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Aerial imagery and structure-from-motion data products from UAS survey of the intertidal zone at Post Point, Bellingham Bay, WA, June 2019

February 22, 2021

An unmanned aerial system (UAS) was used to acquire high-resolution imagery of the intertidal zone at Post Point in Bellingham Bay, Washington on June 6, 2019. This imagery was processed using structure-from-motion (SfM) photogrammetric techniques to derive high-resolution digital surface models (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 20:59 Universal Coordinated Time (UTC) (13:59 Pacific Daylight Time (PDT)), with a predicted water level of -0.61 meters relative to the NAVD88 vertical datum at the Bellingham NOAA tide station (station ID 9449211).

A total of five flights were conducted for the survey between 19:24 and 21:01 UTC (12:24 and 14:01 PDT). Flights F01, F02, and F03 were conducted at an approximate altitude of 70 meters above ground level (AGL), resulting in a nominal ground-sample-distance (GSD) of 1.8 centimeters per pixel. Flights F04 and F05 were conducted at an approximate altitude of 35 meters AGL, resulting in a nominal ground-sample-distance (GSD) of 0.9 centimeters per pixel. Flight F04 was aborted early. The imagery from this flight was not used and is not included in this data release.

The flights were conducted using a Department of Interior-owned 3DR Solo quadcopter fitted with a Ricoh GR II digital camera featuring a global shutter. 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 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.

Nineteen 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 ("BELI") located approximately 5 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) orthomosaic images with a resolutions of 1 and 2 centimeters per pixel, 2) digital surface models (DSM) and hillshade images with resolutions of 2 and 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.