NOAA-National Ocean Service19980606Bathymetric Digital Elevation Models at 30 meter resolutionmap
Series Information
Series Name:
Issue Identification:
Silver Spring, MDNOAA-National Ocean Service
These Bathymetric Digital Elevation Models (DEM) at 30 meter resolution and each
covering a 7 1/2 minute quadrangle in UTM projection using NAD27 were generated from original point
soundings collected during hydrographic surveys conducted by the National Ocean Service and its
predecessors. Mean High Water shoreline as defined by NOAA nautical charts was used as a constraining
boundary and assigned its local elevation relative to the local datum (typically Mean Low Water). DEM
grid values outside the shoreline (on land) were assigned null values (-32676). In the event of
multiple surveys in a region,the most recent survey soundings were retained. Both 7.5 minute and 1
degree DEMs are available. The 1 degree DEMs were generated from the higher resolution 7.5 minute
DEMs which covered the estuary. A Digital Elevation Model(DEM) contains a series of elevations
ordered from south to north with the order of the columns from west to east. The DEM is formatted as
one ASCII header record (A- record), followed by a series of profile records (B- records) each of
which include a short B-record header followed by a series of ASCII integer elevations (typically in
units of 1 centimeter {0.01 meter]) per each profile. The last physical record of the DEM is an
accuracy record (C-record). The 7.5-minute DEM (30- by 30-m data spacing) is cast on the Universal
Transverse Mercator (UTM) projection. It provides coverage in 7.5- by 7.5-minute blocks. Each
product provides the same coverage as a standard USGS 7.5-minute quadrangle but the DEM contains over
edge data. Coverage is available for many estuaries of the contiguous United States but is not
complete.
NOAAServer Codes: [L 50 24 -65 -125] ; nodateline
Bathymetric DEM's can be used as layers in geographic information systems for earth
science analysis. DEM's can also serve as tools for volumetric analysis, for site location of
structures, or for drainage basin delineation. The Source_soundings are collected by the National
Ocean Survey.
The datum for these bathymetric DEMs (Mean Low Water) is not the same
as that used by the US Geological survey (USGS) for land based DEMs (NAV29) which results in a
discontinuity of about 1/2 the tidal range. Moreover, the shoreline for the USGS DEMs is indeterminateand
not the same as that used for the Bathymetric DEMs. Furthermore, the vertical resolution is
smaller and accuracy of the Bathymetric DEMs is better than USGS DEMs. Because of these differences,
extreme care should be used in merging NOAA and USGS DEM data. 7.5-minute DEMs have rows and columns
which vary in length and are staggered. The UTM bounding coordinates form a quadrilateral (no two
sides are parallel to each other), rather than a rectangle. The user will need to pad out the uneven
rows and columns with blanks or flagged data values, if a rectangle is required for the user's
application. Some software vendors have incorporated this function into their software for input of
standard formatted USGS DEMs.
ground condition
Status
Spatial_Domain
West_Bounding_Coordinate -125.00
East_Bounding_Coordinate -65.00
North_Bounding_Coordinate 50.00
South_Bounding_Coordinate 24.00
Keywords
Theme_Keyword_Thesaurus none
Theme_Keyword DEM
Theme_Keyword digital elevation model
Theme_Keyword digital bathymetric model
Theme_Keyword digital terrain model
Theme_Keyword bathymetry
Theme_Keyword altitude
Theme_Keyword height
Theme_Keyword depth
Place
Access_Constraints none
Use_Constraints
Acknowledgment of the National Oceanic and Atmospheric Administration- Nation Ocean Service would be
appreciated in products derived from these data.
The datum for these bathymetric DEMs (Mean Low Water) is not the same as that used by the US
Geological survey (USGS) for land based DEMs (NAV29) which results in a discontinuity of about 1/2
the tidal range. Moreover, the shoreline for the USGS DEMs is indeterminate and not the same as that
used for the Bathymetric DEMs. Furthermore, the vertical resolution is smaller and accuracy of the
Bathymetric DEMs is better than USGS DEMs. Because of these differences, extreme care should be used
in merging NOAA and USGS DEM data.
Data Quality Information
The accuracy of a DEM is dependent upon the level of detail of the Source_soundings and the grid
spacing used to sample that source. The primary limiting factor for the level of detail of the Source_
is the scale of the Source_survey, the technology used to collect soundings, and the resolution of
the Source_soundings. Additional complications is describing accuracy are the ages of the surveys
used to collect sounding in a given area. Care was used to use the most recent surveys covering a
given area but in some instances adjacent surveys may be decades apart in age. In some regions, the
only Source_data were at fathom (6 feet) resolution. In general, most Source_surveys were certified
to one foot or better by the National Ocean Service.
The fidelity of the relationships encoded in the data structure of the DEM are automatically verified
using a NOAA-NOS software program upon completion of the data production cycle. The test verifies
full compliance to the DEM specification.
The DEM is visually inspected for completeness on a DEM view and edit system for the purpose of
performing a final quality control and if necessary, edit of the DEM. The physical format of each
digital elevation model is validated for content completeness and logical consistency during
production quality control and prior to archiving in the [WHERE National Geographic Data Center].
Due to the variable orientation of the 7 1/2 minute quadrilateral in relation to the Universal
Transverse Mercator (UTM) projection grid, profiles that pass within the bounds of the DEM
quadrilateral may be void of elevation grid points and are not represented in the DEM. This condition
occurs infrequently and is always the first or last profile of the dataset. DEM's may contain void
areas caused by elevations being above Mean High Water or on non tidal land. Void elevations are
assigned the value of -32,767. In addition, suspect elevation areas may exist in the DEM but are
not specifically identified.
The horizontal accuracy of the DEM is expressed as an estimated root mean square error (RMSE). The
estimate of the RMSE is based upon horizontal accuracy tests of the Source_soundings used to generate
the DEM. As a first approximation the locational accuracy of the Source_soundings are 0.0015 m at
Source_"Smooth Sheet" scale (120 m @ 1:80,000 to 15 m @ 1:10,000). Smooth Sheets are maps generated
as a principle product of each (historic) hydrographic survey with fully corrected soundings plotted
on them.
Quantitative_Horizontal_Positional_Accuracy Assessment:
Value: 3 meters [estimated]Explanation: Digital elevation models meet
The vertical RMSE statistic is used to describe the vertical accuracy of a DEM. It encompasses both
random and systematic errors introduced during production of the data. The RMSE is encoded in element
number 5 of record C of the DEM. This accuracy estimate includes components related to quantization
of the Source_soundings (1.3 to 0.15 m), the systematic editing of the Source_data (1% or 0.10m),
un-sampled bathymetric features (estimated at less than 5% of depth), time related changes (erosion,
deposition, and seismic shifts), and dredging operations (cut and fill). It is estimated that the
accuracy of the Bathymetric DEMs is 2% of depth or 1 meter for depths grater than 20 meters and 2 %
of depth or 0.20 meters for depths shallower than 20 meters. THESE DEMs SHOULD NOT BE USED FOR
NAVIGATION. There are three types of DEM vertical errors: blunder, systematic, and random. These
errors are reduced in magnitude by editing but cannot be completely eliminated. Blunders are errors
of major proportions and are easily identified and removed during interactive editing. Systematic
errors follow some fixed pattern and are introduced by data collection procedures and systems.
Systematic error artifacts include vertical unsampled elevation shifts, relative spacing of the
Source_soundings, misinterpretation of terrain surface caused by softness or poor reflectivity and by
the resolution of the collected soundings (feet, feet & fractions, fathoms, fathoms & fractions,
meters, tenths of meters etc.). Random errors result from unknown or accidental causes. The 1
degree (DSQ) DEMs are generated from 30 m grids on UTM projection. The rms difference between these
surfaces is an estimate of the vertical accuracy of the DSQ DEMs.
National Oceanic and Atmospheric Administration-1997National Ocean Service- Hydrographic Survey DataBoulder CONational Geophysical Data CenterCD-ROM
Source_Currentness Reference: date of surveys
Abbreviation: GEODAS vol 2 ver 3.3Hydrographic SoundingsNational Oceanic and Atmospheric Administration1994Medium Resolution Vector ShorelineBoulder CONational Ocean SurveyCD-ROMNOAAShoreline position
The production procedures, instrumentation, hardware, and software used in the collection of standard
National Oceanic and Atmospheric Administration (NOAA) Bathymetric Digital Elevation Models (DEM's)
vary depending on systems used at the time of the survey. Documentation Logsheets were kept at all
stages of processing to track file names, dates, hydrographic survey coverage, and soundings or
hydrographic surveys that were deleted as part of the quality control process. A short summary of the
processing steps for each estuary is available on the individual data pages. In addition, a list of
each of the surveys which were included is accessible through the individual data pages. Overview
Original hydrographic data from the National Ocean Service and it predecessors was used exclusively
as Source_data. Processing was performed on desktop computers using a variety of commercial and
custom software systems. The two main software systems used were Digital Optimization of Grid
Systems (DOGS) version 1.5x software developed by NOAA and MapInfo Professional version 4.5 published
by MapInfo augmented by a MapInfo add-on named Vertical Mapper published by Northwood Geosciences.
The processing sequence for each estuary started with the generation of a comprehensive Source_data
set from the NOS archives, These Source_sounding were quality controlled toeliminate outliers and
superseded surveys, optimized to reduce the number of data values, augmented with points representing
the Mean High Water shoreline, and then gridded. The gridded data sets were converted from the
internal proprietary grid format to Digital Elevation Model (DEM) format for public distribution.
Creating Point Sets of Hydrographic Survey Data Sounding data obtained from Hydrographic Surveys were
extracted using DOGS from the GEODAS CD distributed by the National Geophysical Data Center using
each estuary's shoreline as a clipping boundary. Large estuaries were broken into several overlapping
regions and subsets of points were extracted and processed. Most historic hydrographic surveys are
included on the GEODAS CD. For those regions which were missing data (parts of southern Florida &
Chesapeake Bays only), soundings were digitized from a hard copy Smooth sheets generated by the
Hydrographic surveys.
Editing and Quality Control of Bathymetry Point Data
The first step of the quality control was to review the data using the DOGS software. The data were
first examined for surveys or parts of surveys that are redundant. For many areas there are more
surveys than are actually useful. Entire surveys or large portions of surveys were deleted for the
following reasons:
Another more recent survey fully covers the same area.
The survey has questionable values which can not be fixed by way of figuring out a mathematical
update value for the entire survey, and there are too many bad points to pick out probable "good"
values.
Sections of surveys were omitted if they were overlapped by more accurate surveys or by similar yet
denser coverage. The second step was to display the data by depth values and to remove stray points
that were obvious outliers from the surrounding data values. Optimizing the Bathymetry Point Data
using DOGS The data was first triangulated in DOGS in order to optimize the set of points. The
computer program DOGS can analyze a large set of bathymetric data and create from it a smaller set of
optimized points which describes the bathymetry of a geographic region to within a user defined error.
This smaller data set then can be used on its own as a representation of the area's bathymetry, or as
input into other computer programs or Geographical Information Systems.
There are two calculation options for triangulation in DOGS: relative and absolute. Bathymetry point
files were cut into sections for separate processing based on mean depth of 10 meters. A relative
height error criteria of 0.01 was used for the triangulation of regions whose depths averaged above
10 meters. An absolute height error criteria of 0.1 meters was used for areas with average depths
less than 10 meters. The two resultant files were saved as text files. After combining these files,
the vertical error associated with the optimized data set was 1% of depth or 0.1 meters, whichever was greater.
The optimized DOG file was imported into MapInfo's MapInfo Professional desktop mapping
software.
Augmenting the data set with Shoreline Points
The final bathymetry was clipped to NOAA's 1:250,000 Coastal Assessment Framework (CAF) shoreline. A
copy of the shoreline file was edited to create a point file from the vector vertices and optimized
using DOGS to produce a more workable, smaller file, with little compromise to the shape of the
shoreline. Mean High Water tide level was assigned to the shoreline points to give them a height.
These Shoreline data points were added to the set of bathymetry points before doing the final
triangulation in MapInfo.
Generation of a Gridded Bathymetry Dataset
Linear triangulation of the combined point files were done in MapInfo using the Vertical Mapper 2.0
partner product software. The resultant TIN file was then used to create a continuous grid file with
30 meter resolution on a UTM projection using a NAD27 horizontal datum. A second grid was created
from the first by aggregating the 30 meter grid values to 90 meter resolution and exporting the 90m
center points. A new TIN was created after reattaching the shoreline pts. The triangle side lengths
and coincident point distances were small enough to disallow interpolation outside the 90m distance,
so as to emulate a rectangular interpolation and still allow the shoreline to be represented without
averaging out the values. The result of the new file is a geographic 3 arc second resolution grid.
Both grids were cut to the estuary boundary shoreline. 7.5 minute and 1 degree sections were then
created from these grids. The 7.5 minute grids have 30 meter resolution in a UTM projection using
the NAD27 datum. The 1 degree grids have a 3 arc second resolution in a geographic projection
(Latitude/Longitude) using the NAD27 datum. In their native form, both of these grids are in proprietary
formats.
Creating DEM files
The MapInfo grid files were converted to a public domain USGS format DEM files using the NOAA DEM
maker software. The formats available for downloading are 30 meter & 3 arc second DEMs in USGS DEM
format.
DEM's are viewed on interactive editing systems to identify and correct blunder and systematic errors.
DEM's are verified for physical format and logical consistency.
Spatial Data Organization Information
rastergrid cell470470
Spatial Reference Information
Horizontal_Coordinate_System Definition:
Name: Universal Transverse Mercator
UTM_Zone:Number: 18
-75.00.05000000.0
Scale_Factor_at_Central Meridian: 0.999
Planar_Coordinate_Encoding Method: row and column
3030metersNorth American Datum 1927Clark 18666378206.4
Denominator_of_Flattening Ratio: 294.9787
National Geodetic Vertical Datum of 19291metersExplicit elevation coordinate included with horizontal coordinates
Entity and Attribute Information
Entity_Type
Entity_Type_Label
Entity_Type_Definition
Entity_Type_Definition Source:
The digital elevation model is composed of a elevation value linked to a
grid cell location representing a gridded form of a bathymetric map overlay. Each grid cell entity
contains a 6-character integer value between -32,767 and 32,768. The grid is generated from
profiles of data each containing header information (profile identifier, starting point, relative
datum for profile values (deepest value within the DEM), number of values, etc) followed by profile
values relative to the relative datum for the profile. All non-null values in the profile are
positive.
U.S. Department of the Interior, U.S. Geological Survey, 1992, Standards for digital elevation models:
Reston, VA, A hypertext version is available at:
http://www-nmd.usgs.gov/www/ti/DEM/standards_dem.html
Softcopy in ASCII format is available at:
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt1.txt
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt2.txt
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt3.txt
Softcopy in WordPerfect format is available at:
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt1.wp5
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt2.wp5
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt3.wp5
Softcopy in PostScript format is available at:
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt1.ps
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt2.ps
ftp://www-nmd.usgs.gov/www/ti/DEM/stdempt3.ps
Distribution Information
National Oceanic and Atmospheric Administrationmailing address
NOAA-NOS-SPO room 09449 SSMC4
1305 East West Highway
Silver SpringMaryland201921 301 713-30000800-1600 Monday -Friday7.5-minute digital elevation models1 Degree-minute digital elevation models
Distribution:Liability:
Although these data have been processed successfully on a computer system at the
National Oceanic and Atmospheric Administration, no warranty expressed or implied
is made by NOAA regarding the utility of the data on any other system, nor shall the act
of distribution constitute any such warranty. The National Oceanic and Atmospheric Administration
will warrant the delivery of this product in computer-readable format and will offer appropriate
adjustment of credit when the product is determined unreadable by correctly adjusted computer input
peripherals, or when the physical medium is delivered in damaged condition. Requests for adjustment
of credit must be made within 90 days from the date of this shipment from the ordering site.
DEM
Format Information Content:
USGS standard DEM: The standard USGS DEM can be described as an ASCII
formatted elevation file preceded by a metadata header file of one 1024
byte ASCII record.
Digital_Transfer_Option
http://mapfinder.nos.noaa.govhttp://www-orca.nos.noaa.gov/projects/bathymetry/bathymetry.htmlCD-Recordable650megabytes
ISO 9660; the files are placed in a flat directory on the CD with
naming conventions that are ISO 9660 Level 1 compliant (DOS 8.3).
The online copy of the data set is available at no cost. The cost of data sets on CD-ROM
has not been determined and until priced, will not be available.
Reference InformationDate: 19980606
Contact_Organization_Primary
National Ocean Servicemailing address
1305 Eastwest Highway
Silver SpringMaryland20910
Contact_TDD/TTY_Telephone
Contact_Facsimile_Telephone
Contact_Electronic_Mail_Address
FGDC Content Standards for Digital Geospatial MetadataJune 8, 1994