Precision Airborne Camera (PAC) System - Field data from periodic and event-response surveys of the U.S. Atlantic and Gulf Coasts

IMAGE & POSITION ACQUISITION -- C. W. Wright Consulting operates the Precision Airborne Camera (PAC) system to conduct nadir (vertical) aerial imagery surveys along coastal swaths in shore-parallel lines with optimized endlap/overlap to facilitate SfM processing (Westoby and others, 2012). The PAC system is comprised of a high-resolution digital camera fitted with a prime lens, along with a custom integrated survey-grade Global Navigation Satellite System (GNSS) receiver aboard a Piper P28A aircraft. The camera is triggered by a microcontroller that is synchronized to the pulse-per-second (PPS) signal from the survey-grade GNSS. An event mark is then captured from the flash shoe, timestamped by the GNSS and recorded by the microcontroller. The dataset received from the vendor is comprised of the raw imagery, raw dual-frequency carrier phase GNSS data and photo flash event time data. GNSS positions record the phase center of the GNSS antenna at the moment an image is captured (see Positional_Accuracy metadata element for uncertainty discussion). Lever arm distances are used to translate the position from the nodal point of the camera lens to the GNSS antenna reference point (for more details see Over and others, 2021); these are recorded relative to the camera coordinate system, where the camera is 0,0, and the axes run as follows: X axis runs from left (negative) to right (positive) Y axis runs from bottom (negative) to top (positive) Z axis runs from front of lens (negative) to back of camera (positive) The PAC system hardware is updated as new technological advances are available, which may include a new camera and/or new GNSS system, which may also change the lever arm distances; estimated uncertainties in the lever arm distances are 5 cm in each direction. Below are the specifications of the different PAC system versions and the dates they were in use: VERSION 1.1 Survey Dates Valid: 20171130 - 20180807 Camera Model: Ricoh GRII GNSS Survey Equipment: GPS-only system Lever Arm Distances: 0.44 m, -0.31 m, and 1.25 m (x, y, z) VERSION 1.2 Survey Dates Valid: 20180808 - 20180907 Camera Model: Sony A7R 36 Megapixel GNSS Survey Equipment: GPS-only system Lever Arm Distances: 0.44 m, -0.31 m, and 1.25 m (x, y, z) VERSION 2 Survey Dates Valid: 20180908 - Present Camera Model: Sony A7R 36 Megapixel GNSS Survey Equipment: GPS & GLONASS system Lever Arm Distances: 0.1 m, -0.15 m, and 1.25 m (x, y, z)

IMAGE RENAMING -- Images received from the vendor (in raw image file format, ARW) are renamed using IrfanView software to conform to the following naming convention: yyyy-mmdd-hhmmss-DSCnnnnn-N7251F.ARW, where yyyy_mmdd represents the year (yyyy), month (mm) and day (dd) of capture; hhmmss is the time of capture as hours (hh), minutes (mm), seconds(ss) in Universal Coordinated Time (UTC); DSCnnnnn represents a sequential image identifier (ID) generated by the camera; and N7251F is the tail number of the aircraft used for acquisition. The following pattern string is used in the batch renaming tool to rename the images from their raw name format (DSCnnnnn) to the naming convention defined above: $E36868(%Y-%m%d-%H%M%S)-$N-N7251F

IMAGE CONVERSION -- Renamed images are batch-converted to a format compatible with photogrammetry processing software using Adobe Photoshop Camera Raw and applying the Camera Neutral Profile (Joint Photographic Experts Group format, JPEG) and saving at quality 12 (max) using sRGB color space at 8 bits/channel. Software versions, conversion date, and other parameters are recorded in individual imagery header metadata.

IMAGE QA/QC -- Original and converted images are counted, and the beginning and end of the survey are reviewed to cull extraneous images (such as camera testing or inadvertently triggered images). A multi-page contact sheet of images is created to allow quick review ensuring that (a) conversion was successful with no corrupted imagery, and (b) "extra" imagery not capturing the intended view is culled. If corrupted converted images are found, imagery is re-converted until a collection of valid images remains. Note that in some cases, water-only images may be retained if there are visible or potentially visible bathymetric features, aquatic vegetation, and/or objects in the scene that may be useful for future data applications.

AIRCRAFT NAVIGATION PROCESSING -- Raw GNSS data received by the antenna mounted atop the aircraft were recorded at 1 hertz (Hz) by a dual-frequency survey-grade GNSS receiver, which is post-processed and differentially corrected with as many continuously operating reference stations (CORS) within 30 kilometers (km) of any part of the trajectory, or locally-established base station(s) if necessary, using NovAtel's GrafNav software to produce 1-Hz aircraft trajectories. Trajectories are produced using precise ephemerides unless the flight level metadata clearly describes the dataset as PROVISIONAL, in which case a lower accuracy ephemeris is used. This is sometimes necessary if the imagery is collected to rapidly assess damage from natural disasters. The aircraft trajectories, in combination with precisely recorded image capture (flash event) times, are used to generate aircraft GNSS antenna positions at the moment of each image capture and is provided for each collection. Images with missing positions are reconciled through direct interpolation based on the image and GNSS time stamps. The positions in the aircraft positions file represent the position of the aircraft GNSS antenna, not the position of the camera or features photographed. To determine actual image positions, photogrammetric software such as Agisoft Metashape can be used to apply the lever arm offsets (in the camera frame of reference) from the GNSS antenna reference point to the camera lens.

IMAGERY HEADERS -- Georeferencing, copyright, and other relevant information is added to the imagery headers of each image using Phil Harvey's ExifTool. To extract the information from the image headers using ExifTool, the following command can be used: exiftool -n -csv *.jpg > allheaders.csv The -csv flag writes the information out in a comma-delimited format. The -n option formats the latitude and longitude as signed decimal degrees.

KMZ CREATION -- In order to provide a visual map of the survey extent with example images throughout the survey domain, and to aid in completing the geospatial information in the collection-level metadata, a clickable Keyhole Markup Language Zipped file (.kmz) is created for each collection using ExifTool.