Post-Hurricane Florence RGB averaged orthoimagery of coastal North Carolina

On a workstation (AMD Threadripper 3960X /AsRock TRX40 Creator motherboard/256GB 3000MHz RAM) using Win10x64, all Aerial Imagery (and associated positional data) and 21 of 34 available Ground Control Points were brought into an Agisoft Metashape Pro (v. 1.6.5) project based on identification in the imagery. Only one ground control point, #34, was used as a control point to help 'lock' in the camera positions whereas the rest of the points were used as check points. See the Aerial Imagery citation metadata for Metashape Reference Settings inputs. To align in a 4D manner (see Sherwood and others, 2018) additional imagery and positions were added from 2019-08-30, 2019-09-02, 2019-09-08, 2019-09-12, and 2019-09-13 (see contact person below for imagery from the additional dates past 2018, which are in publication preparation/review). The additional imagery improves the horizontal and vertical accuracy but are not used past this initial alignment process step; similar products can be reproduced without them. The following steps were performed on the Metashape project in the geographic coordinate system NAD83(2011) in ellipsoid height following the general guidance in Over and others (2021): 1. Separate camera models and camera groups were created for each flight date. 2. Imagery (with positions) are aligned to create a point cloud using a 'High' alignment setting, keypoint limit of 70,000, tiepoint limit of 0, generic preselection selected, and reference preselection via source selected. The tiepoint accuracy was set to 1 pixel. 3. The resultant point cloud was filtered using one iteration of the 'Reconstruction uncertainty' filter at a level of 10, one iteration of the 'Projection accuracy' filter at a level of 3, and two iterations of the 'Reprojection accuracy' filer at a level of 0.3 (in the second iteration the 'fit additional corrections' option was turned on). With each filter iteration points are selected, deleted, and then the camera model optimized to refine the focal length, cx, cy, k1, k2, k3, p1, and p2 camera model coefficients. 4. Natural breaks in the final alignment product were identified based on bodies of water or the extreme northern and southern ends of a flight (PostFlorence_InletMap.jpg on the larger citation landing page https://www.sciencebase.gov/catalog/item/6037cc78d34eb1203117512d). These 'chunks' are the vicinity of the Virginia-North Carolina border (VA is used to denote this northern extent) to Oregon Inlet, Oregon Inlet to Hatteras Inlet, Hatteras Inlet to Ocracoke Inlet, Ocracoke Inlet to Ophelia Inlet, Ophelia Inlet to Beaufort Inlet, Beaufort Inlet to Bogue Inlet, Bogue Inlet to New River Inlet, and New River Inlet to Oak Island. For each chunk all aligned images and associated tie points in that section were kept and the remainder deleted.
Sherwood, C.R., Warrick, J.A., Hill, A.D., Ritchie, A.C., Andrews, B.D. and Plant, N.G., 2018. Rapid, remote assessment of Hurricane Matthew impacts using four-dimensional structure-from-motion photogrammetry. Journal of Coastal Research, 34(6), pp.1303-1316.

From the previous process step, each 'chunk' is processed as follows to create the final orthorectified products for each flight date: 1. All imagery from October 6, 7, and 8 in the aligned chunk is copied from the workstation to a new Agisoft Metashape Pro. project on the U.S Geological Survey Tallgrass high-performance computing network for processing on CPU and GPU nodes (https://www.usgs.gov/core-science-systems/sas/arc/machine-access). 2. If multiple flight dates were flown for this chunk, imagery from one date is targeted, and the rest are disabled (this is then switched for each flight date in the chunk after the final step and steps 3-7 are repeated) 3. A high quality dense point cloud is generated for the chunk with 'mild' filtering and the options to calculate point color and point confidence selected. 4. Dense point cloud is filtered using the 'Filter by Confidence' tool using the range 0-2, these points selected at this range are classified as low noise. 5. A DEM is built from the dense point cloud with interpolation enabled and without the points classified as low noise (not included in this publication). 6. An orthophoto is built from the interpolated DEM using the blending mode 'average'. 7. The orthorectified product is exported in UTM Zone 18N with a 0.25 meter cell size and integer boundary intervals. 8. Orthorectified product is turned into a cloud-optimized GeoTIFF (COG) using gdal_translate with the following command:for %i in (.\*.tif) do gdal_translate %i .\cog\%~ni_cog.tif -of COG -stats -co BLOCKSIZE=256 -co COMPRESS=LZW -co PREDICTOR=YES -co NUM_THREADS=ALL_CPUS -co BIGTIFF=YES (v. 3.1.4 accessed October 20, 2020 https://gdal.org/). Where i is the name of each geoTIFF section.

The title of the sciencebase page was changed from "RGB averaged orthoimagery" to "Post-Hurricane Florence RGB averaged orthoimagery of coastal North Carolina". The keyword "Hurricane Florence" was added to the None Theasaurus. The broken browse graphic link was fixed. Added doi link to the network address. Updated the metadata contact email to the Woods Hole Data contact in the event the person is no longer with the USGS.