USGS CoastCam at Waiakāne, Moloka'i, Hawai'i: 2018 Timestack Imagery and Coordinate Data

Before mounting the CoastCam (c2) at the beach, the camera (FLIR BFLY-PGE-50S5C-C) was used to take intrinsic camera calibration images. Please refer to the MolokaiHI_c2_20180625_IO zip file and the MolokaiHI_c2_calibration_metadata record included in Harrison and others (2024) for access to the intrinsic camera calibration data and more information on the calibration process. After calibration was complete, the camera was installed and connected to a control computer. The computer and camera were powered by a solar panel and battery. Data was transferred via cellular.

The camera was programmed to collect three-band (RGB) images at 2 Hertz (Hz) for a total of approximately 10 minutes, every hour during daylight hours. The images included in this data release were collected from August 5, 2018, to August 16, 2018. Gaps exist in the dataset because not every image collected by the camera is being used in the dataset. Additionally, some images may have been obscured by water droplets, insects, birds, salt, sand and dirt, poor visibility from adverse weather conditions, or sun glare, and thus may not reflect the true state of the beach and ocean. Each hour the camera was activated, a timestack image product was produced by sampling intensity values at a single transect of pixels, defined by (U,V) pixel coordinates. The (U,V) sampling locations were determined by identifying a cross-shore transect of locations in local horizontal (X,Y) coordinate system, which aligned with the local shoreline orientation. The elevation of samples was chosen as Z = 0 m, which is an elevation assumed to be near the water line. The vertical reference frame was PRVD02 coordinate system. XYZ-sampling locations were projected into (U,V) image coordinates following established methods (Palmsten and Brodie, 2022; Bruder and Brodie, 2020) and using intrinsic (MolokaiHI_c2_20180625_IO) and extrinsic (MolokaiHI_c2_20180625_EO) camera calibrations from Harrison and others (2024).

The timestack images were converted to netCDF timestacks and processed to add additional information using the netCDF4 Python library version 1.6.2. The times of the collection, sampled at 2 Hz, were stored as a one-dimensional (1D) array, where each index of the array contained a time represented as epoch time, seconds elapsed since 00:00:00 UTC on January 1, 1970. The sampling locations, saved as local XYZ values, were stored as a two-dimensional array. Because pixels were stored assuming an elevation of Z = 0 m and the true beach surface may vary, an additional processing step was needed to project the sampling locations onto a topo-bathymetric surface representative of the beach at the time of the image data collection following previously described methods (Palmsten and Brodie, 2023; Holland and others, 1997 ). The topo-bathymetric surface was developed from GNSS survey data (MolokaiHI_XYZ_20180626) collected within the field-of-view of the camera (Harrison and others, 2024) and lidar data (OCM Partners, 2024). The point data was interpolated onto a grid using the method described by Plant and others (2002) with smoothing length scales of 1.5 m in the cross-shore direction and 4 m in the alongshore direction. The local (X,Y) coordinates were rotated and translated into UTM coordinates using information describing the local origin and rotation angle for the site. The (X,Y) coordinates were also translated from UTM to WGS84 latitude and longitude coordinates using the UTM Python library version 0.7.0. One netCDF (.nc) file was created for each timestack image using the netCDF Python library. The data included in each netCDF file includes the timestack image; the UV image coordinates, the XY coordinates in UTM; the XY local coordinates; the latitude and longitude coordinates; the Z-elevation coordinates projected onto the beach surface; and an array of the epoch times as previously described. Additional metadata was included in the netCDF files as global and variable attributes using the Climate and Forecast (CF)-compliant metadata convention (version CF-1.6). These attributes describe the data collection, as well as how the different coordinate systems are used within the data.

A total of 30 netCDF timestack files (*.nc) are included in this release and can be viewed and downloaded on the data release webpage. The netCDF files were renamed using the following convention, with ten period-separated elements: 1) the epoch time--seconds elapsed since 00:00:00 UTC on January 1 1970--the timestack was collected 2) The day of the week the timestack was collected, abbreviated to three letters 3) The month the timestack was collected, abbreviated to three letters 4) The date when the timestack was collected in the format dd_hh_mm_ss where 'dd' is the day of the month, 'hh' is the hour in 24-hour format, 'mm' is the minute, and 'ss' is the second 5) The timezone for date previously described 6) The year the timestack was collected 7) The site nickname where the timestack was collected 8) The camera the timestack was collected from, where 'cx' represents the timestack 9) the name of the transect sampled to create the timestack 10) the file extension, .nc. An example of a filename formatted in this way would be 1533492060.Sun.Aug.05_18_01_00.GMT.2018.molokai.cx.runup03.nc.