Photogrammetry-derived orthoimagery and elevation for Saint Paul, Alaska, collected July 22-25, 2021

Metadata also available as - [Parseable text] - [XML]

Frequently anticipated questions:


What does this data set describe?

Title:
Photogrammetry-derived orthoimagery and elevation for Saint Paul, Alaska, collected July 22-25, 2021
Abstract:
The State of Alaska Division of Geological & Geophysical Surveys (DGGS) collected low-altitude aerial images from an unmanned aerial vehicle (UAV) from July 22 to 25, 2021 and used Structure-from-Motion (SfM) photogrammetry to produce digital surface models (DSMs) and orthoimagery of North Beach, Polovina Beach, and the City of Saint Paul, Alaska. 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 Professional 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/10.14509/30836).
Supplemental_Information:
digital surface model:    The DSM represents surface elevations such as the height of vegetation and buildings. Water bodies can introduce noise. For North, Benson, and Polovina beaches, we manually delineated the ocean boundary to restrict the DSM to the land. For the city DSM, we did not remove water areas because the model includes rocky islets and shallow underwater features of interest (most notably the submerged groins at the northwest breakwater). We filtered the dense cloud to remove low confidence points (most often water surfaces) and prioritize seafloor elevation. However, we do not expect absolute seafloor elevation to be as accurate as above-water features. The DSM is a single-band, 32-bit floating point GeoTIFF file with the "No Data" value set to -3.4028231 x 10^38.	 				
orthoimagery:    The orthoimagery is a three-band (red, green, blue) 8-bit unsigned GeoTIFF file with the "No Data" value set to 0. GSD varies by location, cee the accompanying report for details.
  1. How might this data set be cited?
    Buzard, R.M., Christian, J.E., and Overbeck, J.R., 2022, Photogrammetry-derived orthoimagery and elevation for Saint Paul, Alaska, collected July 22-25, 2021: Raw Data File RDF 2022-1, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 7 p.
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -170.215434
    East_Bounding_Coordinate: -170.210455
    North_Bounding_Coordinate: 57.152016
    South_Bounding_Coordinate: 57.148986
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 22-Jun-2021
    Ending_Date: 25-Jun-2021
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: orthoimage
  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: 2
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.999600
      Longitude_of_Central_Meridian: -171
      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.
  7. How does the data set describe geographic features?
    digital surface model
    The DSM represents surface elevations such as the height of vegetation and buildings. Water bodies can introduce noise. For North, Benson, and Polovina beaches, we manually delineated the ocean boundary to restrict the DSM to the land. For the city DSM, we did not remove water areas because the model includes rocky islets and shallow underwater features of interest (most notably the submerged groins at the northwest breakwater). We filtered the dense cloud to remove low confidence points (most often water surfaces) and prioritize seafloor elevation. However, we do not expect absolute seafloor elevation to be as accurate as above-water features. The DSM is a single-band, 32-bit floating point GeoTIFF file with the "No Data" value set to -3.4028231 x 10^38. (Source: DGGS)
    orthoimagery
    The orthoimagery is a three-band (red, green, blue) 8-bit unsigned GeoTIFF file with the "No Data" value set to 0. GSD varies by location, cee the accompanying report for details. (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 Aleut Community of Saint Paul Island 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 orthoimagery and elevation data are for assessing coastal hazards and changes on Saint Paul Island.

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: 25-Jun-2021 (process 1 of 3)
    Aerial photogrammetric 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 flew the aerial surveys with 70 percent sidelap, 80 percent frontlap, and nadir camera orientation stabilized by a 3-axis gimbal. The survey was flown between 100 and 120 m above the ground at 5.5 to 7.9 m/s, respectively. The resulting images cover 1.21 km2 with ground sampling distance (GSD) of approximately 0.03 m. DGGS surveyed North and Benson beaches on July 22, 2021, from 4:00 PM to 4:30 PM AKDT. The operator returned the UAV once to change batteries. We flew the Black Bluffs in the east area of the City of Saint Paul on July 23, 2021, from 12:55 PM to 1:05 PM, with the camera oriented 10 degrees off-nadir towards the bluff face. This orientation improved mapping of the vertical bluff face. We flew Polovina Beach on July 23, 2021, from 1:50 PM to 2:00 PM AKDT using one battery. We flew the City of St. Paul on July 25, 2021, from 6:40 PM to 7:40 PM AKDT. The operator returned the UAV twice to change batteries. This survey overlapped with the Black Bluffs survey so we combined the final products. The weather throughout all surveys was overcast with no rain and light to moderate wind. No abnormalities were observed during the flights.
    Date: 25-Jun-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/get Datasheet.jsp?PID=BBCN02). This provided real-time kinematic (RTK) corrections to the Trimble R8s GNSS receiver. DGGS measured 22 photo-identifiable points with the R8s. DGGS installed temporary benchmarks using the R8s and subsequently occupied these benchmarks with the base station. We derived the corrected base positions 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 - For the surveys at North Beach, Polovina Beach, and the community of Saint Paul 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. For the city survey, the UAV maintained RTK connection. The lever arm correction is automatically applied and camera GNSS coordinates are written to the image metadata in WGS84 ellipsoid. Yaw, pitch, and roll information are not written to the image metadata. The UAV positions are updated using a X, Y, and Z shift from the initial to the corrected base position. 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. For the city, we used eight GCPs to create the model, leaving four survey check points. For North and Benson beaches, six GCPs and four check points. For Polovina beach, we could only collect elevation points that were not photo-identifiable. We vertically adjust the elevation model using the average offset of eleven vertical points.
  3. What similar or related data should the user be aware of?
    Suleimani, E.N., Nicolsky, D.J., and Salisbury, J.B., 2020, Regional tsunami hazard assessment for communities of Bristol Bay and the Pribilof Islands, Alaska: Report of Investigation RI 2020-1, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 32 p., 6 sheets

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 DSMs and orthoimagery by comparing the known locations of 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. Horizontal accuracy of the Polovina Beach DSM and orthoimage is not assessed. See the accompanying report for tables of checkpoint comparisons.
  3. How accurate are the heights or depths?
    We assess vertical accuracy of the City and North and Benson beaches DSM products using the same check points that are used for horizontal accuracy. For North and Benson beaches, the RMS error of Z offsets is 0.020 m, and the total error of the DSM (X, Y, and Z) is 0.036 m. For the city survey, the RMS error of Z offsets is 0.028 m, and the total error of the DSM is 0.050 m (table 3). Vertical accuracy is not evaluated for offshore and underwater features. We assess vertical accuracy of the Polovina Beach DSM using the RMS error of the fit to eleven GCPs at bare-earth locations. The vertical RMS error of the GCPs is 0.017 m. The RMS error of the vertical correction is 0.054 m (n = 11). The total error is 0.056 m (root-sum-square error of the control and fit RMS errors. See the accompanying report for tables of checkpoint comparisons.).
  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 2022-1
  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: 11-Feb-2022
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 Thu Feb 10 18:37:30 2022