Lidar-derived elevation data for the Twentymile River watershed, Southcentral Alaska, collected August-October 2022

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Frequently anticipated questions:


What does this data set describe?

Title:
Lidar-derived elevation data for the Twentymile River watershed, Southcentral Alaska, collected August-October 2022
Abstract:
Lidar-derived elevation data for the Twentymile River watershed, Southcentral Alaska, collected August-October 2022, Raw Data File 2023-3, uses aerial lidar to produce a classified point cloud, digital surface model (DSM), digital terrain model (DTM), and intensity model of the Twentymile River watershed, Southcentral Alaska, during snow-free ground conditions from August to October 2022. The survey provides snow-free surface elevations for trail planning and assessing avalanche hazards, among other objectives. Ground control data were collected on August 31, 2022, and aerial lidar data were collected on multiple days from August 29 to October 14, 2022, and subsequently processed in a suite of geospatial processing software. These products are released as a Raw Data File with an open end-user license. All files can be downloaded from the Alaska Division of Geological & Geophysical Surveys website (http://doi.org/10.14509/30959).
Supplemental_Information:
classified_points:    Classified point cloud data are provided in LAS format. Data are classified in accordance with ASPRS 2019 guidelines and contain return and intensity information. The average pulse spacing for non-noise returns is 26.9 cm and the average pulse density is 13.8 pts/m2. For ground points, the average point spacing is 59.6 cm and the average density is 2.82 pts/m2. Within the survey area and the scientific priority area, the average pulse spacing and density is 20.7 cm and 23.24 pts/m2, respectively, and the average ground point spacing and density is 64.4 cm and 2.41 pts/m2.	
dsm:    The DSM represents surface elevations, including heights of vegetation, buildings, powerlines, etc. The DSM is a single-band, 32-bit GeoTIFF file of 50-centimeter resolution, delivered in 1 km by 1 km tiles. No Data value is set to -9999.	
dtm:    The DTM represent bare earth elevations, excluding vegetation, bridges, buildings, etc. the DTM is single-band, 32-bit float GeoTIFF file, of 50-centimeter ('DTM'). No Data value is set to -9999.	
dtm_detail:    The detailed DTM represents bare earth elevations, excluding vegetation. The DTM is single-band, 32-bit float GeoTIFF file, of 20-cm resolution. This higher-resolution DTM file covers an area identified as a scientific priority.  No Data value is set to -9999.	
lidar_intensity:    The lidar intensity image describes the relative amplitude of reflected signals contributing to the point cloud. Lidar intensity is primarily a function of scanned object reflectance in relation to the signal frequency, is dependent on ambient conditions, and is not necessarily consistent between separate scans. The intensity image is a single-band, 16-bit unsigned GeoTIFF file of 0.5-meter resolution. No Data value is set to -9999.	
lidar_las_index:    The LAS index file is a shapefile with polygons that serve as an index to the spatial location and extent of each point cloud (LAS) bin.
  1. How might this data set be cited?
    Zechmann, J.M., Wikstrom Jones, Katreen, and Wolken, G.J., 2023, Lidar-derived elevation data for the Twentymile River watershed, Southcentral Alaska, collected August-October 2022: Raw Data File RDF 2023-3, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 8 p.
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -149.074295
    East_Bounding_Coordinate: -148.557069
    North_Bounding_Coordinate: 61.082818
    South_Bounding_Coordinate: 60.787275
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: Aug-2022
    Ending_Date: Oct-2022
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: data
  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?
      Grid_Coordinate_System_Name: Universal Transverse Mercator
      Universal_Transverse_Mercator:
      UTM_Zone_Number: 6
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.999600
      Longitude_of_Central_Meridian: -147
      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 .00000001
      Ordinates (y-coordinates) are specified to the nearest .00000001
      Planar coordinates are specified in Meters
      The horizontal datum used is NAD83 (2011).
      The ellipsoid used is GRS 80.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257222101.
      Vertical_Coordinate_System_Definition:
      Altitude_System_Definition:
      Altitude_Datum_Name: NAVD88, GEOID12B
      Altitude_Resolution: 0.001
      Altitude_Distance_Units: meters
      Altitude_Encoding_Method:
      Explicit elevation coordinate included with horizontal coordinates
  7. How does the data set describe geographic features?
    classified_points
    Classified point cloud data are provided in LAS format. Data are classified in accordance with ASPRS 2019 guidelines and contain return and intensity information. The average pulse spacing for non-noise returns is 26.9 cm and the average pulse density is 13.8 pts/m2. For ground points, the average point spacing is 59.6 cm and the average density is 2.82 pts/m2. Within the survey area and the scientific priority area, the average pulse spacing and density is 20.7 cm and 23.24 pts/m2, respectively, and the average ground point spacing and density is 64.4 cm and 2.41 pts/m2. (Source: DGGS)
    dsm
    The DSM represents surface elevations, including heights of vegetation, buildings, powerlines, etc. The DSM is a single-band, 32-bit GeoTIFF file of 50-centimeter resolution, delivered in 1 km by 1 km tiles. No Data value is set to -9999. (Source: DGGS)
    dtm
    The DTM represent bare earth elevations, excluding vegetation, bridges, buildings, etc. the DTM is single-band, 32-bit float GeoTIFF file, of 50-centimeter ('DTM'). No Data value is set to -9999. (Source: DGGS)
    dtm_detail
    The detailed DTM represents bare earth elevations, excluding vegetation. The DTM is single-band, 32-bit float GeoTIFF file, of 20-cm resolution. This higher-resolution DTM file covers an area identified as a scientific priority. No Data value is set to -9999. (Source: DGGS)
    lidar_intensity
    The lidar intensity image describes the relative amplitude of reflected signals contributing to the point cloud. Lidar intensity is primarily a function of scanned object reflectance in relation to the signal frequency, is dependent on ambient conditions, and is not necessarily consistent between separate scans. The intensity image is a single-band, 16-bit unsigned GeoTIFF file of 0.5-meter resolution. No Data value is set to -9999. (Source: DGGS)
    lidar_las_index
    The LAS index file is a shapefile with polygons that serve as an index to the spatial location and extent of each point cloud (LAS) bin. (Source: DGGS)

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?
    The area of this survey is the traditional homelands of the Dena'ina people. We partnered with U.S. Forest Service to cost-share the acquisition of lidar data in an area of mutual interest. We thank Clearwater Air for their aviation expertise and contribution to these data products and the Alaska Division of Mining, Land, and Water for collecting ground control points. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.
  3. To whom should users address questions about the data?
    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
    Contact_Instructions:
    Please view our website (https://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.

Why was the data set created?

The orthoimage and elevation data were collected for assessing coastal hazards and changes.

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: 14-Oct-2022 (process 1 of 3)
    Aerial photogrammetric survey - DGGS used a Riegl VUX1-LR laser scanner integrated with a global navigation satellite system (GNSS) and Northrop Grumman LN-200C inertial measurement unit (IMU) designed by Phoenix LiDAR Systems. The sensor can collect up to 820,000 points per second at a range of up to 150 m. The scanner operated with a pulse refresh rate of 200,000-400,000 pulses per second over forested terrain, and 50,000-100,000 pulses per second over alpine terrain, with a scan rate between 80 and 220 lines per second. We used a Cessna 180 fixed-wing aircraft to survey from an elevation of approximately 60-600 m above ground level, at a ground speed of approximately 40 m/s, and with a scan angle set from 80 to 280 degrees. The total survey area covers approximately 488 km2. The survey area was accessed by air from the Girdwood-Alyeska and Merrill Field airports in Girdwood and Anchorage. See table 1 for departure and return times and weather conditions.
    Date: 31-Aug-2022 (process 2 of 3)
    Ground survey - The Alaska Division of Mining, Land, and Water (DMLW) collected ground control points on August 31, 2022. They deployed a Trimble R12 GNSS receiver at an unnamed survey marker (0.5 in steel rod in a 4 in pipe casing) offset from the south side of the Alaska Railroad Portage Depot parking lot. It provided a two-hour base station occupation and real-time kinematic (RTK) corrections to points they surveyed with a rover Trimble R12i GNSS receiver (internal antenna). DMLW collected 24 ground control points and check points to use for calibration and to assess the vertical accuracy of the point cloud. Most points were collected on bare earth or paved surfaces. However, ground control points 9, 17, 18, and 24 were collected in forest and shrubland.
    Date: 2022 (process 3 of 3)
    Lidar dataset processing - We processed point data in SDCimport software for initial filtering and multiple-time-around (MTA) disambiguation. MTA errors, corrected in this process, result from ambiguous interpretations of received pulse time intervals and occur more frequently with higher pulse refresh rates. We processed Inertial Measurement Unit (IMU) and Global Navigation Satellite System (GNSS) data in Inertial Explorer, and we used Spatial Explorer software to integrate flightline information with the point cloud. We calibrated the point data at an incrementally precise scale of sensor movement and behavior, incorporating sensor velocity, roll, pitch, and yaw fluctuations throughout the survey. We created macros in Terrasolid software and classified points per the American Society for Photogrammetry and Remote Sensing (ASPRS) 2019 guidelines. Once classified, we applied a geometric transformation and converted the points from ellipsoidal heights to GEOID12B (Alaska) orthometric heights. We used ArcGIS Pro to derive raster products from the point cloud. A 50 cm DSM was interpolated from maximum return values from the ground, vegetation, bridge deck, and building classes using a binning method. A 50 cm DTM was interpolated from all ground class returns using a binning method and minimum values. An additional 20 cm DTM was produced for the scientific priority area using a triangulation interpolation. We produced an intensity image using average binning in ArcGIS Pro.
  3. What similar or related data should the user be aware of?
    Wikstrom Jones, Katreen, Wolken, G.J., Daanen, R.P., and Herbst, A.M., 2020, Lidar-derived elevation data for Kensington Mine, Southeast Alaska: Raw Data File RDF 2020-7, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 5 p
    Wikstrom Jones, Katreen, Wolken, G.J., Daanen, R.P., and Herbst, A.M., 2021, Lidar-derived elevation data for Turnagain Pass, southcentral Alaska, September 2, 2018: Raw Data File RDF 2020-16, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 7 p
    Wikstrom Jones, Katreen, and Wolken, G.J., 2022, Lidar-derived elevation data for Eaglecrest ski area, Southeast Alaska, collected September 6, 2019: Raw Data File RDF 2021-10, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 8 p
    Wikstrom Jones, Katreen, and Wolken, G.J., 2022, Lidar-derived elevation data for Speel Arm, Southeast Alaska, collected September 7, 2019: Raw Data File RDF 2021-11, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 8 p
    Wikstrom Jones, Katreen, and Wolken, G.J., 2022, Lidar-derived elevation data for Mount Juneau, Southeast Alaska, collected September 6, 2019: Raw Data File RDF 2021-12, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 8 p

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

  1. How well have the observations been checked?
    Not applicable
  2. How accurate are the geographic locations?
    We did not measure horizontal accuracy for this collection.
  3. How accurate are the heights or depths?
    We measured a mean offset of -0.0048 m between 24 control points and the point cloud. Because of trivial offset, we did not perform any vertical transformation of the lidar point data. The non-vegetated vertical accuracy (NVA) of the point cloud ground class was found using a Triangulated Irregular Network approach. We calculated the project NVA to have a root mean square error (RMSE) of 6.7 cm. We evaluated the relative accuracy for this dataset as the interswath overlap consistency and measured it at 8.4 cm RMSE.
  4. Where are the gaps in the data? What is missing?
    This data relaease is complete.
  5. How consistent are the relationships among the observations, including topology?
    We visually inspected the imagery for data errors such as shifts, and seamline mismatches. Data quality is consistent throughout the survey, save for a gap in coverage in a deep valley in the northwest part of the survey area, and over some glaciated areas where there were few returns due to ground cover conditions. Ground point density varies throughout the survey and is illustrated in the acconpanying report. We provide higher resolution DTM files for the area of scientific interest.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints:
This report, map, and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below).
Use_Constraints:
Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. The user specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.
  1. Who distributes the data set? (Distributor 1 of 1)
    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
    Contact_Instructions:
    Please view our website (https://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.
  2. What's the catalog number I need to order this data set? RDF 2023-3
  3. What legal disclaimers am I supposed to read?
    The State of Alaska makes no expressed or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 30-Mar-2023
Metadata author:
Alaska Division of Geological & Geophysical Surveys
Attn: Simone Montayne
Metadata Manager
3354 College Road
Fairbanks, AK
USA

(907)451-5020 (voice)
(907)451-5050 (FAX)
dggspubs@alaska.gov
Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:

Generated by mp version 2.9.50 on Mon Apr 17 13:28:40 2023