Orthomosaic imagery of the Los Padres Reservoir delta, Carmel River valley, CA, 2017-11-01

Aerial imagery was collected using a Department of Interior-owned 3DR Solo quadcopter fitted with a Ricoh GR II digital camera featuring a global shutter. The camera was mounted using a fixed mount on the bottom of the UAS and oriented in an approximately nadir orientation. During image 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 percent overlap between images from adjacent lines. The camera was triggered at 1 Hz using a built-in intervalometer and was programmed to acquire imagery in JPG format. Before each flight, the camera 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.

Ground control was established using ground control points (GCPs) consisting of small square tarps with black-and-white cross patterns placed on the ground surface throughout the survey area. The GCP positions were measured using survey-grade GPS receivers operating in real-time kinematic (RTK) mode. For each GCP measurement the GPS receiver was placed on a fixed-height tripod and set to occupy each GCP for a minimum occupation time of one minute. The RTK corrections were referenced to a GPS base station occupying a previously established benchmark designated SFML, located on the Los Padres Reservoir dam approximately 1 kilometer from the survey area. In order to ensure consistency with historic surveys, the previously established position for SFML published in Smith and others, 2009 was used for the real-time surveys. After the survey, the static occupation on SFML was submitted to the National Geodetic Survey (NGS) Online Positioning User Service (OPUS).

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 the Geosetter software package. To improve timestamp accuracy, the image acquisition times were adjusted to true ('corrected') UTC time by comparing the image timestamps with several images taken of a smartphone app ('Emerald Time') showing accurate time from Network Time Protocol (NTP) servers. For this survey, +00:00:01 (1 second) was added to the image timestamp to synchronize with corrected UTC time. 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 information was added to the EXIF using the command-line 'exiftool' software with the following command: exiftool ^ -P ^ -IPTC:Credit="U.S. Geological Survey" ^ -IPTC:Contact="pcmsc_data@usgs.gov" ^ -EXIF:Copyright="Public Domain" ^ -XMP:UsageTerms="Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty." ^ -EXIF:ImageDescription="Low-altitude aerial image of the Los Padres Reservoir delta area, Carmel River valley, California, USA, from USGS field activity 2017-635-FA; https://cmgds.marine.usgs.gov/fan_info.php?fan=2017-635-FA" ^ -XMP:Event="Unoccupied Aircraft System survey of Los Padres Reservoir delta area during USGS field activity 2017-635-FA." ^ -EXIF:GPSAreaInformation="Position from UAS onboard autonomous single-frequency GNSS." ^ -EXIF:GPSMapDatum="EPSG:4979 (WGS 84)" ^ -EXIF:Artist="U.S. Geological Survey, Pacific Coastal and Marine Science Center" ^ -IPTC:CopyrightNotice="Public Domain. Please credit U.S. Geological Survey." ^ -IPTC:Caption-Abstract="Aerial image of the Los Padres Reservoir delta area and Carmel River, Carmel River valley, California, USA, from an Unoccupied Aircraft System (UAS) during USGS field activity 2017-635-FA." ^ -sep ", " ^ -keywords="Carmel River, Los Padres Reservoir, Monterey County, California, 2017-635-FA, Unoccupied Aircraft System, UAS, drone, aerial imagery, U.S. Geological Survey, USGS, Pacific Coastal and Marine Science Center" ^ -comment="Aerial image of the Los Padres Reservoir delta area and Carmel River, Carmel River valley, California, USA, from an Unoccupied Aircraft System (UAS) during USGS field activity 2017-635-FA."^ -Orientation= ^ -XMP:AttributionURL="https://doi.org/10.5066/P9J9CHOH" ^ -OffsetTime*=+00:00 -AllDates+=7 ^ -r f* ^ -ext DNG ^ -ext JPG
Additional metadata tags were populated in the imagery metadata using the following command: exiftool ^ -P ^ "-XMP-photoshop:Credit<IPTC:Credit" ^ "-XMP-iptcCore:CreatorWorkEmail<IPTC:Contact" ^ "-XMP-dc:Rights<EXIF:Copyright" ^ "-XMP-dc:Description<EXIF:ImageDescription" ^ "-XMP-exif:all<GPS:all" ^ "-XMP-exif:GPSLatitude<Composite:GPSLatitude" ^ "-XMP-exif:GPSLongitude<Composite:GPSLongitude" ^ "-XMP-exif:GPSDateTime<Composite:GPSDateTime" ^ "-XMP-photoshop:DateCreated<EXIF:DateTimeOriginal" ^ "-XMP-xmp:ModifyDate<EXIF:ModifyDate" ^ "-XMP-dc:Creator<EXIF:Artist" ^ "-XMP-tiff:Make<EXIF:Make" ^ "-XMP-tiff:Model<EXIF:Model" ^ -overwrite_original ^ -ext JPG

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 with the following parameters - Accuracy: 'high'; Pair selection: 'reference', 'generic'; Key point limit: 0 (unlimited); Tie point limit: 0 (unlimited). 2. Sparse point cloud error reduction was performed using an iterative gradual selection and camera optimization process with the following parameters: Reconstruction Uncertainty: 10; Projection Accuracy: 3. Lens calibration parameters f, cx, cy, k1, k2, k3, p1, and p2 were included in the optimization. Additional sparse points obviously above or below the true surface were manually deleted after the last error reduction iteration. 3. Ground control points (GCPs) were automatically detected using the 'Cross (non-coded)' option. False matches were manually removed, and all markers were visually checked and manually placed or adjusted if needed. 4. Additional sparse point cloud error reduction was performed using an iterative gradual selection and camera optimization process with the following parameters: Reprojection Error: 0.3. Lens calibration parameters f, cx, cy, k1, k2, k3, p1, and p2 were initially included in the optimization, but additional parameters k4, b1, b2, p3, and p4 were included once Reprojection Error was reduced below 1 pixel. Additional 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. A dense point cloud was created using the 'high' accuracy setting, with 'aggressive' depth filtering. 6. A Digital Surface Model (DSM) with a native resolution of 4.8 centimeters per pixel was created using all points in the dense point cloud. 7. An RGB orthomosaic with a native resolution of 2.4 centimeters per pixel was created using the DSM as the orthorectification surface, and then exported to a GeoTIFF format with a 2.5-centimeter pixel resolution.

A uniform adjustment was applied to the orthomosaic to shift the image from the historic reference frame published in Smith and others, 2009 to the updated reference frame observed at the SFML benchmark during the 2017 survey (derived using the National Geodetic Survey (NGS) Online Positioning User Service (OPUS)).
To ensure consistency with historic data sets, the original published position for the SFML benchmark was used for the RTK GPS base station position. Thus, the GCP positions and original SfM data products were consistent with the historic data presented in Smith and others, 2009. The SFML benchmark position published in that report is (in UTM zone 10 coordinates relative to the NAD83 reference frame): Northing: 4027605.397 Easting: 619388.986 Ortho. Ht. [m, NAVD88]: 322.418 (GEOID03)
After the 2017-11-01 UAS survey, the 5:40 static GPS occupation on SFML was submitted to NGS OPS which derived the following current-epoch position for SFML: Northing: 4027605.687 Easting: 619388.761 Ortho. Ht. [m, NAVD88]: 322.359 (using GEOID03 separation = -33.047 m)
Thus, the displacement between the 2009 published SFML position and the 2017-11-01 observed position is: dN: +0.290 meters dE: -0.225 meters dElev.: -0.059 meters
This magnitude and direction of displacement is similar to those estimated for this location by the National Geodetic Survey Horizontal Time-Dependent Positioning (HTDP), shown below: HTDP OUTPUT, VERSION 3.4.0 DISPLACEMENTS IN METERS RELATIVE TO NAD_83(2011/CORS96/2007) FROM 11-05-2008 TO 11-01-2017 (month-day-year) FROM 2008.847 TO 2017.835 (decimal years) NAME OF SITE: SFML LATITUDE: 36 23 10.23945 N LONGITUDE: 121 40 7.85831 W NORTH: 0.341 meters EAST: -0.241 meters UP: -0.012 meters
To produce the final data products for this data release, all data were transformed from the historic reference frame into the current-epoch reference frame consistent with the 2017-11-01 observed position for SFML. This transformation was done using a uniform horizontal and vertical adjustment.
For adjustment of the orthomosaic, the following adjustment process was used: 1. The RGB orthomosaic was re-exported from Agisoft Metashape to a tiled GeoTIFF format with a 2.5-centimeter pixel resolution, with the following export bounds (these bounds allow for the final target raster origin to fall on a multiple of the raster cell size after the adjustment): xmin:619547.225 xmax:620062.225 ymin:4026185.710 ymax:4027051.710 2. The tiled geotiff were combined in a virtual raster (VRT) using gdalbuildvrt. 3. The VRT was shifted by the required dN and dE using gdal_translate: gdal_translate -a_ullr 619547.000 4027052.000 620084.600 4026181.600 input.vrt shifted.vrt 4. The shifted VRT was converted to a cloud optimized GeoTIFF for the final data product using gdal_translate: gdal_translate -b 1 -b 2 -b 3 -mask 4 -ot Byte -scale -of COG ^ -co BLOCKSIZE=256 -co COMPRESS=JPEG -co QUALITY=95 ^ -co RESAMPLING=AVERAGE -a_srs EPSG:6339 --config GDAL_TIFF_INTERNAL_MASK YES ^ --config GDAL_TIFF_OVR_BLOCKSIZE 128 -co NUM_THREADS=ALL_CPUS --config GDAL_CACHEMAX "50" ^ --config GDAL_MAX_DATASET_POOL_SIZE 512 ^ shifted.vrt LosPadresRes_2017-11-01_orthomosaic_25mm_.tif