Elevation of marsh units in Blackwater salt marsh complex, Chesapeake Bay, Maryland

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Frequently anticipated questions:

What does this data set describe?

Title:
Elevation of marsh units in Blackwater salt marsh complex, Chesapeake Bay, Maryland
Abstract:
This data release contains coastal wetland synthesis products for the geographic region of Blackwater salt marsh complex, Chesapeake Bay, Maryland. Metrics for resiliency, including unvegetated to vegetated ratio (UVVR), marsh elevation, and others, are calculated for smaller units delineated from a digital elevation model, providing the spatial variability of physical factors that influence wetland health. The U.S. Geological Survey has been expanding national assessment of coastal change hazards and forecast products to coastal wetlands with the intent of providing Federal, State, and local managers with tools to estimate the vulnerability and ecosystem service potential of these wetlands. For this purpose, the response and resilience of coastal wetlands to physical factors need to be assessed in terms of the ensuing change to their vulnerability and ecosystem services.
  1. How might this data set be cited?
    Ackerman, Kate V., Defne, Zafer, and Ganju, Neil K., 20210518, Elevation of marsh units in Blackwater salt marsh complex, Chesapeake Bay, Maryland: data release DOI:10.5066/P9M7H6XG, U.S. Geological Survey, Coastal and Marine Hazards and Resources Program, Woods Hole Coastal and Marine Science Center, Woods Hole, MA.

    Online Links:

    This is part of the following larger work.

    Ackerman, Kate V., Defne, Zafer, and Ganju, Neil K., 2021, Coastal wetlands of the Blackwater region, Chesapeake Bay, Maryland: data release DOI:10.5066/P9M7H6XG, U.S. Geological Survey, Reston, VA.

    Online Links:

    Other_Citation_Details:
    Suggested citation: Ackerman, K.V., Defne, Z., and Ganju, N.K., 2021, Coastal wetlands of the Blackwater region, Chesapeake Bay, Maryland: U.S. Geological Survey data release, https://doi.org/10.5066/P9M7H6XG.
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -76.3333
    East_Bounding_Coordinate: -75.6812
    North_Bounding_Coordinate: 38.5053
    South_Bounding_Coordinate: 38.2175
  3. What does it look like?
    https://www.sciencebase.gov/catalog/file/get/608aee34d34ef6cfc4aba938/?name=Elev_BW_Browse.png (PNG)
    Graphic that shows elevation of conceptual marsh units of the Blackwater region salt marsh complex.
  4. Does the data set describe conditions during a particular time period?
    Calendar_Date: 2021
    Currentness_Reference:
    publication date
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: Vector Digital dataset (Polygon)
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a Vector data set. It contains the following vector data types (SDTS terminology):
      • G-polygon (8518)
    2. What coordinate system is used to represent geographic features?
      The map projection used is WGS 1984 Web Mercator Auxiliary Sphere (Esri full name: WGS_1984_Web_Mercator_Auxiliary_Sphere).
      Projection parameters:
      False_Easting: 0.0
      False_Northing: 0.0
      Longitude_of_Central_Meridian: 0.0
      Standard_Parallel: 0.0
      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest 0.6096
      Ordinates (y-coordinates) are specified to the nearest 0.6096
      Planar coordinates are specified in meters
      The horizontal datum used is WGS_1984.
      The ellipsoid used is WGS 84.
      The semi-major axis of the ellipsoid used is 6378137.0.
      The flattening of the ellipsoid used is 1/298.257223563.
  7. How does the data set describe geographic features?
    mu_Elev_BW
    Table containing attribute information associated with the dataset. (Source: USGS)
    FID
    Internal feature number. (Source: Esri) Unique whole numbers that are automatically generated.
    Shape
    Feature geometry. (Source: Esri) Coordinates defining the features.
    FID_CMU
    Unique whole numbers used to identify each conceptual marsh unit. (Source: USGS)
    Range of values
    Minimum:0
    Maximum:8532
    Units:none
    ATOT_M2
    Total surface area of a marsh unit. (Source: USGS)
    Range of values
    Minimum:142.611297132
    Maximum:3039925.47635
    Units:m^2
    AVEG_M2
    Surface area of the vegetated part of a marsh unit. (Source: USGS)
    Range of values
    Minimum:0.0
    Maximum:2284096.35511
    Units:m^2
    MU_ELEV
    Mean elevation of a marsh unit. Value is set to -9999 if MU_HFAR is greater than 0.25. No data value is -9999. (Source: USGS)
    Range of values
    Minimum:0
    Maximum:2.35209
    Units:meters
    VG_ELEV
    Mean elevation of the vegetated area in a marsh unit. Value is set to -9999 if there is no vegetated area in the marsh unit. No data value is -9999. (Source: USGS)
    Range of values
    Minimum:0
    Maximum:2.352095
    Units:meters
    MU_HFAR
    Percent (in decimal form) of the marsh unit that contains hydro-flattened bathymetric/elevation values. (Source: USGS)
    Range of values
    Minimum:0.0
    Maximum:1.0
    Units:none
    VG_HFAR
    Percent (in decimal form) of the vegetated portion of the marsh unit that contains hydro-flattened bathymetric/elevation values. No data value is -9999. (Source: USGS)
    Range of values
    Minimum:0.0
    Maximum:1.0
    Units:none
    SKWNSS
    The measure of skewness of the elevation distribution in the vegetated part of a marsh unit. Positive skewness values (i.e., right-skewed distribution) indicate clustering of elevation distribution towards lower elevations. Negative values (left-skewed distribution) indicate clustering towards higher elevations. Values of -9999 indicate no vegetated area within the marsh unit or no elevation value for the vegetated portion. No data value is -9999. (Source: USGS)
    Range of values
    Minimum:-4.94626
    Maximum:7.58949
    Units:none
    LOW3RD
    LOW3RD is the percent of elevation values that fall within the lowest third of the elevation range within the vegetated part of a marsh unit. Values of -9999 indicate no vegetated area within the marsh unit or no elevation value for the vegetated portion. No data value is -9999. (Source: USGS)
    Range of values
    Minimum:0.004229
    Maximum:99.9467
    Units:none
    FLG
    Marsh unit flag indicating anomalous marsh units based on absence of vegetation, elevation and surface area. FLG values indicate: (-1) no vegetated area in the marsh unit; (-10) marsh unit elevation higher than the 99.8 percentile; (-100) marsh unit elevation less than 0.2 percentile; (-200) greater than 25 percent of the marsh unit elevation is hydro-flattened; (-1000) marsh unit surface area less than 900 m^2; and (0) no flag. Combination of negative values indicates a combination of flags; for example, -1001 indicates no vegetated area and area less than 900 m^2. (Source: Producer Defined)
    Range of values
    Minimum:-1201
    Maximum:0
    Units:none
    Entity_and_Attribute_Overview:
    In this dataset, mean elevation for each conceptual salt marsh unit in the Blackwater region salt marsh complex has been calculated based on the 1-meter resolution raster source dataset. Decimal values in the attribute table are a result of double precision calculations while significant digits are considered in defining the attribute measurement resolutions. Therefore, the smallest unit increment for area values and elevation is assumed to be 1 meter and 0.01, respectively.
    Entity_and_Attribute_Detail_Citation: USGS

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • Kate V. Ackerman
    • Zafer Defne
    • Neil K. Ganju
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
    Kate Ackerman
    U.S. Geological Survey
    Geologist
    384 Woods Hole Road
    Woods Hole, MA
    US

    508-548-8700 x2331 (voice)
    508-457-2310 (FAX)
    kackerman@usgs.gov

Why was the data set created?

The purpose of this shapefile is to calculate the mean elevation for each marsh unit and its vegetated component. Analysis of marsh elevation is part of a comprehensive assessment to identify the factors and their weights in determining the vulnerability and resiliency of salt marshes. Elevation distribution in the Blackwater salt marsh complex is given in terms of mean elevation of conceptual marsh units. The elevation data is based on the 1-meter resolution Coastal National Elevation Database (CoNED).

How was the data set created?

  1. From what previous works were the data drawn?
    CoNED (source 1 of 2)
    U.S. Geological Survey, 2016, Topobathymetric Model for Chesapeake Bay Region - District of Columbia, States of Delaware, Maryland, Pennsylvania, and Virginia, 1859 to 2015: U.S. Geological Survey, Reston, VA.

    Online Links:

    Type_of_Source_Media: Digital
    Source_Contribution:
    Downloaded CoNED Digital Elevation Model geodatabase. Projection was NAD 1983 UTM Zone 18N with the North American Vertical Datum of 1988 (NAVD 88). Download date was 2020/01/29.
    CMU_BW (source 2 of 2)
    U.S. Geological Survey, 2021, Conceptual marsh units of Blackwater salt marsh complex, Chesapeake Bay, Maryland: U.S. Geological Survey, Reston, VA.

    Online Links:

    Type_of_Source_Media: Digital
    Source_Contribution:
    Used conceptual marsh units to delineate marsh unit boundaries and areas of vegetation.
  2. How were the data generated, processed, and modified?
    Date: 2020 (process 1 of 1)
    This process step and all subsequent process steps were performed by the same person, Kate Ackerman, in ArcMap (ver. 10.7.1) using tools from ArcToolbox, unless otherwise stated. For complex operations, names of specific tools used are given in CAPITAL letters (any critical parameters used are given in parentheses, separated by a semicolon, immediately after the tool name). The input and output file names are provided in [square brackets] when necessary. Units for length and area calculations are meters (m) and square meters (m^2) unless otherwise stated.
    a) Set the data frame coordinate system and projection to NAD 1983 UTM Zone 18N.
    b) Dissolve the vegetated and unvegetated classes in each marsh unit to have only one class for each marsh unit. DISSOLVE(Input features=[CMU_BW.shp]; Dissolve field=FID_CMU; Statistics field=ATOT_M2 and FLG with Statistics type= First). PROJECT(Input coordinate system=WGS 1984 Web Mercator Auxiliary Sphere; Output coordinate system=NAD 1983 UTM Zone 18N; Geographic transformation=WGS 1984 (ITRF00) to NAD 1983) the feature dataset to obtain dissolved marsh units [mu_diss.shp].
    c) Create a rectangular polygon that covers the domain [extent.shp]. CLIP the CoNED elevation dataset by the boundaries polygon [elev_mosaic.tif].
    d) Remove pixels from elevation raster that are hydro-flattened. Use FOCAL STATISTICS(Input raster=[elev_mosaic.tif], Output raster=[FocalStats_3x3_range.tif], Neighborhood settings=3x3 cell, Statistics type=range) and RASTER CALCULATOR to identify [Map algebra expression: Con("%FocalStats_3x3_range.tif%" == 0,-1,1); Output raster=[elev_mosaic_One_NegOne.tif]] and remove [Map algebra expression: SetNull("%elev_mosaic_One_NegOne.tif%", "%elev_mosaic.tif%", "value = -1"); Output raster=[elev_mosaic_noHF.tif]] all hydro-flattened areas from [elev_mosaic.tif] to create [elev_mosaic_noHF.tif].
    e) Calculate the mean elevation for each marsh unit with ZONAL STATISTICS AS TABLE(Feature zone data=[mu_diss.shp], Zone field=FID_CMU; Input value raster=[elev_mosaic_noHF.tif]; Statistics type=Mean; Output table=[mu]; Ignore Nodata=True). ADD JOIN (Layer name=[mu_diss.shp]; Input join field=FID_CMU; Join Table=[mu]; Output join field=FID_CMU) to marsh units layer based on the "FID_CMU" field.
    f) Export dataset as a shapefile [mu_elev.shp] and keep the 'mu_MEAN" and "mu_ATOT_M2" fields from the join operation.
    g) In [mu_elev.shp], add field (MU_HFAR) to calculate the percent of the marsh unit that has hydro-flattened area. If the percent hydro-flattened area is greater than 25 percent, the marsh unit elevation is set to -9999.
    h) Calculate elevation of the vegetated areas only. SELECT BY ATTRIBUTES FROM [CMU_BW.shp] where "TYP"= 'vegetated.' Follow steps (e) to (g) using the selected features to export [veg_elev.shp].
    i) JOIN FIELD(Input table=[mu_elev.shp]; Input join field=mu_FID_CMU; Join table=[veg_elev.shp]; Output join field=vg_FID_CMU; Join fields=vg_AVEG_M2, vg_ATOT_M2, vg_MEAN). Change the field names to "VG_ELEV" and "MU_ELEV" for the fields with the mean elevation for the vegetated area only and mean elevation for the entire marsh unit, respectively. Name fields "AVEG_M2" and "ATOT_M2" for vegetated and total area, respectively; and "MU_HFAR" and "VG_HFAR" for the percent (in decimal form) of the marsh unit, and of the vegetated portion, that has hydro-flattened area, respectively. Set MU_ELEV to -9999 if MU_HFAR is greater than 0.25; set VG_ELEV to -9999 if there is no vegetated area in the marsh unit or if VG_HFAR is greater than 0.25. Export to a new shapefile [mu_elev_BW.shp].
    j) Run Python script (marsv5.py) in ArcPro (used 2.4.1 for this data release) to calculate metrics that quantify the distribution of marsh unit elevation values. Skewness ("SKWNSS") is the measure of asymmetry of the elevation distribution in the vegetated part of a marsh unit. Positive skewness values, i.e. right-skewed distributions, indicate clustering of elevation distributions towards lower elevations. Negative skewness values indicate a left-skewed distribution and clustering towards higher elevations. Lower third ("LOW3RD") is the percent of elevation values that fall within the lowest third of the elevation range within the vegetated part of a marsh unit. SKWNSS and LOW3RD parameters are calculated by applying the formulation of Raposa et al. (2016) to the elevation raster within the vegetated part of each marsh unit. If SKWNESS and LOW3RD values= 0 and VG_ELEV= -9999, set SKWNESS and LOW3RD values to -9999.
    k) SELECT BY ATTRIBUTES from [CMU_BW.shp] where "TYP"= 'vegetated,’ and join field "APGN_M2" from [CMU_BW.shp] to the feature dataset [mu_elev_BW.shp] and rename this field "AVEG_M2."
    l) Add field "FLG" to flag anomalous marsh units based on absence of vegetation, elevation and surface area. The flags are calculated in their respective shapefile (i.e., elevation flags are calculated in the elevation shapefile). Set FLG to: (-1) no vegetated area in the marsh unit; (-10) marsh unit elevation higher than the 99.8 percentile; (-100) marsh unit elevation less than 0.2 percentile; (-200) greater than 25 percent of the marsh unit elevation is hydro-flattened; (-1000) marsh unit surface area less than 900 m^2; and (0) no flag. Combination of negative values indicates a combination of flags; for example, -1001 indicates no vegetated area and area less than 900 m^2.
    m) Rearrange field names and change the projection for better performance of web services with online base maps. PROJECT(Input coordinate system=NAD 1983 UTM Zone 18N; Output coordinate system=WGS 1984 Web Mercator Auxiliary Sphere; Geographic transformation=WGS 1984 (ITRF00) to NAD 1983) the feature dataset to obtain the final elevation dataset [mu_elev_BW.shp].
    Reference: Raposa, K. B., Wasson, K., Smith, E., Crooks, J. A., Delgado, P., Fernald, S. H., et al. (2016). Assessing tidal marsh resilience to sea-level rise at broad geographic scales with multi-metric indices. Biological Conservation, 204, 263–275. https://doi.org/10.1016/J.BIOCON.2016.10.015. Person who carried out this activity:
    Kate Ackerman
    U.S. Geological Survey
    Geologist
    384 Woods Hole Road
    Woods Hole, MA
    US

    508-548-8700 x2331 (voice)
    508-457-2310 (FAX)
    kackerman@usgs.gov
    Data sources used in this process:
    • CoNED
    • CMU_BW
    Data sources produced in this process:
    • mu_UVVR_BW.shp
  3. What similar or related data should the user be aware of?

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

  1. How well have the observations been checked?
    Marsh units and water and land boundaries inherit their horizontal accuracy from the accuracy of the source data, the Blackwater marsh units, and their vertical accuracy from the 2016 U.S. Geological Survey Coastal National Elevation Database (USGS CoNED).
  2. How accurate are the geographic locations?
    Horizontal accuracy is inherited from the source dataset that delineates the marsh units and can be considered to be +/-6 meters.
  3. How accurate are the heights or depths?
    Because the elevation values are averaged over delineated polygons, vertical accuracy depends on both the horizontal accuracy of the NWI dataset and the vertical accuracy of the source elevation data, CoNED. CoNED vertical elevation accuracy is 0.2 meters.
  4. Where are the gaps in the data? What is missing?
    The results are specific to the marsh polygons as defined within the boundaries of the Blackwater region salt marsh complex. A detailed on-the-ground analysis of a single site may result in a different interpretation of the wetland and marsh unit boundaries.
  5. How consistent are the relationships among the observations, including topology?
    Positive elevation indicates above sea level. Marsh units with extremely low elevation (smaller than 0.2 percentile value), extremely high elevation (larger than 99.8 percentile value) or areas less than 900 square meters were flagged. In two areas (around marsh unit 3094 and marsh unit 4424), there are marsh units that have straight lines for boundaries; this is due to larger areas of hydro-flattened bathymetry/elevation values that do not allow the basin analysis to identify ridge lines between basins. Most of these marsh units have a flag that shows that the hydro-flattened area of the marsh unit is greater than 25 percent of the marsh unit area. In a few cases, there are hydro-flattened values on land, for example in the area of MU 4424.

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:
The mean elevation for each marsh unit is defined for scientific research purposes and should not be used as a sole source of reference for any regulations and policy making. Public domain data from the U.S. Government are freely redistributable with proper metadata and source attribution. Please recognize the U.S. Geological Survey as the source of this information.
  1. Who distributes the data set? (Distributor 1 of 1)
    U.S. Geological Survey - ScienceBase
    Denver Federal Center, Building 810, Mail Stop 302
    Denver, CO

    1-888-275-8747 (voice)
    sciencebase@usgs.gov
  2. What's the catalog number I need to order this data set? mu_Elev_BW.shp (and other files associated with a shapefile), browse graphic (elev_BW_Browse.png), and CSDGM metadata in XML format.
  3. What legal disclaimers am I supposed to read?
    Neither the U.S. Government, the Department of the Interior, nor the USGS, nor any of their employees, contractors, or subcontractors, make any warranty, express or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe on privately owned rights. 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. Not for navigational use.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 08-Jun-2021
Metadata author:
Kate V. Ackerman
U.S. Geological Survey
Geologist
384 Woods Hole Rd
Woods Hole, MA
US

508-548-8700 x2331 (voice)
508-457-2310 (FAX)
kackerman@usgs.gov
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
This page is <https://cmgds.marine.usgs.gov/catalog/whcmsc/SB_data_release/DR_P9M7H6XG/Elev_BW.faq.html>
Generated by mp version 2.9.50 on Tue Sep 21 18:18:56 2021