Orthophotomosaic image (natural color) of the north coast of Barter Island, Alaska acquired on July 01 2014 (GeoTIFF image, 19-cm resolution)

Data acquisition - Fairbanks Fodar collected aerial photographs on July 01 2014, using a small aircraft (Cessna 170B) platform. The aerial survey was planned so flight lines and photograph frequency provided 60 percent side lap and 80 percent end lap photo coverage along with flying height of approximately between 740 m, resulting in a 19- cm ground sample distance (GSD) of the orthophotomosaics. A Nikon D800E with a 24 mm Nikkor f/1.4 lens was used to collect 36-megapixel photographs, in Joint Photographic Experts Group (JPEG) or Nikon Electronic Format (NEF), depending on location. Photographs were collected at 1- to 3-second intervals. On-board global positioning system (GPS) data were acquired by a Trimble 5700 with roof-mounted antenna approximately 1 m above the camera, collecting at 5 Hertz. Each camera shutter trip placed an event marker onto the GPS datastream for precise timing and location.

Data processing - Aerial survey GNSS data were processed using Waypoint's Grafnav commercial GNSS software using GPS constellation. Each project was processed using either post-processing kinematic (PPK) or precise point positioning (PPP) methods, depending on the quality of the solution, which was primarily dependent on the distance from Continually Operating Reference Stations (CORS), such that the flight resulted in data with better than 10 cm separation in forward and reverse trajectory solutions. GPS data were processed to the World Geodetic System 1984 (WGS84) European Petroleum Survey Group Well Known Identification Number (EPSG) 32607 and the WGS84 Ellipsoid. Photographs were individually processed for optimum contrast and exposure using Adobe Camera Raw. To accommodate the large data acquisition volumes, most photographs were shot and processed to JPEG format. Aerial survey GPS data (event marker coordinates) were manually correlated to image filenames using the image timestamp to create a camera external orientation file for import into Agisoft Photoscan Professional (Photoscan) software. The external orientation file provides the X, Y, Z position of the camera for each photograph taken during the survey.

Photogrammetry - Aerial stereophotographs were imported into the photogrammetric software, which uses a Structure-from-Motion (SfM) algorithm to create a three-dimensional terrain model from the stereo-imagery. The terrain model was then used to orthometrically correct the photos and produce the final orthoimage mosaic in Photoscan. Within the Photoscan software application, standard workflow steps were followed: photo-alignment, alignment optimization, dense point cloud building, mesh creation, DSM and orthoimage creation, and exporting the results.

Orthoimage mosaics were adjusted horizontally to the coordinates of a single GCP as measured in September 2016, in order to adjust the data from a relative coordinate system to a fixed ground-based geodetic coordinate system (WGS84 UTM Zone 7). See Gibbs and others (2019) for a thorough explanation of methodology.

Edited metadata to add keywords section with USGS persistent identifier as theme keyword. No data were changed.

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