This data set contains point data that represents individual soundings from hydrographic surveys collected in Coastal bays in 2000 and 2003. Depths are adjusted for tidal stage and referenced to NAVD88 datum
The purpose of this data set is to provide up-to-date bathymetric data in the coastal bays, the Resources Assessment Services, Maryland Geological Survey conducted hydrographic surveys in the Maryland's coastal bays using standard digital bathymetric methods. The objectives for this survey were to: 1) provide consistent, systematic bathymetric coverage of the study areas; and 2) develop a baseline bathymetric data set that may be expanded and/or used for comparison with surveys conducted in the future.
This data set contains the final adjusted soundings from hydrographic surveys collected within the Maryland coastal bays. The surveys were conducted in Assawoman Bay and St. Martin River between May 1and June 28, 2000, in Chincoteague Bay between May 31 and October 1, 2003, and in Isle of Wight Bay between October 2 and October 8, 2003.
ground condition
These data are not for navigational purposes. The Maryland Department of Natural Resources assumes no liability for uses of these data. Acknowledgment of the Maryland Geological Survey, and the Fisheries Service, both part of the Maryland Department of Natural Resources, and NOAA shall be made in products derived from these data.
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Funding for bathymetric data collected in the Northern Coastal Bays of Maryland was provided by the Fisheries Service, Maryland Department of Natural Resources. Financial assistance for the bathymetric survey of Chincoteague Bay was provide by the NOAA Coastal Services Center, NOAA Award No. NA17OC2689.
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Position (Horizontal) Data: The Ashtech GPS unit was programmed to output location coordinates only if the following criteria were met: 1) position derived from at least six satellites with positions of 15 ° or greater above the horizon; 2) Position dilution of precision (PDOP) less than 5; and 3) DGPS signal less than 30 seconds old. The Starlink DGPS tracked the signals from Cape Henry, Virginia, and Cape Henlopen, Delaware. The receiver sent updates to the Ashtech based upon the station with the strongest signal. The position latency, or time lag between the position update and the posted time of day (time of depth sounding), also was recorded and later used to filter out less accurate location data. The Ashtech GPS-Starlink DGPS system provided for ± 1.5-meter accuracy. Water Depths (Soundings): The Knudsen echo sounder was checked against known depths to reduce errors. The echo sounder was calibrated throughout the entire range of water depths measured. The recorded depths were adjusted after collection using a calibration equation derived from the calibrations conducted in the field. The accuracy of the Kundsen echo sounder is +/- 0.01 meters. To determine the actual water level during the time the hydrographic surveys were conducted, water level recorders were installed and operated at several locations within the areas being surveying. Water level recorder systems manufactured by Global Water Instrumentation, Inc. were used for this study. The manufacturer calibrated the water level recorders prior to shipping. Calibrations were confirmed in the field prior to deployment and again at the end of the study. Each recorder, once installed, was surveyed using either traditional differential leveling techniques and/or GPS techniques to determine elevation of water level sensor, referenced to North America Vertical Datum of 1988 (NAVD88).
The area covered by the data set includes navigable portions of Assawoman Bay, including Delaware, St. Martin River, Isle of Wight Bay, Sinepuxent Bay, Newport Bay, and the Maryland portion of Chincoteague Bay. In the northern half of the study area (north of South Point in Sinepuxent Bay), soundings were collected on a grid spacing of 200 meters (east-west) by 1000 meters (north-south). In Chincoteague Bay, soundings were collected on a grid spacing of 400 meters (east-west) by 1000 meters (north-south). Bay bottom features missed by the hydrographic surveys (i.e., features lying between hydrographic survey lines, or features outside of surveyed areas) would not be represented in this data.
The Ashtech GPS-Starlink DGPS system provides for ± 1.5-meter accuracy. The accuracy of the post-processed horizontal GPS data is within 3.5 meters. The figure takes in account of the average boat speed (4 knots or 2 m/sec) and GPS coordinate update latency (1000 msec).
The accuracy of the post-processed bathymetry data is estimated to be ± 0.2 meters (0.7 ft). This estimate takes in account of the error introduced with the water level adjustments and elevation accuracy of the water level recorders.
Hydrographic surveys were conducted using a 17 ft Boston Whaler equipped with a 70 horsepower outboard engine. Track lines followed predetermined NAD83 UTM northing gridlines or easting gridlines, which facilitated navigation of the whaler and allowed survey lines to be easily retraced for quality control and quality assurance (QA/QC) purposes and during future surveys. Track lines were spaced 400 meters apart and extended from shore to shore (generally east-west). Tie-in track lines were run every 1000 meters perpendicular to the shore-to-shore tracks lines. Boundary surveys, following the shoreline, were also conducted to establish boundary (near-shore) conditions. Bathymetric data were collected using an Ashtech Reliance Precision GPS (model SCA-12S; L1 code and carrier) and a Knudsen 320B/P dual frequency echo sounder with sounding frequencies of 200 KHz and 28 KHz. The echo sounder transducer is a KEL771 dual frequency transducer with a 200 KHz beam angle of 4 degrees and a 28 KHz beam angle of 29 degrees. The echo sounder generated acoustic pulses for bottom recognition at a rate of 2 Hz. The pulse width was set to automatically change between 0.2 mS and 0.8 mS depending on the depth of the water. The transmitted acoustic wave reflected off the density gradient separating the water column from the bottom sediment. The returned acoustic wave is received by the transducer, and the time separation between the sent and the returned wave is recorded. This time separation is directly proportional to distance. The recordings were then filtered for points that were outside of the gate window (2 meters) and integrated within the echo sounder to produce an accurate measurement from the transducer to the water/sediment interface. At an average vessel speed of 4 knots, a depth sounding was collected approximately every 1.0 m (3.3 ft) along the survey track-lines. To minimize pitch and roll, the transducer, used for both transmitting the pulse and receiving the echo signal, was mounted off the port side of the whaler, slightly aft of the vessel's center. Because of the size of the boat and safety considerations, surveys were collected during "calm" conditions (waves = 1 ft). The sounding data was stored along with the GPS location and positional latency in a laptop computer. Navigation was provided through a Lowrance GlobalNav 212 GPS interfaced to a Lowrance DGPS beacon receiver. A Starlink MRB-2 DGPS receiver provided DGPS signals to the Ashtech Reliance GPS system. The Ashtech unit was programmed to output location coordinates only if the following criteria were met: 1) position derived from at least six satellites with positions of 15 ° or greater above the horizon; 2) Position dilution of precision (PDOP) less than 5; and 3) DGPS signal less than 30 seconds old. DGPS differential corrections broadcast by the United States Coast Guard provided a real-time horizontal accuracy of 1 to 2 m [3 to 6 feet] using the Cape Henry, Virginia, and Cape Henlopen, Delaware, DGPS sites. The Ashtech GPS, the Lowrance GPS, and the echo sounder were checked against known horizontal and vertical measurements during the survey. The echo sounder was calibrated to obtain speed of sound correction and transducer offset. The HF sounding data from the hydrographic surveys were processed to remove invalid depth data (invalid HF flag), depths less than 0.35 meters, "out of sequence" depths (e.g., fish, water column noise, multiples, etc.), and invalid location coordinates (GPS latency values > 1000 msec). For soundings having the same geographic coordinates, the sounding with the higher latency was removed. Sounding depth values were adjusted to account for the offset of the transducer below the water surface. The offset value was derived from the calibration of the echo sounder. The adjusted sounding depth values were then corrected to account for tide stage and/or wind effects. To determine the actual water level during the time the hydrographic surveys were conducted, water level recorders were installed and operated at several locations within the areas being surveying. Water levels from the water level recorder were interpolated to the exact times depth soundings were taken, and the interpolated water level were subtracted from the adjusted depth soundings, obtaining depths corrected to NAVD88, meters.
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Dataset copied.
Metadata imported.
Internal feature number.
ESRI
Unique number assigned during merging data in MSAccess
ESRI
Identification field
ASIS
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ASIS
Name of Project
ASIS
Coordinate system used to identify exact feature location
ASIS
Coordinate system used to identify exact feature location
ASIS
Depth, in meters, of bay bottom (sediment surface) referenced to North America Vertical Datum of 1988 plane (NAVD88). Depths are given as negative values, i.e. distance below reference plane. In Maryland's coastal bays, the average bay water levels were within ±0.02 meters of NAVD88.
Maryland Geological Survey
Time that sounding was recorded, given in hours since Jan. 1, 2000, 00 hour. Time is reference in UTC
Maryland Geological Survey
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