EAARL Coastal Topography--Dauphin Island, Alabama, 2010: Bare Earth

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Frequently anticipated questions:

What does this data set describe?

EAARL Coastal Topography--Dauphin Island, Alabama, 2010: Bare Earth
ASCII XYZ point cloud data were produced from remotely sensed, geographically referenced elevation measurements by the U.S. Geological Survey (USGS). Elevation measurements were collected over Dauphin Island, post-Tropical Storm Bonnie (July 2010 tropical storm), using the National Aeronautics and Space Administration (NASA) Experimental Advanced Airborne Research Lidar (EAARL), 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 60 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 2-3 meters. The EAARL, developed originally by NASA at Wallops Flight Facility in Virginia, measures ground elevation with a vertical resolution of 3 centimeters. A sampling rate of 3 kilohertz or higher results in an extremely dense spatial elevation dataset. Over 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.
Raw lidar data are not in a format that is generally usable by resource managers and scientists for scientific analysis. Converting dense lidar elevation data into a readily usable format without loss of essential information requires specialized processing. The USGS'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 multi-tiered 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 to georeferenced spot (x,y,z) returns for "first surface" and "bare earth" topography. The "first returns" are indicative of vegetation-canopy height, or bare ground in the absence of vegetation, whereas "last returns" typically represent "bare-earth" elevations under vegetation. The terms first surface and bare earth refer to the digital elevation data of the terrain, but while 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, while the trailing edge algorithm (that is, the algorithm searches for the location prior to 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 portion 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 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 North American Vertical Datum of 1988. The development of custom software for creating these data products has been supported by the USGS CMGP's Lidar for Science and Resource Management project. Processed data products are used by the USGS CMGP's National Assessment of Coastal Change Hazards project to quantify the vulnerability of shorelines to coastal change hazards such as severe storms, sea-level rise, and shoreline erosion and retreat.
  1. How might this data set be cited?
    U.S. Geological Survey, 2016, EAARL Coastal Topography--Dauphin Island, Alabama, 2010: Bare Earth: U.S. Geological Survey Data Release doi:10.5066/F74Q7S3M, U.S. Geological Survey, St. Petersburg, FL.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -88.3510
    East_Bounding_Coordinate: -88.0827
    North_Bounding_Coordinate: 30.2569
    South_Bounding_Coordinate: 30.2260
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Calendar_Date: 24-Jul-2010
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: XYZ point cloud 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 1e-07. Longitudes are given to the nearest 1e-07. 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 1/298.25722210100002.
      Altitude_Datum_Name: North American Vertical Datum of 1988
      Altitude_Resolution: 0.03
      Altitude_Distance_Units: meters
      Explicit elevation coordinate included with horizontal coordinates
  7. How does the data set describe geographic features?
    The file is .xyz ASCII comma-delimited. (Source: Internally devised to maintain naming consistency.)
    decimal degrees (Source: NAD 83)
    Range of values
    decimal degrees (Source: NAD 83)
    Range of values
    Elevation in meters (Source: NAVD 88 (Geoid96))
    Range of values
    Minimum:-8.05 meters
    Maximum:70.94 meters
    Time in seconds of epoch (Source: Seconds of epoch (seconds since January 1, 1970))
    Range of values
    This file consists of bare earth elevation data available for this survey. Data has been processed via the ALPS processing software. Data has been quality controlled though the application of the RCF in ALPS and turns were manually removed. Data has been converted from the ITRF08 reference frame to NAD83, NAVD88 (Geoid96).
    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)
    • U.S. Geological Survey
  2. Who also contributed to the data set?
    Acknowledgment of the USGS, 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 USGS staff. Users should be aware that comparisons with other datasets for the same area from other time periods may be inaccurate due to 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?
    Joseph Long
    600 4th Street South
    St. Petersburg, FL

    772 502-8024 (voice)
    727 502-8182 (FAX)
    Hours_of_Service: M-F, 8:00-4:00 ET

Why was the data set created?

The purpose of this project was to provide highly detailed and accurate elevation data of coastal parks and barrier islands, post-Tropical Storm Bonnie (July 2010 tropical storm), for use as a management tool and to make these data available to natural-resource managers and research scientists. To ensure that St. Petersburg Coastal and Marine Science Center (SPCMSC) data management protocols were followed, this survey was assigned a USGS field activity number (FAN), 10LTS03. Additional survey and data details are available at http://cmgds.marine.usgs.gov/fan_info.php?fan=10LTS03.

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: 27-Jul-2010 (process 1 of 4)
    The data were collected using a Pilatus PC-6 aircraft. The EAARL 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 USGS office in St. Petersburg, Florida. Raw EAARL waveform, GPS, and Inertial Measurement Unit (IMU) data were loaded into ALPS software. ALPS (20100727) was used to interactively process small sections of data to obtain best estimates of aircraft pitch and roll biases. Pitch and roll biases are 0 and -0.75 respectively. Person who carried out this activity:
    Christine Kranenburg
    Cherokee Nation Technologies, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
    600 4th Street South
    St. Petersburg, FL

    772 502-8129 (voice)
    Hours_of_Service: M-F, 8:00-4:00 ET
    Date: 27-Jul-2010 (process 2 of 4)
    ALPS (20100727) batch processed the raw data for last return and first return topography then batch filtered the bare earth topography using the RCF. The input parameters of the random consensus filter were: grid cell size (buffer) = 700 cm x 700 cm; vertical tolerance (vertical width) = 80 cm. Person who carried out this activity:
    Christine Kranenburg
    Cherokee Nation Technologies, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
    Computer Scientist
    600 4th Street South
    St. Petersburg, FL

    (727) 502-8129 (voice)
    Date: 18-Feb-2016 (process 3 of 4)
    ALPS (20160218) was used to convert bare earth topography from ITRF08 to NAD83, NAVD88 using the Geoid96 model. Lastly, ALPS exported the NAD83, NAVD88 (Geoid96) bare earth topography from .pbd format to an ASCII XYZ file. The final .xyz file consists of four columns, longitude, latitude, elevation, and time. Person who carried out this activity:
    Christine Kranenburg
    Cherokee Nation Technologies, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
    Computer Scientist
    600 4th Street South
    St. Petersburg, FL

    (727) 502-8129 (voice)
    Date: 04-Jan-2017 (process 4 of 4)
    Keywords section of metadata optimized for discovery in USGS Coastal and Marine Geology Data Catalog. Person who carried out this activity:
    U.S. Geological Survey
    Attn: Alan O. Allwardt
    Contractor -- Information Specialist
    2885 Mission Street
    Santa Cruz, CA

    831-460-7551 (voice)
    831-427-4748 (FAX)
  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 30(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 0.07 degree, 3 centimeters nominal ranging accuracy, and vertical elevation accuracy of 15 centimeters RMSE for the topographic surface. Quality checks are built into the data-processing software.
  2. How accurate are the geographic locations?
    Raw elevation measurements have been determined to be within 1 meter in horizontal accuracy.
  3. How accurate are the heights or depths?
    Typical vertical elevation accuracies for these data are consistent with the point elevation data; however, a ground-control survey is not conducted simultaneously with every lidar survey. Vertical accuracies may vary based on the type of terrain and the accuracy of the Global Positioning System (GPS) and aircraft-attitude measurements.
  4. Where are the gaps in the data? What is missing?
    These point-cloud data may appear sparse or nonexistent, which is a result of automated filtering, digitizer failure or lack of survey coverage. Turns were manually removed where no topographic features were present.
  5. How consistent are the relationships among the observations, including topology?
    These data were processed for topography ONLY; offshore elevations are invalid.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints: None
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)
    Joe Long
    Research Oceanographer
    600 4th Street South
    St. Petersburg, FL

    (727) 502-8024 (voice)
  2. What's the catalog number I need to order this data set? BONNIE2010_Dauphin_EAARLA_BE_n88g96.xyz
  3. What legal disclaimers am I supposed to read?
    Although these data have been processed successfully on a computer system at the 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 and/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?

Who wrote the metadata?

Last modified: 31-Jul-2017
Metadata author:
Christine Kranenburg
Cherokee Nation Technologies, U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL
Computer Scientist
600 4th Street South
St. Petersburg, FL

(727) 502-8129 (voice)
Metadata standard:
FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)

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