Photogrammetry-derived orthoimagery and elevation for Kotlik, Alaska, collected July 9-10, 2021

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Frequently anticipated questions:


What does this data set describe?

Title:
Photogrammetry-derived orthoimagery and elevation for Kotlik, Alaska, collected July 9-10, 2021
Abstract:
The State of Alaska Division of Geological & Geophysical Surveys (DGGS) collected low-altitude aerial images from an unmanned aerial vehicle (UAV) on July 9-10, 2021 and used Structure-from-Motion (SfM) photogrammetry to produce a digital surface model (DSM) and orthoimage of Kotlik. The orthoimagery and elevation data are for assessing coastal hazards and changes. We used Trimble Business Center to process the Global Navigation Satellite System (GNSS) data used for positional control. We used Agisoft Metashape to process the photogrammetry data. 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/10.14509/30789).
Supplemental_Information:
digital surface model:    The data release provides digital surface models for the city of Kotlik and for the nearby landfill. The DSM represents surface elevations such as the height of vegetation and buildings. Water bodies introduce noise. We manually delineate the river boundary to restrict the DSM to the land. We selectively remove areas where inland water bodies introduced great noise. The DSM is a single-band, 32-bit floating point GeoTIFF file with the "No Data" value set to -3.4028231 x 10^38. The city DSM has a GSD of 0.066 m. The landfill DSM has a GSD of 0.055 m.	
orthoimagery:    The data release provides orthoimages for the city of Kotlik and for the nearby landfill. The orthoimagery is a three-band (red, green, blue) 8-bit unsigned GeoTIFF file with the "No Data" value set to 0. The city orthoimage has a GSD of 0.033 m per pixel. The landfill orthoimage has a GSD of 0.028 m per pixel.
  1. How might this data set be cited?
    Buzard, R.M., Christian, J.E., and Overbeck, J.R., 2021, Photogrammetry-derived orthoimagery and elevation for Kotlik, Alaska, collected July 9-10, 2021: Raw Data File RDF 2021-16, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 5 p.
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -163.570030
    East_Bounding_Coordinate: -163.524427
    North_Bounding_Coordinate: 63.039495
    South_Bounding_Coordinate: 63.022482
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 09-Jul-2021
    Ending_Date: 10-Jul-2021
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: GeoTIFF
  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: 3
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.999600
      Longitude_of_Central_Meridian: -165
      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 1/298.257222101000025.
      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?
    digital surface model
    The data release provides digital surface models for the city of Kotlik and for the nearby landfill. The DSM represents surface elevations such as the height of vegetation and buildings. Water bodies introduce noise. We manually delineate the river boundary to restrict the DSM to the land. We selectively remove areas where inland water bodies introduced great noise. The DSM is a single-band, 32-bit floating point GeoTIFF file with the "No Data" value set to -3.4028231 x 10^38. The city DSM has a GSD of 0.066 m. The landfill DSM has a GSD of 0.055 m. (Source: DGGS)
    orthoimagery
    The data release provides orthoimages for the city of Kotlik and for the nearby landfill. The orthoimagery is a three-band (red, green, blue) 8-bit unsigned GeoTIFF file with the "No Data" value set to 0. The city orthoimage has a GSD of 0.033 m per pixel. The landfill orthoimage has a GSD of 0.028 m per pixel. (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?
    We thank the Native Village of Bill Moore's Slough for funding and supporting the creation of these data products.
  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 are for assessing coastal hazards and changes near Kotlik, Alaska.

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: 10-Jul-2021 (process 1 of 3)
    Airborne survey - DGGS used a DJI Phantom 4 RTK UAV with a FC6310R camera model (8.8 mm lens) to collect 20-megapixel JPEG photographs (5472 x 3648 pixels per image). We conducted two separate surveys: one over the city and one at the landfill. We flew the aerial surveys with 70 percent sidelap, 80 percent frontlap, and nadir camera orientation stabilized by a 3-axis gimbal. The city survey was flown 120 m above the ground at 7.9 m/s. The landfill survey was flown 100 m above the ground at 6.5 m/s. The resulting images cover 1.45 km2 with ground sampling distance (GSD) of approximately 0.03 m. DGGS conducted the landfill flight on July 9, 2021, from 10:15 to 10:30 AM AKDT, and the city flight on July 10, 2021, from 4:45 to 7:30 PM AKDT. During the city survey, the operator returned the UAV five times to change batteries. The weather throughout both surveys was fair with scattered clouds, no rain, and light wind. No abnormalities were observed during the flights.
    Date: 10-Jul-2021 (process 2 of 3)
    Ground survey - DGGS set up a GNSS base station using a Trimble R10 receiver sampling at 5 Hz. The base was installed over a benchmark with a published solution (found at www.ngs.noaa.gov/OPUS/getDatasheet.jsp?PID=BBGX51). This provided real-time kinematic (RTK) corrections to the Trimble R8s GNSS receiver. DGGS measured 18 photo-identifiable points with the R8s. We later derived the corrected base position using the Online Positioning User Service (found at www.ngs.noaa.gov/OPUS/) and post-processed the R8s positions in Trimble Business Center.
    Date: 2021 (process 3 of 3)
    Photogrammetric dataset processing - The UAV did not maintain RTK connection with the Trimble R10 base station. We apply a post-processing kinematic correction using RTKLIB (an open-source GNSS processing software found at www.rtklib.com). The UAV GNSS receiver samples at 5 Hz, rather than at image acquisition times. We interpolate the corrected positions at image timestamps to derive coordinates. The image timestamp metadata also contains orientation to support the lever arm correction that adjusts coordinates from the GNSS receiver to the camera. We compute the interpolation and lever arm correction using the worksheet found at www.aerotas.com/phantom-4-rtk-ppk-processing-workflow. DGGS processed images in Agisoft Metashape Professional software (Version 1.6.3 build 10732). We masked image corners where shadows and image warping were disruptive. Processing steps included aligning images, identifying ground control points (GCPs), manually cleaning the sparse point cloud, optimizing the bundle block adjustment (refining camera positions and lens distortion parameters), constructing the dense point cloud, building the DSM, and creating the orthomosaic image. We used six GCPs to create the model, leaving twelve survey check points. We processed the landfill separately and obtained no points to georeference or check the raw output. We shift the products in X, Y, and Z to fit the 2019 lidar-derived DSM (available for download at elevation.alaska.gov). The horizontal shift is accomplished with one reference point (no rotation or rescale required). The vertical shift is calculated using the median offset of the DSMs in bare earth areas. This offset includes the correction from ellipsoid to orthometric height.
  3. What similar or related data should the user be aware of?
    Buzard, R.M., Overbeck, J.R., Turner, M.M., and Christian, J.E., 2021, Coastal flood impact assessments for Alaska communities: Kotlik: Report of Investigation RI 2021-1C, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 57 p
    Herbst, A.M., and Daanen, R.P., 2020, High-resolution lidar data for Kotlik, Western Alaska: Raw Data File RDF 2020-15, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 7 p
    Overbeck, J.R., Christian, J.E., and Buzard, R.M., 2021, Single-beam bathymetry data near Kotlik, Alaska, collected July 2021: Raw Data File RDF 2021-14, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 6 p
    Overbeck, J.R., Hendricks, M.D., and Kinsman, N.E.M., 2018, Photogrammetric digital surface models and orthoimagery for the continuous coastline, Wales to Platinum, Alaska, segment E: Stebbins to Kotlik: Raw Data File RDF 2017-8E, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 3 p
    Overbeck, J.R., Hendricks, M.D., and Kinsman, N.E.M., 2018, Photogrammetric digital surface models and orthoimagery for the continuous coastline, Wales to Platinum, Alaska, segment F: Kotlik to Nunam Iqua: Raw Data File RDF 2017-8F, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 3 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 quantify the horizontal accuracy of the city DSM and orthoimage by comparing the known locations of 12 photo-identifiable check points measured with GNSS against their modeled locations in the photogrammetric products. X and Y errors are calculated as the root-mean-square (RMS) error of offsets. The total horizontal error is the root-sum-square error of X and Y RMS errors, 0.048 m. Horizontal accuracy of the landfill DSM and orthoimage is not assessed.
  3. How accurate are the heights or depths?
    We assess vertical accuracy of the city DSM using the same twelve check points. The RMS error of Z offsets is 0.070 m. The total error of the DSM (X, Y, and Z) is 0.085 m. We assess vertical accuracy of the landfill DSM using the fit to the lidar (control) DSM. The lidar non-vegetated vertical accuracy is 0.082 m (Herbst and Daanen, 2020). The RMS error of the vertical correction is 0.153 m (n = 22,401). The total error is 0.173 m (root-sum-square error of the control and fit RMS errors).
  4. Where are the gaps in the data? What is missing?
    This data release is complete.
  5. How consistent are the relationships among the observations, including topology?
    DGGS visually inspected the orthoimage for data errors such as shifts, seamline mismatches, and water noise overlapping land. There were no significantly erroneous areas that required repair. Visual errors common to these SfM photogrammetry products include discontinuous powerlines, blurriness near high-angle features like buildings, and distortion at water boundaries. Bright objects like metal roofs and white paint can cause overexposure, leading to spurious elevation points.

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 2021-16
  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: 20-Dec-2021
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 Dec 20 14:39:47 2021