Digital elevation models (DEMs) of the Elwha River delta, Washington, July 2017

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Frequently anticipated questions:


What does this data set describe?

Title:
Digital elevation models (DEMs) of the Elwha River delta, Washington, July 2017
Abstract:
This portion of the USGS data release presents digital elevation models (DEMs) derived from bathymetric and topographic surveys conducted on the Elwha River delta in July 2017 (USGS Field Activity Number 2017-638-FA). Nearshore bathymetry data were collected using two personal watercraft (PWCs) and a kayak equipped with single-beam echosounders and survey-grade global navigation satellite systems (GNSS) receivers. Topographic data were collected on foot with survey-grade GNSS receivers mounted on backpacks. Positions of the survey platforms were referenced to a GNSS base station placed on a benchmark with known horizontal and vertical coordinates relative to the North American Datum of 1983 (CORS96 realization) and North American Vertical Datum of 1988. The final data were projected in Cartesian coordinates using the Washington State Plane North (meters) coordinate system. A total of 1,270,212 individual elevation points were collected within the survey area between July 20 and July 23, 2017. DEM surfaces were produced from all available elevation data using linear interpolation. Two separate DEMs were constructed. A DEM was produced that covered the entire survey area (approximately 511 ha) with 5-m horizontal resolution. A second DEM with 1-m resolution was produced that covered the river mouth and adjacent areas (approximately 131 ha). The DEMs were created by interpolating between measurements as much as 50 meters apart. For this reason, we cannot evaluate the accuracy of each point in the DEM, only the measurements it is based on. The estimated vertical uncertainties of the bathymetric and topographic measurements are 12 and 5 cm, respectively. Digital data files for each DEM are provided in ESRI ARC ASCII (*.asc) format.
Supplemental_Information:
Additional information about the field activities from which these data were derived are available online at: https://cmgds.marine.usgs.gov/fan_info.php?fan=2017-638-FA
  1. How might this data set be cited?
    Stevens, Andrew W., Gelfenbaum, Guy R., Warrick, Jonathan A., Miller, Ian M., and Weiner, Heather M., 2019, Digital elevation models (DEMs) of the Elwha River delta, Washington, July 2017: data release DOI:10.5066/P9AJ8ALB, U.S. Geological Survey, Pacific Coastal and Marine Science Center, Santa Cruz, CA.

    Online Links:

    This is part of the following larger work.

    Stevens, Andrew W., Gelfenbaum, Guy R., Warrick, Jonathan A., Miller, Ian M., and Weiner, Heather M., 2019, Bathymetry, topography, and sediment grain-size data from the Elwha River delta, Washington, July 2017: data release DOI:10.5066/P9AJ8ALB, U.S. Geological Survey, Pacific Coastal and Marine Science Center, Santa Cruz, CA.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -123.591538
    East_Bounding_Coordinate: -123.520296
    North_Bounding_Coordinate: 48.159538
    South_Bounding_Coordinate: 48.135183
  3. What does it look like?
    https://www.sciencebase.gov/catalog/file/get/5c9bf7bae4b0b8a7f62c32cb?name=ew17_july_dem_5m.png&allowOpen=true (PNG)
    image map showing 5-m resolution digital elevation model (DEM) of Elwha River delta, color-coded to show elevation
    https://www.sciencebase.gov/catalog/file/get/5c9bf7bae4b0b8a7f62c32cb?name=ew17_july_dem_1m.png&allowOpen=true (PNG)
    image map showing 1-m resolution digital elevation model (DEM) of Elwha River mouth, color-coded to show elevation
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 20-Jul-2017
    Ending_Date: 23-Jul-2017
    Currentness_Reference:
    ground condition at time data were collected
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: Esri ARC/INFO ASCII GRID
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a Raster data set.
    2. What coordinate system is used to represent geographic features?
      The map projection used is Lambert Conformal Conic.
      Projection parameters:
      Standard_Parallel: 47.5
      Standard_Parallel: 48.73333333333333
      Longitude_of_Central_Meridian: -120.8333333333333
      Latitude_of_Projection_Origin: 47.0
      False_Easting: 500000.0
      False_Northing: 0.0
      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest 0.001
      Ordinates (y-coordinates) are specified to the nearest 0.001
      Planar coordinates are specified in METERS
      The horizontal datum used is NAD83 (CORS96).
      The ellipsoid used is GRS_1980.
      The semi-major axis of the ellipsoid used is 6378137.0.
      The flattening of the ellipsoid used is 1/298.257222101.
      Vertical_Coordinate_System_Definition:
      Altitude_System_Definition:
      Altitude_Datum_Name: North American Vertical Datum of 1988
      Altitude_Resolution: 0.01
      Altitude_Distance_Units: meters
      Altitude_Encoding_Method:
      Explicit elevation coordinate included with horizontal coordinates
  7. How does the data set describe geographic features?
    Attribute Table
    Table containing attribute information associated with the data set (Source: Producer defined)
    Elevation
    Elevation relative to the North American Vertical Datum of 1988 at the base station (-20.11 m above the reference ellipsoid). (Source: Producer Defined)
    Range of values
    Minimum:-22.07
    Maximum:5.24
    Units:meters

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • Andrew W. Stevens
    • Guy R. Gelfenbaum
    • Jonathan A. Warrick
    • Ian M. Miller
    • Heather M. Weiner
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
    U.S. Geological Survey, Pacific Coastal and Marine Science Center
    Attn: PCMSC Science Data Coordinator
    2885 Mission Street
    Santa Cruz, CA

    831-427-4747 (voice)
    pcmsc_data@usgs.gov

Why was the data set created?

Data were obtained to assess the coastal geomorphic response following the removal of two dams on the Elwha River. These data are intended for science researchers, students, policy makers, and the general public. These data can be used with geographic information systems or other software to identify topographic and shallow-water bathymetric features.

How was the data set created?

  1. From what previous works were the data drawn?
    bathymetry data (source 1 of 2)
    Stevens, Andrew W., Gelfenbaum, Guy R., Warrick, Jonathan A., Miller, Ian M., and Weiner, Heather M., 2019, Nearshore bathymetry data from the Elwha River delta, Washington, July 2017: U.S. Geological Survey, Pacific Coastal and Marine Science Center, online.

    Online Links:

    Type_of_Source_Media: digital dataset
    Source_Contribution:
    Bathymetry data were combined with topography data, and then interpolated into a gridded surface
    topography data (source 2 of 2)
    Stevens, Andrew W., Gelfenbaum, Guy R., Warrick, Jonathan A., Miller, Ian M., and Weiner, Heather M., 2019, Topography data from the Elwha River delta, Washington, July 2017: U.S. Geological Survey, Pacific Coastal and Marine Science Center, online.

    Online Links:

    Type_of_Source_Media: digital dataset
    Source_Contribution:
    Topography data were combined with bathymetry data, and then interpolated into a gridded surface
  2. How were the data generated, processed, and modified?
    Date: 01-Sep-2017 (process 1 of 2)
    Quality control was applied to raw topography and bathymetry data. Comparisons were performed to ensure internal consistency between the survey platforms. DEMs were produced by linear interpolation of all available elevation point data with the computer program MATLAB. Data were exported in ESRI ARC ascii grid format for distribution. Data sources used in this process:
    • all sources cited were used
    Date: 20-Jun-2019 (process 2 of 2)
    Edits were made to standardize a few of the "None" keywords. Person who carried out this activity:
    Susan A Cochran
    U.S. Geological Survey, Pacific Coastal and Marine Science Center
    2885 Mission St.
    Santa Cruz, CA
    United States

    831-460-7545 (voice)
    scochran@usgs.gov
  3. What similar or related data should the user be aware of?
    International Hydrographic Organization (IHO), 2008, IHO standards for hydrographic surveys (5th ed.): International Hydrographic Bureau Special Publication 44, 28p., Monaco.

    Online Links:


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

  1. How well have the observations been checked?
    Repeatability tests were conducted across different survey platforms, but no comparisons to "true" values were conducted.
  2. How accurate are the geographic locations?
    Digital elevation models (DEMs) are created by interpolating between elevation measurements as much as 50 meters apart, for this reason we cannot evaluate the accuracy of each point in a DEM, only the original data from which it is generated. The horizontal accuracy of the individual points was assessed based on the combined uncertainty of GNSS base stations and kinematic (rover) data points. Manufacturer-reported accuracy for the differentially corrected horizontal positions for the Trimble R7, R8 and R10 receivers used to collect the topographic and bathymetric measurements is 0.8 cm + 0.5 ppm. Baselines from the GNSS base station were typically less than 5 km, suggesting a horizontal accuracy of survey-vessel positions to be 0.825 cm relative to the base station. The combined horizontal uncertainty from the base station coordinate solutions and rover trajectories is between 1.5 cm and 7 cm, with a mean horizontal uncertainty of 3 cm. Uncertainty in the horizontal positions associated with pitch and roll of the survey vessels and variable surveyor posture during the backpack surveys is unknown.
  3. How accurate are the heights or depths?
    Digital elevation models (DEMs) are created by interpolating between elevation measurements as much as 50 meters apart, for this reason we cannot evaluate the accuracy of each point in a DEM, only the original data from which it is generated. An estimated vertical accuracy of 5 cm for the topographic GNSS backpack survey platforms was estimated by combining uncertainty in the base station coordinates and the manufacturer reported vertical accuracy of 1.5 cm + 1 ppm for differential corrected kinematic data. Additional uncertainty in the final computed elevations in the bathymetry data is related to unmeasured variability in the speed of sound used to compute depths from bathymetric soundings and, thus, are depth dependent. Based on analysis of all available sound velocity casts, we estimate the uncertainty in the final elevations to be 0.6 percent of water depth. The total vertical uncertainty is calculated using the standard equation sigmat=v(a^2+(b×d)^2), where sigmat is the total vertical uncertainty, a is the uncertainty of the GNSS vertical position, b is the depth-dependent uncertainty factor (0.006), and d is water depth (International Hydrographic Organization, 2008). Maximum water depths surveyed by the personal watercraft are approximately 18 m, yielding a maximum total vertical uncertainty of 12 cm.
  4. Where are the gaps in the data? What is missing?
    Dataset is considered complete for the information presented, as described in the abstract. Users are advised to read the rest of the metadata record carefully for additional details.
  5. How consistent are the relationships among the observations, including topology?
    All data fall within expected ranges.

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:
USGS-authored or produced data and information are in the public domain from the U.S. Government and are freely redistributable with proper metadata and source attribution. Please recognize and acknowledge the U.S. Geological Survey as the originator(s) of the dataset and in products derived from these data. This information is not intended for navigational purposes.
  1. Who distributes the data set? (Distributor 1 of 1)
    U.S. Geological Survey - Science Base
    U.S. Geological Survey
    Denver Federal Center, Building 810, Mail Stop 302
    Denver, CO
    USA

    1-888-275-8747 (voice)
    sciencebase@usgs.gov
  2. What's the catalog number I need to order this data set? DEM grids are available in Esri ARC/INFO ASCII GRID format (ew17_july_dem_5m.asc; ew17_july_dem_1m.asc), along with low-resolution browse graphics of each DEM in png format (ew17_july_dem_5m.png; ew17_july_dem_1m.png) and associated metadata (ew17_july_dem.xml).
  3. What legal disclaimers am I supposed to read?
    Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 20-Jun-2019
Metadata author:
U.S. Geological Survey, Pacific Coastal and Marine Science Center
Attn: PCMSC Science Data Coordinator
2885 Mission Street
Santa Cruz, CA

831-427-4747 (voice)
pcmsc_data@usgs.gov
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

This page is <https://cmgds.marine.usgs.gov/catalog/pcmsc/westcoast/washington/elwha/ew17_july_dem.faq.html>
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