2023-2024 NOAA NGS Topobathy Lidar: Virginia Rivers

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This graphic shows the lidar footprint for this dataset.

Topobathy Lidar Data Report

Data were processed to an initial LAS format using Teledyne CARIS software. CARIS was also utilized for GPS and inertial processing, and Terrasolid and LAStools were used for data visualization, 3D editing, and export to final LAS/LAZ. Data were processed using NAD83(2011) horizontal and vertical datums. The data are in UTM Zone 17 coordinates and ellipsoid elevations in meters. The data classifications are: unclassified (1); ground (2); noise (7); bridge deck (17); high noise (18); bathymetric bottom (40); water surface (41); derived water surface (42); submerged object (43); and water column (45) in accordance with project specifications.

The boresight for each lift was done individually as the solution may change slightly from lift to lift. The initial points for each mission calibration were inspected for flight line errors, flight line overlap, slivers or gaps in the data, point data minimums, or issues with the lidar unit or GPS. Roll, pitch and scanner scale were optimized during the calibration process until the relative accuracy was met.

Dewberry utilized Bayesmap StripAlign for this alignment procedure. This alignment procedure corrected systematic issues globally, per aircraft lift, per flightline, and finally based on local errors along the flight trajectory. Error adjustments included internal sensor parameters. Due to the complex geometric relationship of the elliptical scan pattern the forward and reverse directions must be aligned independently. Additionally, since the green and NIR scanner map different surfaces, they were also aligned independently, then corrected to match each other.

Difference rasters (DZ orthos) were generated, adjustment parameters were reviewed, and registration/match regions were reviewed to ensure data quality.

A final vertical accuracy check of the boresighted flight lines was completed against the surveyed check points after the z correction to ensure the requirement of NVA = 19.6 cm 95% Confidence Level was met.

Point classification was performed according to USGS Lidar Base Specification 2.1. Bare earth DEMs were exported from the classified point cloud. Synthetic points generated by CZMIL refraction correction algorithms are present in this dataset. Please see the final project report for more details on the synthetic points

>Dewberry used algorithms in TerraScan to create the initial ground/submerged topography surface.

Dewberry used rasterized aggregate extents of refracted points to create automated 2-D breaklines with LAStools and ArcGIS. Light travels at different speeds in air versus water and its speed and direction of travel change when it enters the water column. The refraction correction process accounts for this difference by adjusting the depth (distance traveled) and horizontal position (change of angle/direction) of the lidar points acquired within water. These breaklines delineate areas where the refraction correction was applied to the lidar data by CZMIL's automated refraction correction software based on the software's detection of water. The class 42 synthetic surface is generated by the software as a reference surface from which to perform the correction.

Dewberry used the 2-D refraction extents and additional bathy features to classify the bathymetric bottom and ground points properly in TerraScan.

Synthetically generated class 42 (synthetic water surface) points were flagged using the synthetic bit. The withheld bit was set on class 7 and class 18 in TerraScan after all classification was complete.

All lidar data was peer-reviewed. Dewberry's QAQC also included creating void polygons for use during review. All necessary edits were applied to the dataset. LASTools software was used to update LAS header information, including all projection and coordinate reference system information. The final lidar data are in LAS format 1.4 and point data record format 6.

All data was then verified by an Independent QC department within Dewberry. The independent QC was performed by separate analysts who did not perform manual classification or editing. The independent QC involved quantitative and qualitative reviews.

The NOAA Office for Coastal Management (OCM) received files in laz format. The files contain lidar elevation and intensity measurements.

OCM performed the following processing on the data for Digital Coast storage and provisioning purposes: 1. Converted from UTM to geographic coordinates 2. Sorted by gps time 3. Reorganized the files to cloud-optimized point cloud (COPC) format.