Process_Description:
Aerial imagery was collected using a Department of Interior-owned quadcopters fitted with Ricoh GR II digital cameras featuring global shutters. The cameras were mounted using fixed mount on the bottom of the UAS and oriented in an approximately nadir orientation. During acquisition the UAS was flown on pre-programmed autonomous flight lines at an approximate altitude of 100 meters above ground level (AGL), resulting in a nominal ground-sample-distance (GSD) of 2.6 centimeters per pixel. The flight lines were spaced to provide approximately 70-80 percent overlap between images from adjacent lines. The camera was triggered at 1 Hz using either a built-in or external intervalometer. Before each flight, the camera’s digital ISO, aperture and shutter speed were manually set to adjust for ambient light conditions. Although these settings were changed between flights, they were not permitted to change during a flight; thus, the images from each flight were acquired with consistent camera settings. The images were recorded in raw Adobe DNG format. The flights were conducted on 2021-10-13, 2021-11-04, 2022-10-25, and 2023-11-13.
Process_Date: 20231113
Process_Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: Joshua Logan
Contact_Organization:
U.S. Geological Survey, Pacific Coastal and Marine Science Center
Contact_Position: Physical Scientist
Contact_Address:
Address_Type: mailing address
Address: 2885 Mission Street
City: Santa Cruz
State_or_Province: CA
Postal_Code: 95060
Country: US
Contact_Voice_Telephone: 831-460-7519
Contact_Facsimile_Telephone: 831-427-4748
Contact_Electronic_Mail_Address: jlogan@usgs.gov
Process_Description:
Ground control was established using ground control points (GCPs) consisting of a combination of small square tarps with black-and-white cross patterns and temporary "X" marks placed with chalk on the ground surface throughout the survey area before each flight. The GCP positions were measured using post-processed kinematic (PPK) GPS, using corrections from a GPS base station located on a temporary benchmark ("HZ01") established near the El Dorado Irrigation District Hazel Creek Day Use Area. The coordinates for HZ01 were derived using the average of three National Geodetic Survey OPUS-S solutions from the static GNSS observations collected during the 2021 surveys. For each GCP measurement the GPS receiver was placed on a fixed-height tripod and set to occupy the GCP for a minimum occupation time of one minute. Post-processing was conducted using the Trimble Business Center software package.
Process_Date: 20231113
Process_Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: Joshua Logan
Contact_Organization:
U.S. Geological Survey, Pacific Coastal and Marine Science Center
Contact_Address:
Address_Type: mailing and physical
Address: 2885 Mission Street
City: Santa Cruz
State_or_Province: CA
Postal_Code: 95060
Contact_Voice_Telephone: 831-460-7519
Contact_Electronic_Mail_Address: jlogan@usgs.gov
Process_Description:
The image files were renamed using a custom python script. The file names were formed using the following pattern Fx-YYYYMMDDThhmmssZ_Ryz.*, where:
- Fx = Flight number
- YYYYMMDDThhmmssZ = date and time in the ISO 8601 standard, where 'T' separates the date from the time, and 'Z' denotes UTC ('Zulu') time.
- Ry = RA or RB to distinguish camera 'RicohA' from 'RicohB'
- z = original image name assigned by camera during acquisition
- * = file extension (JPG or DNG)
The approximate image acquisition coordinates were added to the image metadata (EXIF) ('geotagged') using the image timestamp and the telemetry logs from the UAS onboard single-frequency 1-Hz autonomous GPS. The geotagging process was done using either a custom python script which calls the exiftool utility, or by using the exiftool utility via the command line. To improve timestamp accuracy, the image acquisition times were adjusted to true ('corrected') UTC time as necessary by comparing the image timestamps with several images taken on the day of the flights of a smartphone app ('Emerald Time') showing accurate time from Network Time Protocol (NTP) servers. The positions stored in the EXIF are in geographic coordinates referenced to the WGS84(G1150) coordinate reference system (EPSG:4979), with elevation in meters relative to the WGS84 ellipsoid.
Additional pertinent metadata were added to the EXIF headers using the command-line 'exiftool' software.
Process_Date: 2023
Process_Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: Joshua Logan
Contact_Organization:
U.S. Geological Survey, Pacific Coastal and Marine Science Center
Contact_Address:
Address_Type: mailing and physical
Address: 2885 Mission Street
City: Santa Cruz
State_or_Province: CA
Postal_Code: 95060
Contact_Voice_Telephone: 831-460-7519
Contact_Electronic_Mail_Address: jlogan@usgs.gov
Process_Description:
Structure-from-motion (SfM) processing techniques were used to create point clouds, DSMs, and orthomosaic images in the Agisoft Photoscan/Metashape software package using the following workflow:
1. Initial image alignment was performed in "4D" whereby the images from all survey dates were aligned together to improve survey-to-survey precision. The following parameters were used:
Accuracy: 'high'
Pair selection: 'reference', 'generic'
Key point limit: 40,000
Tie point limit: 4,000
2. Ground control point (GCP) positions were imported and markers were manually identified and placed in the images.
3. Sparse point cloud error reduction was performed using an automated python script (Logan and others, 2022), to sequentially apply the Reconstruction Uncertainty and Projection Accuracy gradual selection filters to remove 50 percent of the sparse points, followed by camera optimization. This resulted in the following final gradual selection filter values:
Final Reconstruction Uncertainty: 20.0
Final Projection Accuracy: 4.0
Lens calibration parameters for optimization: f, cx, cy, k1, k2, k3, p1, and p2
4. Additional sparse point cloud error reduction was performed using the automated python script to iteratively apply the Reprojection Error gradual selection filter and camera optimization such that no more than 10 percent of the remaining sparse points are deleted at a time. Between each iteration of the filter, camera optimization was performed with the following lens calibration parameters: f, cx, cy, k1, k2, k3, p1, and p2. Once Reprojection Error was reduced below 1 pixel, additional lens calibration parameters (k4, b1, b2, p3, and p4) were included during optimization. This process was repeated until the following final Reprojection Error filter levels were achieved:
Final Reprojection Error: 0.5
Lens calibration parameters for optimization: f, b1, b2, cx, cy, k1, k2, k3, k4, p1, p2, p3 and p4
Additional remaining sparse points obviously above or below the true surface were manually deleted after the last error reduction iteration, and a final optimization was performed.
5. After final error reduction, images from each survey date were separated into individual chunks for each date.
6. Dense point clouds were created for each survey-date chunk using the 'high' accuracy setting, with 'aggressive' depth filtering.
7. Digital Surface Models (DSM) for each survey date were created using the dense point clouds and exported to GeoTIFF files with 10-centimeter pixel resolutions and LZW compression.
8. A clipping mask polygon shapefile was derived in QGIS using the 1056 meter elevation contour as an approximate upper bound. The lower bound was hand digitized to exclude areas below the reservoir water surface. Additional areas with standing water or excessive noise were also excluded by the clipping mask.
9. The GDAL "gdalwarp" utility was used to clip the GeoTIFF files using the shapefile polygons to produce GeoTIFFs in cloud-optimized GeoTIFF (COG) format. The "target aligned pixels (-tap)" parameter was used to ensure cell alignment between the DSMs from each survey date.
Process_Date: 2023
Process_Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: Joshua Logan
Contact_Organization:
U.S. Geological Survey, Pacific Coastal and Marine Science Center
Contact_Position: Physical Scientist
Contact_Address:
Address_Type: mailing address
Address: 2885 Mission Street
City: Santa Cruz
State_or_Province: CA
Postal_Code: 95060
Country: US
Contact_Voice_Telephone: 831-460-7519
Contact_Facsimile_Telephone: 831-427-4748
Contact_Electronic_Mail_Address: jlogan@usgs.gov
