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Aerial imagery and structure-from-motion data products from UAS survey of the intertidal zone at West Whidbey Island, WA, June 2019

September 17, 2021

An unmanned aerial system (UAS) was used to acquire high-resolution imagery of the intertidal zone at West Whidbey Island, Washington on June 4, 2019. This imagery was processed using structure-from-motion (SfM) photogrammetric techniques to derive a high-resolution digital surface model (DSM), orthomosaic imagery, and topographic point clouds.

In order to maximize the extent of the subaerially exposed area, the survey was timed to coincide with a spring low tide occurring at approximately 18:02 Universal Coordinated Time (UTC) (11:02 Pacific Daylight Time (PDT)), with a predicted water level of -0.74 meters below mean lower-low water (MLLW) at the Sunset Beach NOAA subordinate tide station (station ID 9447951).

A total of eleven flights were conducted for the survey. Two flights (F01 and F02) were not used for systematic data collection, and data from these flights are not included in this data release. Flights F03, F09, F10 and F11 were oblique image data collection flights, during which the camera was placed on a fixed mount in an oblique orientation to collect imagery of the face of the large coastal bluff on the east side of the survey area. Flights F04, F05, F06, F07, and F08 were nadir image data collection flights during which the camera was fixed in an approximately nadir orientation. The nadir flights were conducted at an approximate altitude of 70 meters above-ground-level, resulting in a nominal ground-sample-distance (GSD) of 1.8 centimeters per-pixel.

All flights were conducted using a Department of Interior-owned 3DR Solo quadcopter fitted with a Ricoh GR II digital camera featuring a global shutter. For the nadir flights, the UAS was flown on pre-programmed autonomous flight lines which were oriented roughly shore-parallel and were spaced to provide approximately 70 percent overlap between images from adjacent lines. The oblique acquisition flights were flown manually with the UAS in a sideways-facing orientation facing the bluff. For all flights, the camera was triggered at 1 Hz using a built-in intervalometer. Before each flight, the camera's digital ISO, aperture, and shutter speed were adjusted for ambient light conditions.

Twenty-five temporary ground control points (GCPs) were distributed throughout the area to establish survey control. The GCPs consisted of a combination of small square tarps with black-and-white cross patterns and "X" marks placed on the ground using temporary chalk. The GCP positions were measured using post-processed kinematic (PPK) GPS, using corrections from a GPS base station ("COUP") located approximately 7 kilometers from the study area operated by the Washington State Reference Network (WSRN).

This data release presents five data products derived from these surveys which are available for download: 1) an orthomosaic image with a resolution of 2 centimeters per-pixel, 2) a digital surface model (DSM) and a hillshade image with a resolution of 4 centimeters per-pixel, 3) topographic point clouds in LAZ format, 4) the geographic positions of the ground control points, and 5) the raw aerial imagery in JPG format.