at Ocean City Inlet. The jetties block the southward transport of 

sediment, causing Assateague Island to migrate westward. 
Digital Elevation Model of Ocean City, Maryland

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3. Methodology

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

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

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

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

and vertical datums: North American Datum of 1983 (NAD 83)2 geographic and mean high water (MHW), for

modeling of maximum flooding, respectively. Data processing and evaluation, and DEM assembly and assessment are

described in the following subsections.

Table 1: PMEL specifications for the Ocean City DEM.

Grid Area Ocean City, Maryland

Coverage Area 74.71o to 75.58o W; 37.68o to 38.87o N

Coordinate System Geographic decimal degrees

Horizontal Datum World Geodetic System of 1984 (WGS 84)

Vertical Datum Mean high water (MHW)

Vertical Units Meters

Grid Spacing 1/3 arc-second

Grid Format ESRI Arc ASCII grid

3.1 Data Sources and Processing

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

federal agencies: NOAA’s NGDC, Office of Coast Survey (OCS) and Coastal Services Center (CSC); the U.S.

Geological Survey (USGS); and the U.S. Army Corps of Engineers (USACE). Safe Software’s FME data

translation tool package was used to shift datasets to NAD 83 geographic horizontal datum. The datasets were

then displayed with ESRI’s ArcGIS, ESRI Imagery World 2D Online World Imagery 2D, and Applied Imagery’s

Quick Terrain Modeler software (QT Modeler) to assess data quality and manually edit datasets. Vertical datum

transformations to MHW were accomplished using NOAA National Geodetic Survey’s Vertical Datum (VDatum)

model software, and FME and ArcGIS 3D Analyst tool, based upon data from NOAA tide stations (see Sec. 3.2.1).

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

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

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

tsunamis may originate most anywhere around the world, tsunami modelers require a global datum, such as WGS 84 geographic, for their DEMs

so that they can model the wave’s passage across ocean basins. This DEM is identified as having a WGS 84 geographic horizontal datum even

though the underlying elevation data were typically transformed to NAD 83 geographic. At the scale of the DEM, WGS 84 geographic and NAD

83 geographic are identical and may be used interchangeably.
Grothe et al., 2010

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Figure 3. Source and coverage of datasets used in building the Ocean City DEM. Red box denotes DEM extents.
Digital Elevation Model of Ocean City, Maryland

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3.1.1 Coastline

	 Coastline datasets of the Ocean City region were obtained from NOAA’s OCS, and USGS. (Table 2; Fig. 4).

Table 2: Coastline datasets used in building the Ocean City DEM.

Source Year Data Type Spatial 

Resolution

Original Horizontal 

Datum/Coordinate 

System

Original 

Vertical 

Datum

URL

OCS 2007-

2008

ENC

Coastline

1:20,000 to

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

USGS 2007 Jetty

Outline 1 meter NAD 83 UTM Zone18N Not defined http://pubs.usgs.gov/of/2007/1388/data/Jetty

Figure 4. Digital coastline datasets used in building the Ocean City DEM. ENCs with an “_2” refer 

to a higher resolution inset on the ENC.
Grothe et al., 2010

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1) Office of Coast Survey ENCs

Five Electronic Navigational Charts (ENCs) are available for the Ocean City area (Table 3). They were

downloaded from NOAA’s OCS web site in S-57 format and include coastline data files referenced to MHW.

The nautical charts are also available as georeferenced Raster Nautical Charts (RNCs; digital images of

the charts) and were used in ArcMap to quality control (QC) bathymetric and topographic datasets, the

bathymetric surface, and the final DEM.

Table 3: ENCs available in the Ocean City region.

Chart Title Edition Edition Date Format Scale

12210 Chincoteague Inlet to Great Machipongo

Inlet 38 5/1/2008 ENC/RNC 1:80,000 with 1:20,000 inset

12211 Fenwick Island to Chincoteague Inlet 43 10/1/2007 ENC/RNC 1:80,000 with 1:20,000 inset

12214 Cape May to Fenwick 48 10/1/2007 ENC/RNC 1:80,000

12216 Cape Henlopen to Indian River Inlet 28 4/1/2008 ENC/RNC 1:80,000

12304 Delaware Bay 45 2/1/2008 ENC/RNC 1:80,000

2) United States Geological Survey Ocean City Inlet Jetty

NGDC digitized the Ocean City Inlet jetty on Assateague Island using ArcMap, based on the USGS jetty

shapefile, and included it the final coastline (Fig 5).

Figure 5. USGS Ocean City Inlet jetty shapefile used as a trace for the final coastline. (RNC #12211 in background)
Digital Elevation Model of Ocean City, Maryland

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The ENC coastlines and the USGS Ocean City Inlet jetty outline were merged in ArcMap to create a final

coastline for the Ocean City region. NGDC edited the final coastline to include morphologic changes captured in

the more recent lidar data (e.g., Fig 6). The final coastline was also draped over the world imagery layer in ArcMap,

available from the ArcGIS Online Resource Center, and edited to include all tidal bays, channels, fingers, jetties, and

breakwaters that were not included in some of the lower resolution ENC coastlines (e.g., Fig. 7). Piers and bridges

were excluded from the final coastline.

Figure 6. Final coastline based on 2008 USGS topographic lidar survey.

Figure 7. The final coastline including fingers, inlets, and channels visible in satellite imagery 

but not represented in the ENC coastlines.
Grothe et al., 2010

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3.1.2 Bathymetry

	 Bathymetric datasets available in the Ocean City region include 94 National Oceanographic Survey (NOS)

hydrographic surveys, four USACE surveys located at the Ocean City Inlet, and one multibeam swath sonar survey

from NGDC’s multibeam bathymetry database (Table 4; Fig. 6). The multibeam swath sonar survey AT1L3 was

evaluated but not used in gridding the DEM because the data coverage was insignificant, covering only 11 km by 0.1

km in a linear feature near the south-east corner of the DEM.

Table 4: Bathymetric datasets used in building the Ocean City DEM.

Source Year Data Type Spatial Resolution

Downloaded 

Horizontal Datum/

Coordinate System

Downloaded 

Vertical 

Datum

URL

NGDC 1880 to

2004

NOS

Hydrographic

survey

soundings

Ranges from 1:5,00

to 1:120,000 (varies

with scale of survey,

depth, traffic, and

probability of

obstructions)

NAD 83

geographic

Mean lower

low water

and mean low

water

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

bathymetry/hydro.html

USACE 2006 to

2007

Hydrographic

survey

soundings

Line spacing ranging

from 15 to 30 meters

apart and point

spacing 3 to 4 meters

NAD 83 Maryland

State Plane (feet) MLLW (feet) http://www.nab.usace.army.mil/

Navigation/DepthRpts.htm#052

NGDC 2009 Digitized

surveys

WGS 84

geographic MHW

Figure 8. Spatial coverage of bathymetric datasets 

used in building the Ocean City DEM. White areas 

denote no data and red box denotes DEM extent.
Digital Elevation Model of Ocean City, Maryland

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1) NGDC hydrographic survey database

A total of 94 NOS hydrographic surveys conducted between 1880 and 2004 were available for use in

developing the Ocean City DEM (Table 5; Fig. 9). The hydrographic survey data were downloaded from

NGDC’s online NOS hydrographic database using GEODAS3. The data are vertically referenced to mean

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

Survey data were downloaded in an area 0.05 degree (~5%) larger than the Ocean City DEM extent to

support data interpolation across grid edges.

Only 76 of the 94 surveys were used in building the Ocean City DEM (Table 5), as some older surveys

have been superseded by newer surveys. Data point spacing for the NOS surveys varies by collection date. In

general, earlier surveys have greater point spacing than more recent surveys.

After converting all NOS survey data to MHW using either VDatum or a Tide station offset (see Sec.

3.2.1), the data were displayed in ESRI ArcMap and reviewed for digitizing errors against scanned original

survey smooth sheets and edited as necessary. The surveys were also compared to the topographic and

bathymetric, the final coastline, and RNCs. The surveys were clipped to remove soundings that overlap the

more recent USACE surveys located within the Ocean City Inlet, along the coastline, and where soundings

from older surveys have been superseded by more recent NOS surveys.

Table 5: Digital NOS hydrographic surveys available in the Ocean City region.

Survey ID Year Scale Original Vertical 

Datum

Downloaded 

Horizontal Datum Vertical Datum Conversion Used

D00023 1984 40,000 MLLW NAD 83 geographic Tide station offset

F00383 1993 20,000 MLLW NAD 83 geographic Tide station offset

F00437 1997 10,000 MLLW NAD 83 geographic Tide station offset

F00453 1999 10,000 MLLW NAD 83 geographic Tide station offset

H01455A 1880 20,000 MLW NAD 83 geographic VDatum

H01455B 1887 20,000 MLW NAD 83 geographic VDatum

H01816 1887 20,000 MLW NAD 83 geographic VDatum

H05230** 1932 5,000 MLW NAD 83 geographic Tide station offset

H05346* 1933 20,000 MLW NAD 83 geographic Tide station offset

H05347* 1934 20,000 MLW NAD 83 geographic Tide station offset

H05348* 1933 40,000 MLW NAD 83 geographic Tide station offset

H05349* 1933 20,000 MLW NAD 83 geographic Tide station offset

H05350* 1933 120,000 MLW NAD 83 geographic Tide station offset

H05351* 1933 40,000 MLW NAD 83 geographic Tide station offset

H05353 1934 40,000 MLW NAD 83 geographic Tide station offset

H05354* 1933 20,000 MLW NAD 83 geographic Tide station offset

H05355 1934 40,000 MLW NAD 83 geographic Tide station offset

H05356 1934 40,000 MLW NAD 83 geographic Tide station offset

H05357* 1933 20,000 MLW NAD 83 geographic Tide station offset

H05358* 1933 20,000 MLW NAD 83 geographic Tide station offset

H05673 1934 40,000 MLW NAD 83 geographic Tide station offset

H05675 1934 10,000 MLW NAD 83 geographic VDatum

H05770 1934 40,000 MLW NAD 83 geographic Tide station offset

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

by NOAA’s National Geodetic Survey (NGS) to convert NOS hydrographic survey data from NAD 27 geographic to NAD 83 geographic. NADCON is the U.S. Federal Standard for NAD 27 geographic to NAD 83 geographic datum transformations.
Grothe et al., 2010

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Survey ID Year Scale Original Vertical 

Datum

Downloaded 

Horizontal Datum Vertical Datum conversion used

H05771 1934 40,000 MLW NAD 83 geographic Tide station offset

H05702 1934 40,000 MLW NAD 83 geographic Tide station offset

H05703 1934 20,000 MLW NAD 83 geographic Tide station offset

H05713 1938 120,000 MLW NAD 83 geographic Tide station offset

H05714* 1934 20,000 MLW NAD 83 geographic Tide station offset

H05715 1934 40,000 MLW NAD 83 geographic Tide station offset

H05716 1934 10,000 MLW NAD 83 geographic VDatum

H05769 1934 10,000 MLW NAD 83 geographic VDatuml

H06232* 1937 10,000 MLW NAD 83 geographic Tide station offset

H06264* 1937 40,000 MLW NAD 83 geographic Tide station offset

H06272* 1937 40,000 MLW NAD 83 geographic Tide station offset

H06344* 1938 40,000 MLW NAD 83 geographic Tide station offset

H07034 1945 10,000 MLW NAD 83 geographic Tide station offset

H07035* 1945 10,000 MLW NAD 83 geographic Tide station offset

H07946** 1951 20,000 MLW NAD 83 geographic Tide station offset

H08710 1962 10,000 MLW NAD 83 geographic VDatum

H08711* 1962 10,000 MLW NAD 83 geographic Tide station offset

H08596* 1963 10,000 MLW NAD 83 geographic Tide station offset

H09136 1970 20,000 MLW NAD 83 geographic Tide station offset

H09153 1971 20,000 MLW NAD 83 geographic Tide station offset

H09154 1970 10,000 MLW NAD 83 geographic Tide station offset

H09175 1970 10,000 MLW NAD 83 geographic Tide station offset

H09176 1970 10,000 MLW NAD 83 geographic Tide station offset

H09202 1971 20,000 MLW NAD 83 geographic Tide station offset

H09203 1971 10,000 MLW NAD 83 geographic Tide station offset

H09204 1971 5,000 MLW NAD 83 geographic Tide station offset

H09311 1972 10,000 MLW NAD 83 geographic Tide station offset

H09312 1972 20,000 MLW NAD 83 geographic Tide station offset

H09578 1975 20,000 MLW NAD 83 geographic Tide station offset

H09579 1975 20,000 MLW NAD 83 geographic Tide station offset

H09629 1976 40,000 MLW NAD 83 geographic Tide station offset

H09639 1976 40,000 MLW NAD 83 geographic Tide station offset

H09640 1976 40,000 MLW NAD 83 geographic Tide station offset

H09663 1976 80,000 MLW NAD 83 geographic Tide station offset

H09699* 1977 20,000 MLW NAD 83 geographic Tide station offset

H09700* 1977 20,000 MLW NAD 83 geographic Tide station offset

H09714 1977 20,000 MLW NAD 83 geographic Tide station offset

H09715 1978 10,000 MLW NAD 83 geographic VDatum

H09722 1977 5,000 MLW NAD 83 geographic Tide station offset
Digital Elevation Model of Ocean City, Maryland

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Survey ID Year Scale Original Vertical 

Datum

Downloaded 

Horizontal Datum Vertical Datum conversion used

H09723* 1977 20,000 MLW NAD 83 geographic Tide station offset

H09727 1977 20,000 MLW NAD 83 geographic Tide station offset

H09759 1978 20,000 MLW NAD 83 geographic Tide station offset

H09764 1978 20,000 MLW NAD 83 geographic Tide station offset

H09780 1978 20,000 MLW NAD 83 geographic Tide station offset

H09788 1978 20,000 MLW NAD 83 geographic Tide station offset

H09796 1978 20,000 MLW NAD 83 geographic Tide station offset

H10044 1982 20,000 MLW NAD 83 geographic Tide station offset

H10045 1982 20,000 MLW NAD 83 geographic Tide station offset

H10046 1982 20,000 MLW NAD 83 geographic Tide station offset

H10234 1994 10,000 MLLW NAD 83 geographic Tide station offset

H10241 1994 10,000 MLLW NAD 83 geographic Tide station offset

H10439 1992 20,000 MLLW NAD 83 geographic Tide station offset

H10440 1992 20,000 MLLW NAD 83 geographic Tide station offset

H10444 1993 20,000 MLLW NAD 83 geographic Tide station offset

H10446 1993 20,000 MLLW NAD 83 geographic Tide station offset

H10464 1993 20,000 MLLW NAD 83 geographic Tide station offset

H10475 1993 20,000 MLLW NAD 83 geographic Tide station offset

H10476 1993 20,000 MLLW NAD 83 geographic Tide station offset

H10489 1993 20,000 MLLW NAD 83 geographic Tide station offset

H10533 1994 10,000 MLLW NAD 83 geographic Tide station offset

H10573 1994 10,000 MLLW NAD 83 geographic Tide station offset

H10854 1999 10,000 MLLW NAD 83 geographic Tide station offset

H10917 1999 10,000 MLLW NAD 83 geographic Tide station offset

H10926 2000 10,000 MLLW NAD 83 geographic Tide station offset

H10931 1999 10,000 MLLW NAD 83 geographic Tide station offset

H10935 1999 20,000 MLLW NAD 83 geographic Tide station offset

H10936 1999 20,000 MLLW NAD 83 geographic Tide station offset

H10989 2000 40,000 MLLW NAD 83 geographic Tide station offset

H11081 2002 20,000 MLLW NAD 83 geographic Tide station offset

H11104 2002 20,000 MLLW NAD 83 geographic Tide station offset

H11243 2004 20,000 MLLW NAD 83 geographic Tide station offset

*Surveys not used in the final DEM because they were superseded by newer surveys

**Surveys not used in the final DEM because they are located in interior creeks outside the western extent of the DEM
Grothe et al., 2010

12

Figure 9. Digital NOS hydrographic survey coverage in the Ocean City region. Some earlier surveys were not used as they 

have been superseded by more recent surveys. Three surveys outside the western extent of the DEM (H05230, H07947, and 

H07946) also were not used. DEM boundary in red.
Digital Elevation Model of Ocean City, Maryland

13

2) United States Army Corps of Engineers hydrographic surveys

Four hydrographic surveys are available from the USACE, Baltimore District (Table 6; Fig 10). Two

surveys (s52waug08a, s052i026mar09a) were downloaded as xyz data and two surveys (s052i08may09,

s052s37mar09a) were provided to NGDC by John Hill from the USACE Baltimore District. The surveys

were collected in 2008 and 2009, and referenced to NAD 83 geographic Maryland State Plane (feet) and

MLLW (feet) datums. The files were converted to NAD 83 geographic and MHW (meters) using VDatum.

Point spacing averages 4 meters along profiles approximately 150 meters long and averaging 20 meters apart.

Table 6: USACE hydrographic surveys used in building the Ocean City DEM.

Survey File Year of 

Survey Survey Scale Original Vertical Datum Original Horizontal Datum

s52waug08a 2008

5700 x 180 meter area with

~30 meter line spacing and

~3 meter point spacing

NAD 83 geographic

Maryland State Plane (feet) MLLW (feet)

s052i026mar09a 2009

920 x 140 meter area with ~30

meter line spacing and

~4 meter point spacing

NAD 83 geographic

Maryland State Plane (feet) MLLW (feet)

s052i08may09 2009

140 x 75 meter area with ~15

meter line spacing and

~3 meter point spacing

NAD 83 geographic

Maryland State Plane (feet) MLLW (feet)

s052s37mar09a 2009

1775 x 175 meter area with

~30 meter line spacing and

~3 meter point spacing

NAD 83 geographic

Maryland State Plane (feet) MLLW (feet)

Figure 10. Digital USACE hydrographic survey 

coverage in the Ocean City region.
Grothe et al., 2010

14

2) Digitized Inlets and harbors

The Ocean City Harbor consists of many man-made inlets and harbors. Most of these inlets and harbors have

not been surveyed by NOS or USACE. Therefore, the grid does not represent accurate depths for these areas. To

maintain negative depths in these areas, a “pre-surface” bathymetric grid (see Sec. 3.3.2) was created with the

coastline values at -1 meters. For most places, this maintained negative depths but in some of the narrow finger

channels, gridding introduced artificial bridges in the waterways. NGDC digitized soundings with values of -2

meters to properly represent these inlets in the DEM. There is also one boat harbor right inside the Ocean City

Inlet that does not exist in the data or on RNCs but is visible in satellite imagery. NGDC manually adjusted the

coastline and digitized depths of -3 meters in the harbor (Fig. 11).

Figure 11. New boat harbor 

shown in satellite imagery 

but not included on the 2007 

RNC #12211 or other bathymetric data. NGDC adjusted 

the coastline using the satellite imagery and digitized -3 

meters depth. 
Digital Elevation Model of Ocean City, Maryland

15

3.1.3 Topography

	 The topographic datasets used to build the Ocean City DEM include: USGS NED 1/3 arc-second DEM,

National Elevation Dataset (NED) 1/9 arc-second DEM, and 2008 bare-earth topographic lidar; 2002 bare-earth

Worchester County lidar, 2005 bare-earth Sussex County lidar and 2005 USACE non-bare-earth lidar available from

CSC’s web site; and NGDC digitized hard structures (jetties and breakwaters; Table 7; Fig. 12). Many lidar surveys are

available through the CSC web site for the Ocean City region dating back to 1996, but NGDC only downloaded and

evaluated surveys post 2000. NGDC also evaluated but did not use the Shuttle Radar Topography Mission (SRTM)

Elevation 1 arc-second DEM available from USGS.

Table 7: Topographic datasets used in building the Ocean City DEM.

Source Year Data Type Spatial Resolution

Original Horizontal 

Datum/Coordinate 

System

Original Vertical 

Datum URL

USGS 1999-2007 NED DEM 1/3 arc-second NAD 83 geographic NAVD 88 (meters) http://ned.usgs.gov

USGS 2002-2007 NED DEM 1/9 arc-second NAD 83 geographic NAVD 88 (meters) http://ned.usgs.gov

USGS 2008 Bare-earth lidar 2 meters NAD 83 UTM meters

Zone 18N NAVD 88 (meters) http://pubs.usgs.gov/

ds/447

CSC 2005 Sussex County bareearth lidar 3 meters NAD 83 geographic NAVD 88 (meters) http://www.csc.noaa.gov

CSC 2005 Non-bare-earth lidar 2 meters NAD 83 geographic NAVD88 (meters) http://www.csc.noaa.gov

CSC 2002 Worchester County

bare-earth lidar 2 meters NAD 83 geographic NAVD 88 (meters) http://www.csc.noaa.gov

NGDC 2009 Points NAD 83 geographic MHW
Grothe et al., 2010

16

Figure 12. Spatial coverage of 

topographic datasets used in building 

the Ocean City DEM.
Digital Elevation Model of Ocean City, Maryland

17

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

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

City DEM region4. Data are in NAD 83 geographic coordinates and NAVD 88 vertical datum (meters), and

are available for download as raster DEMs. The bare-earth elevations have a vertical accuracy of +/- 7 to 15

meters depending on source data resolution. See the USGS Seamless web site for specific source information

(http://seamless.usgs.gov). The dataset was derived from USGS quadrangle maps, aerial photographs based

on topographic surveys, and topographic lidar; it has been revised using data collected in 1999 to 2007. The

NED DEM includes “zero” elevation values over the open ocean, which were removed from the dataset by

clipping to the final coastline.

Regions of the NED 1/3 arc-second DEM were sub-sampled from the NED 1/9 arc-second DEM, which

is derived from lidar data. NGDC used the NED 1/3 arc-second DEM in those regions since the higher

resolution data were already incorporated (see Fig. 13). The NED 1/3 arc-second DEM was also used where

no NED 1/9 arc-second data or lidar data existed.

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

USGS provides limited high-resolution NED 1/9 arc-second DEMs, derived from 3 meter pointspacing lidar data. Data are in NAD 83 geographic coordinates and NAVD 88 vertical datum (meters), and

are available for download as raster DEMs. The horizontal accuracy is 3 meters and the vertical accuracy,

depending on input of source data, is less than 22 centimeters. The NED DEM included “zero” elevation

values over the open ocean, which were removed from the dataset by clipping to the final coastline.

NGDC only used the NED 1/9 arc-second DEM where it was not incorporated in the NED 1/3 arc-second

DEM (Fig. 13). The data were clipped to the coastline to remove values over the open water. Problems in the

data included vertical stripes in the north of the DEM derived from the original lidar data (Fig. 14).

4. The USGS National Elevation Dataset (NED) has been developed by merging the highest-resolution, best quality elevation data available across

the United States into a seamless raster format. NED is the result of the maturation of the USGS effort to provide 1:24,000-scale Digital Elevation

Model (DEM) data for the conterminous U.S. and 1:63,360-scale DEM data for Georgia. The dataset provides seamless coverage of the United

States, HI, AK, and the island territories. NED has a consistent projection (Geographic), resolution (1 arc second), and elevation units (meters). The

horizontal datum is NAD 83 geographic, except for AK, which is NAD 27 geographic. The vertical datum is NAVD 88, except for AK, which is

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

covers the U.S., then this will also be a seamless dataset. [Extracted from USGS NED web site]	

Figure 13. Spatial coverage of the NED 1/9 arcsecond DEM available versus used in the final 

Ocean City DEM. The data was not used because 

it was already incorporated in the NED 1/3 arcsecond DEM, which is sufficient for the 1/3 arcsecond DEM of Ocean City
Grothe et al., 2010

18

Figure 14. Vertical stripes in marshy areas in the NED 1/9 arc-second DEM. 
Digital Elevation Model of Ocean City, Maryland

19

3) United States Geological Survey 2008 bare-earth topographic lidar

USGS provided NGDC with 2008 bare-earth lidar of Assateague Island National Seashore (Fig. 15). The

survey was divided into tiles 2 km by 2 km and is available for download or by DVD. Emily Klipp provided

NGDC with a DVD of all tiles in las and xyz format. The horizontal accuracy of the survey is one meter and

the vertical accuracy is +/- 15 centimeters.

The original horizontal and vertical datums are NAD 83 UTM Zone 18N and NAVD 88. NGDC used

FME to transform the datums to NAD 83 geographic and MHW, and to convert the xyz files into shapefiles

for editing in ArcMap. This survey supersedes all other topographic data and older data were clipped where

they overlap this survey. Values over the open water were manually deleted before converting the data back

into xyz for the final gridding process.

Figure 15. USGS 2008 bare-earth lidar data on Assateague Island.
Grothe et al., 2010

20

4) Coastal Services Center 2005 bare-earth topographic lidar

A 2005 bare-earth topographic lidar survey was downloaded from the CSC web site. The topographic

lidar, collected for the State of Delaware in March 2005, was provided to CSC by Delaware Natural

Resources and Environmental Control, Delaware Coastal Programs. The survey’s ground spacing is 3 meters,

and has a vertical accuracy of 10.2 centimeters and a horizontal accuracy that has not been tested. The data

were downloaded in NAD 83 geographic horizontal datum and NAVD 88 vertical datum. FME was used to

transform the data to MHW and convert to shapefiles for editing in ArcMap.

	 		 NGDC only used the tiles from this dataset where there are no NED DEMs (1/3 arc-second or 1/9 arcsecond) with lidar already incorporated into the data. Problems in the data include returns over the open water

and a missing strip of data (Fig. 16). The values over the open water were manually deleted in ArcMap. The

data were then converted back to xyz using FME for the final gridding process.

Figure 16. Problems in the CSC 2005 bare-earth lidar illustrated in a gridded image. A strip of missing data caused a large gap. Water returns, 

seen as stripes, needed to be deleted before the final gridding process.
Digital Elevation Model of Ocean City, Maryland

21

5) 2005 Coastal Services Center non-bare-earth topographic lidar

A 2005 USACE topographic non-bare earth lidar survey was downloaded from the CSC web site. The

data were collected by the Joint Airborne Lidar Bathymetry Technical Center of Expertise (JALBTCX).

Metadata states this is a topographic-bathymetric survey, but there were no hydrographic data in the survey.

The survey has a vertical accuracy of 0.2 meters and horizontal accuracy of 0.75 meters with 2-meter point

spacing. The data were downloaded as xyz files in NAD 83 geographic horizontal datum and NAVD 88

vertical datum. The xyz files were transformed to MHW and converted to a shapefile using FME.

This survey covers the entire coastline in the Ocean City DEM, but because the data were not processed

to bare earth, the survey was only used to fill in the missing strip of data in the CSC 2005 bare-earth lidar

survey. NGDC manually removed rooftops of houses in ArcMap (Fig. 17).

6) Coastal Services Center 2002 bare-earth topographic lidar

A 2002 bare-earth topographic lidar survey, contracted by the Maryland Department of Natural

Resources, was downloaded from the CSC web site. The survey’s ground spacing is 2 meters, and has a

vertical accuracy of 21.3 centimeters and a horizontal accuracy appropriate for a 1:24,000 scale map. The

data were download as xyz files in NAD 83 geographic and NAVD 88. They were transformed to MHW

and converted to shapefiles for editing in ArcMap using FME. NGDC only used this data where there are no

NED DEMs with lidar already incorporated into the data or newer topographic lidar data (see Fig. 12). Water

returns were manually removed before converting the data back to xyz files for the final gridding process.

7) NGDC digitized structures

NGDC digitized positive elevations of breakwaters and jetties in the Ocean City DEM which were not

represented in any topographic data (Fig. 18). Two breakwaters at the mouth of Delaware Bay were digitized

at 5 meters elevation. Several jetties (Delaware Bay, Indian Rivet Inlet, and Ocean City Inlet) were digitized

at 1 meter elevation.

Figure 17. Spatial coverage of the 2005 

non-bare-earth topographic dataset used 

in building the Ocean City DEM with the 

resulting shapefile after rooftops were 

deleted to approximate bare earth.
Grothe et al., 2010

22

Figure 18. NGDC-digitized breakwaters and 

jetties at A) Delaware Bay, B) Ocean City 

Inlet, and C) India River Inlet.
Digital Elevation Model of Ocean City, Maryland

23

3.2 Establishing Common Datums

3.2.1 Vertical datum transformations

	 Datasets used in the compilation and evaluation of the Ocean City DEM were originally referenced to

several vertical datums including MLLW, MLW, and NAVD 88. All datasets were transformed to MHW to provide the

maximum flooding for inundation modeling. Units were converted from feet to meters as appropriate.

1) Bathymetric data

The NOS and USACE hydrographic surveys were transformed from MLLW and MLW to MHW, using

the VDatum transformation tool developed by OCS and NGS or FME software by adding a constant taken

from the Ocean City, Fishing Pier tide station # 8570280 (see Fig. 20).

VDatum coverage was only available for the intertidal region of the Ocean City DEM (Fig. 19). NOS

and USACE surveys that fell in this range were converted from MLLW or MLW to MHW using VDatum. In

some cases, the surveys extended farther inshore than the VDatum coverage, in which case NGDC manually

changed the vertical datum to MHW by comparing the conversion of nearby soundings.

No VDatum exists in the open ocean. NGDC used a constant offset of the differences from MLW (1.025

meters) and MLLW (1.073 meters) to MHW from the Ocean City, Fishing Pier tide station # 8570280.

2) Topographic data

Figure 19. Spatial coverage of the VDatum 

transformation tool.
Grothe et al., 2010

24

The USGS NED 1/3 arc-second DEM, NED 1/9 arc-second DEM, 2008 bare-earth topographic lidar

data, and the CSC topographic lidar data were downloaded and referenced to NAVD 88 vertical datum.

Transformations to MHW, using FME software or ArcGIS, was accomplished by adding an averaged constant

offset of -0.19 meters (Table 8) as measured at various tide stations in the Ocean City region (Fig. 20).

Figure 20. Location of NOAA tide stations near Ocean City.

Table 8. Relationship between MHW and NAVD 88 vertical datums at tide stations located within the Ocean City 

DEM extent.

Tide Station Station ID Difference from NAVD 88 to MHW

Keydash, Isle of Wight Bay 8570255 0.13

Ocean City 8570282 0.194

Ocean City Inlet 8570283 0.184

Lewes 8557380 0.488

Indian River Inlet 8558690 0.257

Chincoteague, USCG Station 8630249 0.189
Digital Elevation Model of Ocean City, Maryland

25

3.2.2 Horizontal datum transformations

	 Datasets used to build the Ocean City DEM were downloaded and referenced to WGS 84 geographic, NAD

83 UTM Zone 18 North, NAD 83 Maryland State Plane, or NAD 83 geographic horizontal datums. The relationships

and transformational equations between these horizontal datums are well established. Data were converted to a

horizontal datum of NAD 83 geographic using FME software or ArcGIS.

3.3 Digital Elevation Model Development

3.3.1 Verifying consistency between datasets

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

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

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

preparation for gridding. Problems included:

•	 Missing strip of data in the CSC 2005 bare-earth topographic lidar survey.

•	 Inconsistent elevation values in the NED 1/3 arc-second DEM and higher resolution lidar data.

•	 Topographic lidar dataset not processed to bare-earth. The dataset required manual editing of individual

features.

•	 Bathymetric values in older NOS surveys dating back over 70 years are inconsistent with newer NOS and

USACE surveys.

•	 Water returns in NED DEMs and topographic lidar data.
Grothe et al., 2010

26

3.3.2 Smoothing of bathymetric data

	 The NOS hydrographic survey data are generally sparse at the resolution of the 1/3 arc-second Ocean City

DEM (see Fig. 24): in deep water, the NOS survey data have point spacing up to 2000 meters apart, and some shallow

water up to 1000 meters apart. In order to reduce the effect of artifacts in the form of lines or “pimples” in the DEM

due to low resolution datasets, and to provide effective interpolation in the deep water and into the coastal zone, a

1 arc-second-spacing “pre-surface” bathymetric grid was generated using GMT5, a NSF-funded shareware software

application designed to manipulate data for mapping purposes.

	 The NOS hydrographic point data were clipped to remove overlap with USACE soundings, and older NOS

surveys with newer NOS surveys. The coastline elevation value was set at -1 meters to ensure a bathymetric surface

below zero in areas where data are sparse or non-existent.

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

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

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

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

areas). The resulting surface was compared with original soundings to ensure grid accuracy (e.g., Figs. 21 and 22), and

then exported as an xyz file for use in the final gridding process (see Table 9).

Figure 21. Histogram of the differences between all NOS hydrographic surveys and the 1 arc-second pre-surface 

bathymetric grid.

Figure 22. Histogram of the differences between all USACE hydrographic surveys and the 1 arc-second pre-surfaced 

bathymetric grid.

5.GMTisanopensourcecollectionof~60toolsformanipulatinggeographicandCartesiandatasets(includingfiltering,trendfitting,gridding,projecting,

etc.) and producing Encapsulated PostScript File (EPS)illustrationsranging from simple x-y plots via contour mapsto artificially illuminated surfaces

and 3-Dperspective views.GMTsupports ~30map projections and transformations and comeswith support data such asGSHHScoastlines,rivers, and

politicalboundaries.GMTisdevelopedandmaintainedbyPaulWesselandWalterH.F.Smithwithhelpfromaglobalsetofvolunteers,andissupportedby

theNationalScienceFoundation.ItisreleasedundertheGNUGeneralPublicLicense.URL:http://gmt.soest.hawaii.edu[ExtractedfromGMTwebsite.]
Digital Elevation Model of Ocean City, Maryland

27

3.3.3 Integrating topographic datasets

	 Many different topographic surveys are available for the Ocean City region. To represent the most current

morphology and integrate the data smoothly, NGDC used a 50 meter overlap between different high-resolution

datasets to allow averaging of five cells, creating a seamless border. When integrating lower resolution datasets next

to higher resolution datasets, NGDC used a 50 meter buffer to allow smoothing from interpolation. NGDC used the

buffer to fill the CSC 2005 bare-earth topographic lidar survey data gap with the NED 1/3 arc-second DEM and the

CSC 2005 non-bare-earth topographic lidar survey. Figure 23 illustrates there is still a noticeable difference between

the three datasets, but no other higher resolution data were available of that area.

Figure 23. Problems in the final grid due to a missing strip of lidar data filled in with the NED 1/3 arc-second DEM.
Grothe et al., 2010

28

3.3.4 Gridding the data with MB-System

MB-System6 was used to create the 1/3 arc-second Ocean City DEM. MB-System is an NSF-funded shareware

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 apply a tight spline

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

algorithm, as relative gridding weights, is listed in Table 9. Equal weight was given to all datasets except the NED 1/3

arc-second DEM and the “pre-surface” bathymetric grid. Gridding was performed in quadrants with the resulting Arc

ASCII grids seamlessly merged in ArcCatalog to create the final 1/3 arc-second Ocean City DEM. Figure 24 illustrates

cells in the DEM that have interpolated values (shown as white) versus data contributing to the cell value (shown in

black).

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

Dataset Relative Gridding Weight

NOS hydrographic surveys 100

USACE hydrographic surveys 100

Digitized features 100

CSC 2002 bare-earth lidar 100

CSC 2005 bare-earth lidar 100

CSC 2005 non-bare-earth lidar 100

USGS 2008 bare-earth lidar 100

USGS NED 1/9 arc-second topographic DEM 100

USGS NED 1/3 arc-second topographic DEM 10

Pre-surfaced bathymetric grid 0.01

6. MB-System is an open source software package for the processing and display of bathymetry and backscatter imagery data derived from multibeam,

interferometry, and sidescan sonars. The source code for MB-System is freely available (for free) by anonymous ftp (including “point and click”

accessthrough these web pages).Acomplete description is provided in web pages accessed through the web site. MB-System was originally developed

at the Lamont-Doherty Earth Observatory of Columbia University (L-DEO) and is now a collaborative effort between the Monterey Bay Aquarium

Research Institute (MBARI) and L-DEO. The National Science Foundation has provided the primary support for MB-System development since

1993. The Packard Foundation has provided significant support through MBARI since 1998. Additional support has derived from SeaBeam Instruments (1994-1997), NOAA (2002-2004), and others. URL: http://www.ldeo.columbia.edu/res/pi/MB-System[Extracted from MB-System web site.]

Figure 24. Ocean City DEM data distribution plot. White 

denotes no data contributed to the cell value; black denotes data 

contributed to the cell value. Final coastline in red.
Digital Elevation Model of Ocean City, Maryland

29

3.4 Quality Assessment of the DEM

3.4.1. Horizontal accuracy

	 The horizontal accuracy of topographic and bathymetric features in the Ocean City DEM is dependent

upon the datasets used to determine corresponding DEM cell values and the cell size of the DEM. For topographic

features, the horizontal accuracy is 10 meters (see Sec. 3.1.3 for individual topographic datasets horizontal accuracy).

Bathymetric features are resolved only to within a few tens of meters in deep-water areas. Shallow, near-coastal

regions, rivers, and harbor surveys have an accuracy approaching that of sub aerial topographic features. Positional

accuracy is limited by: the sparseness of deep-water soundings; potentially large positional uncertainty of pre-satellite

navigated (e.g., GPS) NOS hydrographic surveys; and by the morphologic change that occurs in this dynamic region.

3.4.2 Vertical accuracy

	 Vertical accuracy of the Ocean City DEM is also highly dependent upon the source datasets contributing

to DEM cell values. Topographic areas have an estimated vertical accuracy between 15 to 21.3 centimeters for lidar

derived data, and 7 to 15 meters for NED 1/3 arc-second topography. Bathymetric areas have an estimated accuracy

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

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

values between sparse, poorly-located NOS soundings degrades the vertical accuracy of elevations.
Grothe et al., 2010

30

3.4.3 Slope maps and 3-D perspectives

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

and identification of artificial slopes along boundaries between datasets (e.g., Fig. 23). 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 (e.g., Fig. 25). 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 26 shows a perspective view image of the 1/3 arcsecond Ocean City DEM in its final version.

Figure 25. Slope map of the Ocean 

City DEM. Flat-lying slopes are white; 

dark shading denotes steep slopes; final 

coastline in red.
Digital Elevation Model of Ocean City, Maryland

31

Figure 26. Perspective view from the southeast of the Ocean City DEM. Vertical exaggeration–times 20.
Grothe et al., 2010

32

3.4.4 Comparison with source data files

	 To ensure grid accuracy, the Ocean City DEM was compared to source data files. All bathymetric data and

select topographic data files from each dataset were chosen for comparison to the Ocean City DEM using Fledermaus, 

FME and ArcMap. A histogram of the differences between all NOS hydrographic surveys and the Ocean City DEM

is shown in Figure 27 and all the USACE hydrographic surveys and the Ocean City DEM is shown in Figure 28.

Differences cluster around zero. The major differences in elevations in NOS surveys with the grid (-16 meters and

+10 meters) are from digitized breakwaters and jettys that were only represented in the final DEM. There is only one

anomalous point (-23 meters) between the USACE hydrographic surveys and the grid, which is due to a bad sounding.

NGDC manually deleted the sounding.

Figure 27. Histogram of the differences between all the NOS surveys and the Ocean City DEM.

Figure 28. Histogram of the differences between all the USACE surveys and the Ocean City DEM.

	 Histograms to compare the differences between the topographic datasets and the Ocean City DEM were

created for gridding evaluation (Figs. 29 - 34). All data points for the CSC 2002 bare-earth lidar survey and the CSC

2005 bare-earth lidar were used for comparison. Only select data points from the CSC 2005 bare-earth lidar survey, the

2008 USGS lidar survey, and the NED 1/3 and 1/9 arc-seconds DEM were used for comparison against the elevation

of the grid because the data points were too dense to run a comparison on all data points in the DEM. Differences

cluster around zero for all surveys, with the differences ranging from -2.25 meters to 3.7 meters. The CSC 2005 nonbare-earth survey was the only survey with a difference greater than 3 meters, and this is due to crude processing of

the data to bare-earth.
Digital Elevation Model of Ocean City, Maryland

33

Figure 29. Histogram of the differences between all the CSC 2002 bare-earth lidar data and the Ocean City DEM.

Figure 30. Histogram of the differences between select CSC 2005 bare-earth lidar data and the Ocean City DEM.

Figure 31. Histogram of the differences between all CSC 2005 non-bare-earth topographic-bathymetric lidar data and the 

Ocean City DEM.
Grothe et al., 2010

34

Figure 32. Histogram of the differences between select USGS 2008 bare-earth lidar data and the Ocean City DEM.

Figure 33. Histogram of the differences between select NED 1/3 arc-second topographic DEM data points and the Ocean 

City DEM.

Figure 34. Histogram of the differences between select NED 1/9 arc-second topographic DEM data points and the Ocean 

City DEM.
Digital Elevation Model of Ocean City, Maryland

35

3.4.5 Comparison with the National Geodetic Survey geodetic monuments

	 The elevations of 1,221 NOAANGS geodetic monuments were extracted from online shapefiles of monument

datasheets (http://www.ngs.noaa.gov/cgi-bin/datasheet.prl), which give monument positions in NAD 83 geographic

(typically sub-mm accuracy) and elevations in NAVD 88 (in meters). Elevations were shifted to MHW vertical datum

for comparison with elevations of the Ocean City DEM (Fig. 35). Differences between the Ocean City DEM and the

NGS geodetic monument elevations range from -10 to 25 meters, with the majority of them being within +/-1 meter

(Fig. 36). Negative values indicate that the monument elevation is less than the DEM elevation. Only 16 monuments

out of 1221 total showed deviations greater than 5 meters from the DEM. After examination, it was determined that

those monuments 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 36. Histogram of the differences between NGS geodetic monument elevations and the Ocean City DEM.

Figure 35. Location of NGS geodetic monuments, shown as 

green circles. NGS monument elevations were used to evaluate 

the DEM.
Grothe et al., 2010

36

4. Summary and Conclusions

	 A bathymetric–topographic digital elevation model of the Ocean City, Maryland region, with cell spacing of

1/3 arc-second, was developed for the Pacific Marine Environmental Laboratory (PMEL) NOAA Center for Tsunami

Research. The best available digital data from U.S. federal, state 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, Quick Terrain Modeler, and Fledermaus

software.

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

•	 Conduct up-to-date topographic lidar surveys for all near-shore regions.

•	 Conduct NOS hydrographic surveys in hydrographic data gaps and the new harbor.

•	 Process CSC 2005 non-bare-earth topographic lidar data to bare-earth.

•	 Complete processing of the USACE hydrographic surveys in Sinepuxent Bay.

5. Acknowledgments

	 The creation of the Ocean City DEM was funded by the NOAA Pacific Marine Environmental Laboratory.

The authors thank Nazila Merati,Chris Chamberlin, Marie Eble, and Vasily Titov (PMEL); Emily Klipp from the

USGS for sending the 2008 bare-earth lidar on DVD; and John Hill from the USACE Philadelphia District office for

providing USACE surveys that were not available on their web site.

6. References

Nautical Chart #12210, 38th Edition, 2008. Chincoteague Inlet to Great Machipongo Inlet. Scale 1:80,000. U.S.

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

Nautical Chart #12211, 43rd Edition, 2007. Fenwick Island to Chincoteague Inlet. Scale 1:80,000 and 1:20,000. U.S.

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

Nautical Chart #12214, 48th Edition, 2007. Cape May to Fenwick. Scale 1:80,000. U.S. Department of Commerce,

NOAA, National Ocean Service, Coast Survey.

Nautical Chart #12216, 28th Edition, 2008. Cape Henlopen to Indian River Inlet. Scale 1:80,000. U.S. Department of

Commerce, NOAA, National Ocean Service, Coast Survey.

Nautical Chart #12304, 45th Edition, 2008. Delaware Bay. Scale 1:40,000 and 1:10,000. U.S. Department of

Commerce, NOAA, National Ocean Service, Coast Survey.
Digital Elevation Model of Ocean City, Maryland

37

7. Data Processing Software

ArcGIS v. 9.3, developed and licensed by ESRI, Redlands, California, http://www.esri.com

ESRI Imagery World 2D Online World Imagery 2D – ESRI ArcGIS Resource Centers, http://resources.esri.com/

arcgisonlineservices

Fledermaus v. 7.0 – developed and licensed by Interactive Visualization Systems (IVS 3D), Fredericton, New

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

FME 2007 GB – Feature Manipulation Engine, developed and licensed by Safe Software, Vancouver, BC, Canada,

http://www.safe.com

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

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

GMT v. 4.4.0 – Generic Mapping Tools, shareware developed and maintained by Paul Wessel and Walter Smith,

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

MB-System v. 5.1.1, shareware developed and maintained by David W. Caress and Dale N. Chayes, funded by the

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

Quick Terrain Modeler v. 6.0.6, Lidar processing software developed by John Hopkins University’s Applied Physics

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

Datum Transformation Tool, Delaware, Maryland, Virginia Embayment V. 01 – developed and maintained by

NOAA’s National Geodetic Survey (NGS), Office of Coast Survey (OCS), and Center for Operational Oceanographic Products and Services (CO-OPS), http://vdatum.noaa.gov/welcome.html