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Bathymetry, substrate and circulation in Westcott Bay, San Juan Islands, Washington, Data catolgue

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Frequently-anticipated questions:


What does this data set describe?

Title:
Bathymetry, substrate and circulation in Westcott Bay, San Juan Islands, Washington, Data catolgue
Abstract:
Nearshore bathymetry, substrate type, and circulation patterns in Westcott Bay, San Juan Islands, Washington, were mapped using two acoustic sonar systems, video and direct sampling of seafloor sediments producing these data.
Supplemental_Information:
To explore sources for this fine sediment and turbidity, a dual-frequency Biosonics sonar operating at 200 and 430 kHz was used to map seafloor depth, morphology and vegetation along 69 linear kilometers of the bay. The higher frequency 430 kHz system also provided information on particulate concentrations in the water column. A boat-mounted 600 kHz RDI Acoustic Doppler Current Profiler (ADCP) was used to map current velocity and direction and water column backscatter intensity along another 29 km, with select measurements made to characterize variations in circulation with tides. An underwater video camera was deployed to ground-truth acoustic data. Seventy one sediment samples were collected to quantify sediment grain size distributions across Westcott Bay. Sediment samples were analyzed for grain size at the Western Coastal and Marine Geology Team sediment laboratory in Menlo Park, CA.
  1. How should this data set be cited?

    Grossman. E.E., Stevens, A., Curran, C., Smith, C., and , 2007, Bathymetry, substrate and circulation in Westcott Bay, San Juan Islands, Washington, Data catolgue:.

    This is part of the following larger work.

    Grossman. E.E., Stevens, A., Curran, C., Smith, C., and , 2007, Bathymetry, substrate and circulation in Westcott Bay, San Juan Islands, Washington: Open-File Report 2007-1305, U.S. Geological Survey, Coastal and Marine Geology Program, Menlo Park, CA.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -123.17290
    East_Bounding_Coordinate: -123.13793
    North_Bounding_Coordinate: 48.60642
    South_Bounding_Coordinate: 48.58775

  3. What does it look like?

    <http://walrus.wr.usgs.gov/infobank/b/b607ps/html/b-6-07-ps.meta.html> (GIF)
    Index map of field activity B-6-07-PS.

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 31-May-2007
    Ending_Date: 05-Jun-2007
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: tabular digital data

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a Point data set.

    2. What coordinate system is used to represent geographic features?

      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.00001. Longitudes are given to the nearest 0.00001. Latitude and longitude values are specified in Decimal degrees.

      The horizontal datum used is North American Datum of 1983.
      The ellipsoid used is Geodetic Reference System 80.
      The semi-major axis of the ellipsoid used is 6378137.000000.
      The flattening of the ellipsoid used is 1/298.257222.

      Vertical_Coordinate_System_Definition:
      Altitude_System_Definition:
      Altitude_Datum_Name: North American Vertical Datum of 1988
      Altitude_Resolution: 0.1
      Altitude_Distance_Units: meters
      Altitude_Encoding_Method:
      Explicit elevation coordinate included with horizontal coordinates
      Depth_System_Definition:
      Depth_Datum_Name: local surface
      Depth_Resolution: 0.1
      Depth_Distance_Units: meters
      Depth_Encoding_Method: Explicit depth coordinate included with horizontal coordinates

  7. How does the data set describe geographic features?

    line[number].xyz
    digital surface elevation data (Source: USGS)

    X
    Location X-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:486997.617
    Maximum:489962.263
    Units:meters
    Resolution:0.001

    Y
    Location Y-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:5381456.319
    Maximum:5383932.558
    Units:meters
    Resolution:0.001

    Z
    Location Z-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:-10.00
    Maximum:0.00
    Units:meters
    Resolution:0.01

    WESC[day]_[number].xy
    digital Acoustic Doppler Current Profiler data (Source: USGS)

    X
    Location X-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:486997.617
    Maximum:489962.263
    Units:meters
    Resolution:0.001

    Y
    Location Y-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:5381456.319
    Maximum:5383932.558
    Units:meters
    Resolution:0.001

    sediment grain size data
    surface elevation data (Source: USGS)

    Lon
    Location X-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:-123.58611
    Maximum:-123.13611
    Units:decimal degrees
    Resolution:0.00001

    Lat
    Location Y-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:48.58611
    Maximum:48.60833
    Units:decimal degrees
    Resolution:0.00001

    Depth_m
    Location Z-coordinate of the data point. (Source: USGS)

    Range of values
    Minimum:0.00
    Maximum:10.00
    Units:meters
    Resolution:0.01

    Entity_and_Attribute_Overview:
    Sample of digital surface elevation data XYZ ASCII file, bathy/line95.xyz:
    X             Y              Z
    487989.879521 5381998.198351 -3.31
    487990.116289 5381998.892258 -3.34
    487989.943243 5382001.029467 -3.40
    
    Sample of Acoustic Doppler Current Profiler XY ASCII file, ADCP/RAW/WESC053107_010p.xy:
    X                Y
    487869.695129    5382286.342641
    487870.234235    5382287.490111
    487871.174033    5382288.284718
    
    Sample of sediment sampling log:
    Station Local_Time       UTC           Lon             Lat           Depth_m    Comment
    1    6/1/07 20:44    6/2/07    3:44    -123.1430877    48.60642247    2.286     -
    2    6/1/07 20:35    6/2/07    3:35    -123.1416559    48.60596625    2.1336    18.6 C
    3    6/1/07 20:28    6/2/07    3:28    -123.1396819    48.60476647    2.4384    22.4 C
    
    Entity_and_Attribute_Detail_Citation: none


Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

  3. To whom should users address questions about the data?

    Eric Grossman
    U.S. Geological Survey, Coastal and Marine Geology
    Geologist
    400 Natural Bridges Dr
    Santa Cruz, CA 95060-5792
    USA

    (831) 427-4725 (voice)
    (831) 427-4748 (FAX)
    egrossman@usgs.gov


Why was the data set created?

The goal of the project was to characterize nearshore habitat and conditions influencing eelgrass (Z. marina) where extensive loss has occurred since 1995. A principal hypothesis for the loss of eelgrass is a recent decrease in light availability for eelgrass growth due to increase in turbidity associated with either an increase in fine sedimentation or biological productivity within the bay.


How was the data set created?

  1. From what previous works were the data drawn?

  2. How were the data generated, processed, and modified?

    Date: 2007 (process 1 of 3)
    A) Bathymetric mapping

    The combined bathymetric and topographic data were merged to create a digital elevation model representing the surface topography of Westcott Bay. This surface is referenced to the WGS 84 datum in the UTM Zone 10 North projection with a horizontal accuracy of 2.3 cm. Elevations are referenced to NAVD88 with an estimated vertical accuracy ranging 2.6-9.8 cm. This includes error from the RTK-DGPS and Biosonics sonar, and errors introduced in data processing.

    Date: 2007 (process 2 of 3)
    B) Sediment Samples for Grain Size Analyses

    Results from the tubes and laser particle analyzer were merged using standard USGS methods found in the USGS particle analysis program pcSedSize.

    Date: 2007 (process 3 of 3)
    C) Nearshore Currents

    The raw ADCP data were vertically binned at 0.25 m and include a blanking distance of 0.25 m (no data in uppermost 0.25 m below the transducer). The RDI ADCP is accurate to within 0.25% of boat + water velocity, resulting in an error for our velocity data of 0.25 to 0.45 cm/s based on our survey speed of 2 kts and measured velocities ranging 0 to 1.5 m/s.

  3. What similar or related data should the user be aware of?

    U.S. Geological Survey, Coastal and Marine Geology Program, 2007, InfoBank.

    Online Links:


How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

  2. How accurate are the geographic locations?

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    A) Bathymetric mapping

    Sixty-nine (69) kilometers of acoustic bathymetry/substrate data were collected with a dual-frequency (200 and 430 kHz) Biosonics system along 142 transects in Westcott and Garrison Bays between 5/31/07 and 6/2/07. The Biosonics sonar generates a 6-degree cone of sound, which translates into a footprint on the seafloor ranging 0.2 to 0.8 m for the water depths surveyed (2 - 8 m). Ship speed generally ranged 4.0-4.5 kts and data were merged with GPS positions at 1 Hz. Resulting data therefore represent 0.2-0.8 m pixels on the seafloor spaced approximately 1-2 m apart along track lines. Track lines were spaced 25 m apart. Twenty (20) km of topographic elevation data were also collected across the upper intertidal region by walking the shoreface with a portable Trimble 4700 RTK-DGPS receiver. This receiver utilized a Trimble Zepher Antenna and Pacific Crest radio receiver to obtain real time position corrections from a base station operating a Trimble 4400 receiver and L1/L2 antenna with a Pacific Crest 35 Watt radio transmitter. Elevation data over emergent beaches and tide flats collected by walking are 2-3 times denser as a result of survey speeds ranging 0.5-1.0 kts.

    B) Sediment Samples for Grain Size Analyses

    Sediment samples were collected using a van veen grab sampler at 71 stations on a 200-250 m grid to characterize grain size distribution throughout Westcott Bay and sources of fine material for suspension and transport.

    The shallow stratigraphy was well-preserved in the sediment grabs so that a 1-3 cm thick aerated surface layer could be observed at most stations dominated by fines.

    In the field, a sub-sample of the uppermost 1-3 cm of each grab was collected and placed into storage bags, labeled, logged, and frozen. Several 10-cm push cores were also collected.

    In the laboratory, sediment samples were split for grain size and carbon analyses and later archived at the Western Coastal and Marine Geology Team sediment laboratory in Menlo Park, CA. Samples were disaggregated and sieved through 2000 and 62 micron screens to separate gravel, sands and mud after the organic component was removed by treatment with hydrogen peroxide. Sediment grain size of sands was determined using the settling methods and principles of Stokes Law on the USGS 3-m long settling tubes. The fine fraction (silts and clays) were analyzed on a Coulter 230 Laser Particle Analyzer. Three samples had less than 3% intermediate fraction and were analyzed on the Coulter. Single sample runs were made on the tubes, while results from the laser particle analyzer are averages of triplicates with standard deviations around the mean ranging 18 to 24 microns due to instrument errors.

    C) Nearshore Currents

    Current velocity, direction and backscatter amplitude were collected along 65 transects in Westcott Bay with a 600 kHz RDI ADCP (Acoustic Doppler Current Profiler) and recorded with WinRiver software. ADCP were recorded with GPS position at 1 hz with a ship speed of 2.0-2.5 kts, so raw ADCP data cover a lateral distance of 1.00 to 1.25 m.

  5. How consistent are the relationships among the observations, including topology?

    A) Bathymetric mapping

    The root mean square (RMS) error of the processed surface elevation values were derived from survey line crossings. Line crossings did not always capture the same point on the seafloor (because of GPS accuracy, navigation, currents, and timing of data recording). Therefore, variability in elevation values at crossings is likely in part derived from variability in the bathymetry of the seafloor where line crossings spanned 0.5 to 2 m apart in the horizontal.

    Because there can be significant natural variability in depth/elevation across 0.5 to 2.0 m of the seafloor, the vertical error of the survey was determined from analysis of three classes of elevation values at line crossings:

    1) values within 2 m of each other from all areas of Westcott Bay including vegetated, rocky and smooth areas (RMS=9.8 cm, n=66)

    2) values within 2 m of each other from the smooth area between Bell Point and the head of Westcott Bay (RMS=3.6 cm, n=22)

    3) values within 0.5 m of each other within the smooth area between Bell Point and the head of Westcott Bay (RMS=2.6 cm, n=6)

    The overall error for the entire survey is therefore 9.8 cm, although particular areas, especially characterized by low relief likely have much smaller errors.

    B) Sediment Samples for Grain Size Analyses

    Triplicate samples were collected at sites WBN and BP for error analyses.

    Analyses of triplicate samples collected at stations WBN and BP show that inter-station variability around the mean grain size ranged ±0.054 mm. The results were then gridded across the bathymetric surface with a nearneighbor gridding algorithm averaging between the three nearest points.

    Video of the seafloor was collected along several principal transects in the western and central portions of Westcott Bay to ground-truth the sonar data.

    C) Nearshore Currents

    Select transects at the mouth of Westcott Bay, Bell Point and immediately east of the Westcott Sea Farm were repeated ~2-3 times under flooding and ebbing tides.


How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints: None
Use_Constraints:
Public domain data from the U.S. government is freely redistributable with proper metadata and source attribution. Please recognize the U.S. Geological Survey (USGS) as the source of this information.

  1. Who distributes the data set? (Distributor 1 of 1)

    U.S. Geological Survey, Coastal and Marine Geology
    c/o Eric Grossman
    Geologist
    400 Natural Bridges Dr
    Santa Cruz, CA 95060-5792
    USA

    (831) 427-4725 (voice)
    (831) 427-4748 (FAX)
    egrossman@usgs.gov

  2. What's the catalog number I need to order this data set?

  3. What legal disclaimers am I supposed to read?

    Although this data set has been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and/or related materials. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials.

    Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

  4. How can I download or order the data?


Who wrote the metadata?

Dates:
Last modified: 17-Sep-2007
Metadata author:
U.S. Geological Survey, Coastal and Marine Geology
c/o Eric Grossman
Geologist
400 Natural Bridges Dr
Santa Cruz, CA 95060-5792
USA

(831) 427-4725 (voice)
(831) 427-4748 (FAX)
egrossman@usgs.gov

Metadata standard:
FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)


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