2017 GA DNR Lidar DEM: Middle Chattahoochee - Walter F Watershed

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Link to data set report

Link to the hydro breaklines.

This graphic displays the footprint for this lidar data set.

Link to custom download the lidar point data from which these raster Digital Elevation Model (DEM) data were created.

Survey Data Acqisition:

Project Introduction: Platinum Geomatics, LLC entered into contract with the Atlantic Group on March 1st, 2017 to perform geodetic control surveying for the calibration and quality control of 9 counties of LiDAR in South Georgia. This is a report of the methodology used and calculated accuracy of the survey performed by Platinum Geomatics, LLC.

Procedures Used: The Trimble Virtual Reference System (VRS) was employed to obtain real-time GNSS corrections for most of the points. When real-time corrections could not be obtained, due to either poor cellular connection or aerial obstructions affecting the geometric dilution of precision (GDOP), a static GNSS session was run and receiver independent exchange format (RINEX) files of the navigated position were downloaded from the VRS. Static GNSS baselines were post-processed using Trimble Business Center (TBC) software. Real-time VRS observations consisted of a minimum of 180 1-second epochs and point tolerances were set to record within 0.049’ horizontal and 0.066’ vertical accuracies.

Equipment Used: Trimble R-7, R-8, and R-10 dual frequency GNSS receivers were used.

Project Datum: Individual point coordinates were reported to NAD83 (2011) Georgia West zone. Coordinates and elevations were reported in US Survey Feet. Reported Elevations are relative to NAVD88 as derived from Geoid 12B.

Point Description Quantity Non-vegetated LiDAR Quality Control 50, Vegetated LiDAR Quality Control 37, and LiDAR Calibration 26

Flight Acquisition: Atlantic acquired one hundred eighty-one (181) passes of the AOI as a series of perpendicular and/or adjacent flight-lines. Differential GNSS unit in aircraft recorded sample positions at 2 Hz or more frequency. Lidar data was only acquired when a minimum of 6 satellites were in view. Atlantic lidar sensors are calibrated at a designated site located at the Fayetteville Municipal Airport (FYM) in Fayetteville, TN and are periodically checked and adjusted to minimize corrections at project sites. Atlantic operated a Cessna 210TL (N732JE) and Partenavia P68 (N775MW) outfitted with a Leica ALS70-HP lidar system during the collection of the project area. Data acquisition was completed between 24th November 2017 and 24th January 2018.

Lidar Pre-processing: Atlantic used a combination of Waypoint and Leica software products to extract the lidar swath data from the raw flight records. Waypoint Inertial Explorer is used to extract the raw IPAS ABGPS/IMU data, which is further processed in combination with controlled base stations to provide the final Smoothed Best Estimate Trajectory (SBET) for each mission. The SBET’s are combined with the raw laser scan files to export the (*.las) formatted swath point clouds.

LiDAR ranging data were initially calibrated using previous best parameters for this instrument and aircraft. Using a combination of GeoCue, and TerraSolid’s TerraScan and TerraMatch the overlapping swath point clouds are corrected for any orientation or linear deviations to obtain the best fit swath-to-swath calibration. Relative calibration was evaluated using advanced plane-matching analysis and parameter corrections derived. This process was repeated interactively until residual errors between overlapping swaths, across all project missions, was reduced to 2 cm or less. A final analysis of the calibrated LiDAR is preformed using a TerraMatch Tie Line report for an overall statistical model of the project area.

Upon completion of the data calibration, Atlantic runs a complete set of Delta-Z (dZ) ortho images. A user-defined color ramp is applied depicting the offsets between overlapping swaths based on project specifications. The dZ orthos provide an opportunity to review the data calibration in a qualitative manner. Atlantic assigns green to all offset values that fall below the required interswath accuracy RMSDz requirement of the project. A yellow color is assigned for offsets that fall between the RMSDz value and 1.5x of that value. Finally, red values are assigned to all values that fall beyond 1.5x of the RMSDz requirements of the project.

The calibrated point cloud data from the laser sensor was merged to produce processed (*.las) file(s) including but not limited to 3D position, intensity, and time-stamp. A filtering methodology was utilized to produce a multi-return surface elevation model dataset with bare-earth conditions. GeoCue, TerraScan, and TerraModel software was used for the initial batch processing and manual editing of the (*.las) point clouds. Atlantic utilized collected breakline data to perform classification for classes’ 9-Water and 10-Ignored Ground in LP360.

Lidar Intensity Imagery Creation: All lidar intensity imagery was created from the final calibrated and classified lidar point cloud. Intensity images were produced from all classified points.

Hydro line Collection: Hydro lines were compiled as water bodies (ponds and lakes) 2 acres or greater, and streams and rivers (“double-line”) 100 nominal feet wide or greater, using the lidar intensity data and surface terrain model of the entire project area. After the collection of hydro lines all features were validated for monotonicity and vertical variance, to ensure that no points were floating above ground. The hydro break lines were encoded into the lidar surface and used to hydro-enforce/flatten all significant water bodies. These final hydro lines were then used in the production of bare Earth digital models to hydro flatten significant water bodies.

Bare-Earth Hydro-flattened DEMs were created in LP-360 from the hydro lines and bare-earth (ground) LiDAR data. All DEMs were created with a cell size of 6 feet. Delivery in an industry-standard, GIS-compatible, 32-bit ERDAS GeoTIFF raster format and cut to the project index tiles. Each surface is reviewed using Global Mapper to check for any surface anomalies or incorrect elevations found within the surface.

The NOAA Office for Coastal Management (OCM) received 3052 raster DEM files in GeoTiff format from Tetra Tech. The data were in GA State Plane West, NAD83(2011), US Survey feet, coordinates and NAVD88 (Geoid12b) elevations in meters. The bare earth raster files were at a 6 ft grid spacing.

OCM performed the following processing on the data for Digital Coast storage and provisioning purposes:

1. Used internal script to assign the EPSG codes.

2. Copied to the files to https.