Lidar-Derived Point Cloud for EAARL-B Submerged Topography–—Saint Thomas, U.S. Virgin Islands, 2014

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Frequently anticipated questions:

What does this data set describe?

Title:
Lidar-Derived Point Cloud for EAARL-B Submerged Topography–—Saint Thomas, U.S. Virgin Islands, 2014
Abstract:
ASCII XYZ point cloud data for a portion of the submerged environs of Saint Thomas, U.S. Virgin Islands, was produced from remotely sensed, geographically referenced elevation measurements collected on March 7, 8, 11, 12, 13, 14, 17, 18, and 24, 2014 by the U.S. Geological Survey, in collaboration with the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Conservation Program. Elevation measurements were collected over the area using the second-generation Experimental Advanced Airborne Research Lidar (EAARL-B), a pulsed laser ranging system mounted onboard an aircraft to measure ground elevation, vegetation canopy, and coastal topography. The system uses high-frequency laser beams directed at the Earth's surface through an opening in the bottom of the aircraft's fuselage. The laser system records the time difference between emission of the laser beam and the reception of the reflected laser signal in the aircraft. The plane travels over the target area at approximately 55 meters per second at an elevation of approximately 300 meters, resulting in a laser swath of approximately 240 meters with an average point spacing of 0.5-1.6 meters. The nominal vertical elevation accuracy expressed as the root mean square error (RMSE) is 13.5 centimeters. A peak sampling rate of 15-30 kilohertz results in an extremely dense spatial elevation dataset. More than 100 kilometers of coastline can be surveyed easily within a 3- to 4-hour mission. When resultant elevation maps for an area are analyzed, they provide a useful tool to make management decisions regarding land development.
Supplemental_Information:
Raw lidar data are not in a format that is generally usable by natural-resource managers and research scientists for scientific analysis. Converting dense lidar elevation data into a readily usable format without loss of essential information requires specialized processing. The U.S. Geological Survey's Coastal and Marine Geology Program (CMGP) has developed custom software to convert raw lidar data into a GIS-compatible map product to be provided to GIS specialists, managers, and scientists. The primary tool used in the conversion process is Airborne Lidar Processing System (ALPS), a multitiered processing system developed originally by a USGS-NASA collaborative project. Specialized processing algorithms are used to convert raw waveform lidar data acquired by the EAARL-B to georeferenced spot (x,y,z) returns for "first surface" and "bare earth" topography. The terms first surface and bare earth refer to the digital elevation data of the terrain, although first-surface data include vegetation, buildings, and other manmade structures, bare-earth data do not. The zero crossing of the second derivative (that is, detection of stationary points) is used to detect the first return, resulting in "first surface" topography, whereas the trailing edge algorithm (that is, the algorithm searches for the location before the last return where direction changes along the trailing edge) is used to detect the range to the last return, or "bare earth" (the first and last returns being the first and last significant measurable part of the return pulse). Statistical filtering, known as the Random Consensus Filter (RCF), is used to remove false bottom returns and other outliers from the EAARL-B topography data. The filter uses a grid of non-overlapping square cells (buffer) of user-defined size overlaid onto the original point cloud. The user also defines the vertical tolerance (vertical width) based on the topographic complexity and point-sampling density of the data. The maximum allowable elevation range within a cell is established by this vertical tolerance. An iterative process searches for the maximum concentration of points within the vertical tolerance and removes those points outside of the tolerance (Nayegandhi and others, 2009). These data are then converted to the North American Datum of 1983 and the Virgin Islands Vertical Datum of 2009 (using GEOID12B); please note that these data are not suitable for determining absolute elevation measurements.
  1. How might this data set be cited?
    Fredericks, Xan, Kranenburg, Christine J., and Nagle, David B., 20151008, Lidar-Derived Point Cloud for EAARL-B Submerged Topography–—Saint Thomas, U.S. Virgin Islands, 2014: U.S. Geological Survey Data Release doi:10.5066/F7G15XXG, U.S. Geological Survey, St. Petersburg, FL.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -65.27130878
    East_Bounding_Coordinate: -64.79665876
    North_Bounding_Coordinate: 18.36943200
    South_Bounding_Coordinate: 18.20183633
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Calendar_Date: 07-Mar-2014
    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?
      Grid_Coordinate_System_Name: Universal Transverse Mercator
      Universal_Transverse_Mercator:
      UTM_Zone_Number: 20
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.999600
      Longitude_of_Central_Meridian: -63.000000
      Latitude_of_Projection_Origin: 0
      False_Easting: 500000.000000
      False_Northing: 0
      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest 2.0
      Ordinates (y-coordinates) are specified to the nearest 2.0
      Planar coordinates are specified in meters
      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 1/298.25722210100002.
      Vertical_Coordinate_System_Definition:
      Altitude_System_Definition:
      Altitude_Datum_Name: Virgin Islands Vertical Datum of 2009 (GEOID12B)
      Altitude_Resolution: 0.20
      Altitude_Distance_Units: meters
      Altitude_Encoding_Method:
      Explicit elevation coordinate included with horizontal coordinates
  7. How does the data set describe geographic features?
    *.xyz
    The "*" represents the file name. The file is .xyz ASCII space-delimited. (Source: Internally devised to maintain naming consistency.)
    X
    UTM easting in meters (Source: NAD 83) varying value range
    Y
    UTM northing in meters (Source: NAD 83) varying value range
    Z
    Altitude in meters (Source: VIVD 09) varying value range
    Entity_and_Attribute_Overview:
    The input parameters for the random consensus filter (RCF) were: grid cell size (buffer) = 1000 centimeters x 1000 centimeters; vertical tolerance (vertical width) = 200 centimeters.
    Entity_and_Attribute_Detail_Citation: http://pubs.usgs.gov/of/2009/1078/

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • Xan Fredericks
    • Christine J. Kranenburg
    • David B. Nagle
  2. Who also contributed to the data set?
    Acknowledgment of the U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, as a data source would be appreciated in products developed from these data, and such acknowledgment as is standard for citation and legal practices for data source is expected. Sharing of new data layers developed directly from these data would also be appreciated by the U.S. Geological Survey staff. Users should be aware that comparisons with other datasets for the same area from other periods may be inaccurate because of inconsistencies resulting from changes in photointerpretation, mapping conventions, and digital processes over time. These data are not legal documents and are not to be used as such.
  3. To whom should users address questions about the data?
    Xan Fredericks
    Cherokee Nation Technology Solutions, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
    Lidar Validation and Processing Analyst
    600 4th Street South
    St. Petersburg, FL
    USA

    727 502-8086 (voice)
    727 502-8182 (FAX)
    afredericks@usgs.gov
    Hours_of_Service: M-F, 8:00-4:00 ET

Why was the data set created?

The purpose of this project was to produce a highly detailed and accurate digital elevation map for a portion of the submerged environs of Saint Thomas, U.S. Virgin Islands, for use as a management tool and to make these data available to natural-resource managers and research scientists. To ensure that SPCMSC data management protocols were followed, this survey was retroactively assigned a USGS field activity number (FAN), 14CNT03. Additional survey and data details are available at http://cmgds.marine.usgs.gov/fan_info.php?fan=14CNT03.

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: 28-May-2015 (process 1 of 4)
    The data were collected using a Cessna 310 aircraft. The EAARL-B laser scanner collects the data using a green-wavelength (532-nanometer) raster scanning laser, while a digital camera acquires a visual record of the flight. The data are stored on hard drives and archived at the U.S. Geological Survey office in St. Petersburg, Florida. The navigational data are processed and then, along with the raw data, are downloaded into ALPS, or the Airborne Lidar Processing System (20140311-20150528). Data are converted from units of time to x,y,z points for elevation, which are then manually edited and quality checked to ensure anomalies and noise have been addressed to specification. Person who carried out this activity:
    Xan Fredericks
    Cherokee Nation Technology Solutions, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
    Lidar Validation and Processing Analyst
    600 4th Street South
    St. Petersburg, FL
    USA

    727 502-8086 (voice)
    afredericks@usgs.gov
    Hours_of_Service: M-F, 8:00-4:00 ET
    Date: 23-Apr-2015 (process 2 of 4)
    The data were depth calibrated in ALPS (20150423) using a linear model with a slope of 0.98103 and an intercept of -0.000681. Person who carried out this activity:
    Christine Kranenburg
    Cherokee Nation Technology Solutions, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
    Programmer/Analyst
    600 4th Street South
    St. Petersburg, FL
    USA

    727 502-8129 (voice)
    ckranenburg@usgs.gov
    Hours_of_Service: M-F, 7:30-3:30 ET
    Date: 09-Sep-2015 (process 3 of 4)
    Metadata imported into ArcCatalog 10.1.1.3143 from XML file. Person who carried out this activity:
    Xan Fredericks
    Cherokee Nation Technology Solutions, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
    Lidar Validation and Processing Analyst
    600 4th Street South
    St. Petersburg, FL
    USA

    727 502-8086 (voice)
    afredericks@usgs.gov
    Hours_of_Service: M-F, 8:00-4:00 ET
    Date: 13-Oct-2020 (process 4 of 4)
    Added keywords section with USGS persistent identifier as theme keyword. Person who carried out this activity:
    U.S. Geological Survey
    Attn: VeeAnn A. Cross
    Marine Geologist
    384 Woods Hole Road
    Woods Hole, MA

    508-548-8700 x2251 (voice)
    508-457-2310 (FAX)
    vatnipp@usgs.gov
  3. What similar or related data should the user be aware of?
    Nayegandhi, A., Brock, J.C., and Wright, C.W., 2009, Small footprint, waveform-resolving lidar estimation of submerged and subcanopy topography in coastal environments: International Journal of Remote Sensing v. 30 no. 4, p. 861-878.

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

  1. How well have the observations been checked?
    The expected accuracy of the measured variables is as follows: attitude within 0.05 degree and 3 centimeters nominal ranging accuracy. Quality checks are built into the data-processing software.
  2. How accurate are the geographic locations?
    Raw lidar measurements have been determined to be within 1 meter in horizontal accuracy.
  3. How accurate are the heights or depths?
    The nominal vertical elevation accuracy of the point elevation data expressed as the root mean square error (RMSE) is 13.5 centimeters. Typical vertical elevation accuracies for these data are consistent with the point elevation data.
  4. Where are the gaps in the data? What is missing?
    Missing areas are a result of the survey not covering a particular region, optical water depth of greater than 1.5 Secchi disc depths, or the manual removal of lidar processing artifacts.
  5. How consistent are the relationships among the observations, including topology?
    These data are located in UTM Zone 20.

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 are freely redistributable with proper metadata and source attribution. The U.S. Geological Survey requests to be acknowledged as originator of these data in future products or derivative research.
  1. Who distributes the data set? (Distributor 1 of 1)
    U.S. Geological Survey
    Attn: Xan Fredericks
    Lidar Validation and Processing Analyst, Cherokee Nation Technology Solutions
    600 4th Street South
    St. Petersburg, FL
    USA

    727 502-8086 (voice)
    Hours_of_Service: M-F, 8:00-4:00 ET
  2. What's the catalog number I need to order this data set? STTH2014_EAARLB_v09g12B.xyz
  3. What legal disclaimers am I supposed to read?
    Although these data have been processed successfully on a computer system at 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. The USGS shall not be held liable for improper or incorrect use of the data described or contained herein. Any use of trade, firm, or product 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?
  5. Is there some other way to get the data?
    Contact U.S. Geological Survey for details.

Who wrote the metadata?

Dates:
Last modified: 02-Nov-2020
Metadata author:
Xan Fredericks
Cherokee Nation Technology Solutions, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
Lidar Validation and Processing Analyst
600 4th Street South
St. Petersburg, FL
USA

727 502-8086 (voice)
afredericks@usgs.gov
Hours_of_Service: M-F, 8:00-4:00 ET
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
This page is <https://cmgds.marine.usgs.gov/catalog/spcmsc/STTH2014_EAARLB_v09g12B_metadata.faq.html>
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