2019 NOAA Topobathy Lidar DEM (Voids): Morro Bay, CA

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This graphic displays the footprint for this lidar data set.

Link to downloadable project report.

Data for the NOAA Morro Bay project was acquired by Quantum Spatial (QSI) using a Riegl VQ-880GII Topobathy LiDAR system. Sonar data was collected by Merkel and Associates using a SEA SWATHplus-H sonar system.

QSI reviewed all acquired flight lines to ensure complete coverage and positional accuracy of the laser points. QSI creates an initial product call Quick Look Coverage Maps. These Quick Looks files are not fully processed data or final products. The collected LiDAR data is immediately processed in the field by QSI to a level that will allow QA\QC measures to determine if the sensor is functioning properly and assess the coverage of submerged topography. An initial SBET was created in POSPAC MMS 8.1 and used in RiProcess which applies pre-calibrated angular misalignment corrections of scanner position to extract the raw point cloud into geo-referenced LAS files. These files were inspected for sensor malfunctions and then passed through automated classification routines (TerraScan) to develop a rough topobathymetric ground model for an initial assessment of bathymetric coverage.

To correct the continuous onboard measurements of the aircraft position recorded throughout the missions, QSI concurrently conducted multiple static Global Navigation Satellite System (GNSS) ground surveys (1 Hz recording frequency) over established monuments located in or around the project area. After the airborne survey, the static GPS data were triangulated with nearby Continuously Operating Reference Stations (CORS) using the Online Positioning User Service (OPUS) for precise positioning. Multiple independent sessions over the same monument were processed to confirm antenna height measurements and to refine position accuracy. QSI then resolved kinematic corrections for aircraft position data using kinematic aircraft GPS and static ground GPS data. A final smoothed best estimate trajectory (SBET) was developed that blends post-processed aircraft position with attitude data. Sensor head position and attitude are calculated throughout the survey. The SBET data are used extensively for laser point processing. The software Trimble Business Center v.3.90, Blue Marble Geographic Calculator 2017, and PosPac MMS 8.1 SP3 are used for these processes.

Next, QSI used RiProcess 1.8.5 to calculate laser point positioning of the Riegl VQ-880GII data by associating SBET positions to each laser point return time, scan angle, intensity, etc. A raw laser point cloud is created in Riegl data format and erroneous points are filtered. Data was exported to LAS 1.4 format and are combined into 500 m x 500 m tiles. Data was then further calibrated using TerraScan, TerraModeler, and TerraMatch and a refraction correction was applied to all sub-water surface returns using QSI proprietary LAS Monkey software. QSI used custom algorithms in TerraScan to create the initial ground/submerged topography surface. Relative accuracy of the green swaths was compared to overlapping and adjacent swaths and verified through the use Delta-Z (DZ) orthos created using QSI's DZ Ortho creator. Absolute vertical accuracy of the calibrated data was assessed using ground RTK survey data and complete coverage was again verified.

QSI then performed manual editing to review all classification and improve the final topobathymetric surface. A final bathymetric void shape was created after final editing and provided to Merkel and Associates to target for sonar data collection. The acquired sonar was then integrated into the LiDAR dataset to provide a seamless bathymetric model. As a general rule, in areas of overlap sonar data was prioritized in deeper areas of the channel while LiDAR data was prioritized in shallower areas of the channel including all areas not submerged during the LiDAR collection. As the lidar laser approaches the laser’s extinction point bathymetric surface profiles and point density degrade thus the prioritization of sonar data in these areas. Conversely side-scan sonar does better when water depths are greater than 1m as areas shallower than this are prone to increased noise making the lidar data more reliable in these areas.

Within the Morro Bay site there was one notable area of significant temporal change. The sand spit at the mouth of the bay displayed a difference as great as 5 meters from the lidar survey. Despite this area being collected at true ground during the lidar survey, the sonar data was used in this area being the most temporally recent data and collected at high tide with high confidence in the surface.

All data from either sensor that was not used in model creation is still preserved in the point cloud as Ignored Ground/Bathymetry (class 20). Please see Appendix B of the NOAA Morro Bay Report for more information regarding the sonar acquisition and processing.

Final topobathymetric DEMs were created at 1m pixel resolution using ground (class 2) and bathymetry (class 40).

Void DEM dataset- A void shapefile was created to indicate areas where there was a lack of bathymetric returns. This shape was created by triangulating bathymetric bottom points with an edge length maximum of 4.56m to identify all areas greater then 9 square meters without bathymetric returns. This shapefile was used to exclude interpolated elevation data from this dataset.