Digital Elevation Model of Ocean City, Maryland:

Procedures, Data Sources and Analysis

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

Research by the NOAA National Geophysical Data Center (NGDC)

October 15, 2009

Medley, P.R.1, L.A. Taylor2, B.W. Eakins1, R.R. Warnken2, K.S. Carignan1, E. Lim1, R.J. Caldwell1, and D.Z. Friday1

Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder1

NOAA, National Geophysical Data Center, Boulder, Colorado2

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/

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..........................................................................................................14

3.2 Establishing Common Datums .................................................................................................22

3.2.1 Vertical datum transformations ...........................................................................22

3.2.2 Horizontal datum transformations.......................................................................24

3.3 Digital Elevation Model Development .....................................................................................24

3.3.1 Verifying consistency between datasets..............................................................24

3.3.2 Smoothing of bathymetric data ...........................................................................25

3.3.3 Integrating topographic datasets..........................................................................26

3.3.4 Gridding the data with MB-System......................................................................27

3.4 Quality Assessment of the DEM...............................................................................................28

3.4.1 Horizontal accuracy.............................................................................................28

3.4.2 Vertical accuracy .................................................................................................28

3.4.3 Slope maps and 3-D perspectives........................................................................29

3.4.4 Comparison with source data files.......................................................................31

3.4.5 Comparison with NGS geodetic monuments......................................................34

4. Summary and Conclusions...............................................................................................................35

5. Acknowledgments............................................................................................................................35

6. References........................................................................................................................................35

7. Data Processing Software ................................................................................................................36

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 ....................15

Figure 13. Spatial coverage of the NED 1/9 arc-second available in the Ocean City region ....................16

Figure 14. Water returns in marshy areas of the NED 1/9 arc-second DEM.............................................17

Figure 15. USGS 2008 bare-earth lidar data on Assateague Island ...........................................................18

Figure 16. Problems in the CSC 2005 bare-earth lidar illustrated in a gridded image ..............................19

Figure 17. Spatial coverage of the 2005 non-bare-earth topographic dataset............................................20

Figure 18. NGDC digitized breakwaters and jetties...................................................................................21

Figure 19. Spatial coverage of the VDatum transformation tool................................................................22

Figure 20. Location of NOAA tide stations near Ocean City ....................................................................23

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

1 arc-second pre-surface bathymetric grid ........................................................................25
Medley et al., 2009

ii

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

1 arc-second pre-surfaced bathymetric grid ......................................................................25

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..........................................................................................................26

Figure 24. Ocean City DEM data distribution plot ....................................................................................27

Figure 25. Slope map of the Ocean City DEM ..........................................................................................29

Figure 26. Perspective view from the southeast of the Ocean City DEM..................................................30

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

DEM ..................................................................................................................................31

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

City DEM...........................................................................................................................31

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

and the Ocean City DEM...................................................................................................32

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

and the Ocean City DEM............................................................................................................................32

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

lidar data and the Ocean City DEM...................................................................................32

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

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

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

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

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

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

Figure 35. Location of NGS geodetic monuments.....................................................................................34

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

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

List of Tables

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

Table 2. Coastline datasets used in 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 DEM extent............................9

Table 6. USACE 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 MHW and NAVD 88 vertical datums at tide 

 stations located in the Ocean City DEM extent.................................................................23

Table 9. Data hierarchy used to assign gridding weight in MB-System..................................................27
Digital Elevation Model of Ocean City, Maryland:

Procedures, Data Sources and Analysis

1. Introduction

The National Geophysical Data Center (NGDC), an office of the National Oceanic and Atmospheric Administration 

(NOAA), has developed a 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 SIFT (Short-term Inundation Forecasting for Tsunamis) 

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.

1
Medley et al., 2009

2

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 starve sediment on the south side, 

causing Assateague Island to migrate towards the shore.
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 America Datum 1983 (NAD 83)2 and mean high water (MHW), for modeling of maximum flooding, 

respectively. Data were gathered in an area slightly larger (~5%) than the DEM extents. This data “buffer” ensures that 

gridding occurs across rather than along the DEM boundaries to prevent edge effects. Data processing and evaluation, 

and DEM assembly and assessment are described in the following subsections.

Table 1: PMEL specifications for the 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 1984 (WGS 84)

Vertical Datum 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 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’s Vertical Datum Transformation (VDatum) software, FME, and ArcGIS, based upon data from NOAA tide 

stations (see Section 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. Many GIS applications treat the two datums as identical, so do not actually transform data between them, and the error introduced by not converting between 

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

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

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

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

interchangeably.
Medley et al., 2009

4

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 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 Zone 18N 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.

75°30'W 75°0'W

38°30'N

38°0'N

Coastline Data

Sources

ENC 12304

ENC 12214

ENC 12216

ENC 12211

ENC 122111_2

USGS Jetty

ENC 12210

ENC 12210_2

Final coastline
Medley et al., 2009

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1) Office of Coast Survey electronic navigational charts

Five electronic navigational charts (ENCs) are available for the Ocean City area (Table 3). They were 

downloaded from NOAA’s OCS web site (http://nauticalcharts.noaa.gov/mcd/enc/) 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: Electronic navigational charts 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

7

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 ESRI 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.
Medley et al., 2009

8

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

MLLW and 

MLW

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

mgg/bathymetry/hydro.

html

USACE 2006 to 

2007

Hydrographic 

survey 

soundings

Profiles ranging from 

15 to 30 m apart and 

point spacing 3 to 

4 m

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; red box denotes DEM extent.
Digital Elevation Model of Ocean City, Maryland

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1) National Geophysical Data Center 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 NOS Hydrographic Survey Database (http://www.ngdc.noaa.gov/mgg/bathymetry/hydro.html) 

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 DEM extent.

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 to NAD 83. NADCON is the U.S. Federal 

Standard for NAD 27 to NAD 83 datum transformations.
Medley et al., 2009

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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 VDatum

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
Medley et al., 2009

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

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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 Maryland State Plane (feet) and MLLW (feet) 

datums. The files were converted to NAD 83 and MHW (meters) using VDatum. Point spacing averages 4 

meters along profiles approximately 150 meters long and averaging 20 meters apart.

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

Survey File Year of Survey Survey Scale Original Vertical Datum Original Horizontal 

Datum

s52waug08a 2008

Profiles with ~30 m line 

spacing and ~3 m point 

spacing

NAD 83 Maryland State 

Plane (feet) MLLW (feet)

s052i026mar09a 2009

Profiles with ~30 m line 

spacing and ~4 m point 

spacing

NAD 83 Maryland State 

Plane (feet) MLLW (feet)

s052i08may09 2009

Profiles with ~15 m line 

spacing and ~3 m point 

spacing

NAD 83 Maryland State 

Plane (feet) MLLW (feet)

s052s37mar09a 2009

Profiles with ~30 m line 

spacing and ~3 m point 

spacing

NAD 83 Maryland State 

Plane (feet) MLLW (feet)

Figure 10. Digital USACE hydrographic survey 

coverage in the Ocean City region.
Medley et al., 2009

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 RNC’s but is visible in satellite imagery. NGDC manually adjusted the 

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

3.1.3 Topography

Topographic datasets used to build the Ocean City DEM include: USGS NED 1/3 arc-second DEM, 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 website; and NGDC digitized solid 

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.

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 

meter depths in the harbor. 
Digital Elevation Model of Ocean City, Maryland

15

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 

bare-earth 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

Figure 12. Spatial coverage of 

topographic datasets used in building 

the Ocean City DEM.
Medley et al., 2009

16

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

The USGS 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 website 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 from 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 (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 cm. 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 include water returns in the north of the DEM 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, except for AK, which is NAD 27. The vertical datum is NAVD 88, except for AK, which is NGVD29. NED is a living 

dataset that is updated bimonthly to incorporate the “best available” DEM data. As more 1/3 arc second (10 m) data covers the U.S., then this will 

also be a seamless dataset. [Extracted from USGS NED web site]

Figure 13. Spatial coverage of the NED 1/9 

arc-second available in the Ocean City region. 

Some data was not used because it was already 

incorporated in the NED 1/3 arc-second DEM. 

Green is NED 1/3 arc-second DEM coverage.
Digital Elevation Model of Ocean City, Maryland

17

Figure 14. Water returns in marshy areas of the NED 1/9 arc-second DEM. Returns are derived from the original lidar data.
Medley et al., 2009

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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 2 km by 2 km tiles 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 cm.

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 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.
Digital Elevation Model of Ocean City, Maryland

19

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 cm and a horizontal accuracy that has not been tested. The data were downloaded in 

NAD 83 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 were 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.
Medley et al., 2009

20

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

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

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

Metadata states that this was a topographic-bathymetric survey, though there were no hydrographic data in 

the survey. The survey has a vertical accuracy of 0.2 m 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 website. The survey’s ground spacing is 2 meters, and has a vertical accuracy 

of 21.3 cm 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 were 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) National Geophysical Data Center 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 nonbare-earth topographic dataset. This survey was 

used in building the Ocean City DEM with the 

resulting shapefile after rooftops were deleted to 

approximate bare earth.
Digital Elevation Model of Ocean City, Maryland

21

Figure 18. NGDC digitized breakwaters and 

jetties at: A) Delaware Bay, B) Ocean City 

Inlet, and C) India River Inlet.
Medley et al., 2009

22

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 within this range were converted from MLLW, or MLW, to MHW using the 

VDatum transformation tool. 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.

Figure 19. Spatial coverage of the VDatum 

transformation tool.
Digital Elevation Model of Ocean City, Maryland

23

2) Topographic data

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

The CSC topographic lidar data were downloaded referenced to NAVD 88 vertical datum. Transformations 

to MHW, using FME software or ArcGIS, were 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 in 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
Medley et al., 2009

24

3.2.2 Horizontal datum transformations

Datasets used to build the Ocean City DEM were downloaded 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 data not processed to bare-earth; the data 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
Digital Elevation Model of Ocean City, Maryland

25

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 of “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., Fig. 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. GMT is an open source collection of ~60 tools for manipulating geographic and Cartesian data sets (including filtering, trend fitting, gridding, 

projecting, etc.) and producing Encapsulated PostScript File (EPS) illustrations ranging from simple x-y plots via contour maps to artificially illuminated surfaces and 3-D perspective views. GMT supports ~30 map projections and transformations and comes with support data such as GSHHS 

coastlines, rivers, and political boundaries. GMT is developed and maintained by Paul Wessel and Walter H. F. Smith with help from a global set 

of volunteers, and is supported by the National Science Foundation. It is released under the GNU General Public License. URL: http://gmt.soest.

hawaii.edu/ [Extracted from GMT web site.]
Medley et al., 2009

26

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.
Digital Elevation Model of Ocean City, Maryland

27

3.3.4 Gridding the data with MB-System

MB-System was used to create the 1/3 arc-second Ocean City DEM. MB-System6 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 at 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” access through these web pages). A complete 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 MBSystem 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.
Medley et al., 2009

28

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 cm 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.
Digital Elevation Model of Ocean City, Maryland

29

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.
30

Figure 26. Perspective view from the southeast of the Ocean City DEM. Vertical exaggeration–times 20.

Medley et al., 2009
Digital Elevation Model of Ocean City, Maryland

31

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. 
Medley et al., 2009

32

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.
Digital Elevation Model of Ocean City, Maryland

33

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.
Medley et al., 2009

34

3.4.5 Comparison with the National Geodetic Survey geodetic monuments

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

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

sub-mm accuracy) and elevations in 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, lighthouse 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.
Digital Elevation Model of Ocean City, Maryland

35

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 NOAA PMEL. The authors thank Chris Chamberlin 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 website to date.

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.
Medley et al., 2009

36

7. Data Processing Software

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

ESRI World Imagery, 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 2009 GB – Feature Manipulation Engine, developed and licensed by Safe Software, Vancouver, BC, Canada, 

http://www.safe.com/

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

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

GMT v. 4.3.4 – 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/

VDatum – Vertical 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.