2015 Cook & Tift County (GA) Lidar

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This graphic shows the lidar coverage for 2016 lidar acquisition of Cook & Tift County, GA.

Date that the source FGDC record was last modified.

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Using one Leica ALS80 (lidar) system on board a Cessna 404 Titan fixed-wing aircraft, lidar data, at a nominal pulse spacing (NPS) of 0.7 meters, was collected for this task order (approximately 515 square miles). AGL = 6500 feet - Aircraft Speed = 150 Knots, Field of View (Full) = 40 degrees, Pulse Rate = 545.0 kHz, Scan Rate = 51.0 Hz, with an average side lap of 25%. Multiple returns were recorded for each laser pulse along with an intensity value for each return. Two (2) missions were flown between February 14, 2016 and February 16, 2016. One (1) Global Navigation Satellite System (GNSS) Base Station was used in support of the lidar data acquisition. Specific information regarding latitude, longitude, and ellipsoid height to the L1 phase center is included in the lidar processing report. Multiple returns were recorded for each laser pulse along with an intensity value for each return. The lidar data acquisition parameters for this mission are detailed in the lidar processing report for this task order. For all acquired lidar data as part of entire NOAA Lidar for Tift and Cook Counties, GA task order, the geoid used to reduce satellite derived elevations to orthometric heights was GEOID12B. Data for the task order is referenced to the State Plane Zone of Georgia West, North American Datum of 1983 (2011) US Feet, and NAVD88, in US Feet. Once the data acquisition and GPS processing phases are complete, the lidar data was processed immediately to verify the coverage had no voids. The GPS and IMU data was post processed using differential and Kalman filter algorithms to derive a best estimate of trajectory. The quality of the solution was verified to be consistent with the accuracy requirements of the project. The SBET was used to reduce the lidar slant range measurements to a raw reflective surface for each flight line. The coverage was classified to extract a bare earth digital elevation model (DEM) and separate last returns. The ALS80 calibration and system performance is verified on a periodic basis using Woolpert's calibration range. The calibration range consists of a large building and runway. The edges of the building and control points along the runway have been located using conventional survey methods. Inertial measurement unit (IMU) misalignment angles and horizontal accuracy are calculated by comparing the position of the building edges between opposing flight lines. The scanner scale factor and vertical accuracy is calculated through comparison of lidar data against control points along the runway. Field calibration is performed on all flight lines to refine the IMU misalignment angles. IMU misalignment angles are calculated from the relative displacement of features within the overlap region of adjacent (and opposing) flight lines. The raw lidar data is reduced using the refined misalignment angles.

The field crew utilized Real-Time Kinematic (RTK) GPS surveying throughout most of the ground control data collection process. For Tift-Cook Counties- Using RTK GPS techniques, observations were performed on a total of 17 LiDAR control points and 90 ground control quality check points. The survey was conducted using a 1-second epoch rate, in a fixed solution RTK mode, with each observation lasting between 60 to 180 seconds. Each station was occupied twice to insure the necessary horizontal and vertical accuracies were being met for this photogrammetric project.

The individual flight lines were inspected to ensure the systematic and residual errors have been identified and removed. Then, the flight lines were compared to adjacent flight lines for any mismatches to obtain a homogeneous coverage throughout the project area. The point cloud underwent a classification process to determine bare-earth points and non-ground points utilizing "first and only" as well as "last of many" lidar returns. This process determined Default (Class 1), Ground (Class 2), Noise (Class 7), Water (Class 9), Ignored Ground (Class 10), Overlap Default (Class 17) and Overlap Ground (Class 18). The bare-earth (Class 2 - Ground) lidar points underwent a manual QA/QC step to verify the quality of the DEM as well as a peer-based QC review. This included a review of the DEM surface to remove artifacts and ensure topographic quality. Classification of water (class 9) and ignored ground (class 10) was completed via the use of the hydrologic breaklines collected for the hydro-flattening phase. The overlap classes were determined by first identifying the overlapping areas and reclassifying the LAS data by offset from a corridor. This allows the returns located on the edge of the swath to be removed from the bare earth coverage in an effort to produce a more uniform data density. The returns determined to be overlap are then further classified to produce overlap default (class 17) and overlap ground (class 18). The surveyed ground control points are used to make vertical adjustments to the data set and to perform the accuracy checks and statistical analysis of the lidar dataset. Supervisory QC monitoring of work in progress and completed editing ensured consistency of classification character and adherence to project requirements across the entire project area. The resulting deliverables for this task order consist of classified LAS file in LAS 1.2 format, 4 feet pixel size DEM files in ESRI Grid format, and Hydrologic Breakline, Flightline Vector, Tile Index, and Control Point data in ESRI Geodatabase format.

The first delivery of this dataset (August, 2015) to the NOAA Office for Coastal Management arrived via hard-drive device from Woolpert, the contracting the company for this project. An internal review from the CGSC data team at NOAA OCM generated a report with data concerns and alterations required before accepting the data as final. A second delivery arrived in March, 2016 which was subsequently reviewed again with feedback provided once again. In July of 2016, a final dataset delivered and accepted for distribution on the Digital Coast. The lidar data were projected to State Plane Georgia West (1002, in feet) and vertically to NAVD88 heights (in feet). OCM performed the following processing for data storage and Digital Coast provisioning purposes:

1. The pilot was only a small portion. The 2nd and 3rd deliveries included the entire project area. A cursory review was performed on the final tiles to insure all tiles meet specification before releasing to the public.

2. Erroneous points were removed and LAS files were compressed for data storage.

3. The data were converted from State Plane (ft) horizontal coordinates to geographic coordinates (decimal degrees) and vertical heights were converted from orthometric (NAVD88, ft) heights to ellipsoidal heights using Geoid12b (meters).