2013 USGS-NRCS Lidar: Maine (Cumberland, Kennebec and York)

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This graphic shows the lidar coverage for the 2013 USGS acquisition on behalf of the NRCS of Cumberland, Kennebec and York counties in Maine.

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Converted from FGDC Content Standards for Digital Geospatial Metadata (version FGDC-STD-001-1998) using 'fgdc_to_inport_xml.pl' script. Contact Tyler Christensen (NOS) for details.

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Using Leica ALS70 (LiDAR) system on board a Cessna 402 aircraft, 171 flight lines of high density data, at a nominal pulse spacing (NPS) of 0.7 meters, were collected for this task order (approximately 2279 square miles of southern Maine). AGL = 6500 feet - Aircraft Speed = 150 Knots, Field of View (Full) = 40 degrees, Pulse Rate = 272 kHz, Scan Rate = 42.3 Hz, with an average side lap of 25 %. Multiple returns were recorded for each laser pulse along with an intensity value for each return. A total of eleven (11) missions were flown between November 05, 2013 and December 04, 2013. Three (3) Global Navigation Satellite System (GNSS) Base Stations (Waterville Robert LeFleur Airport, Portland International Jetport, and MEGO CORS) were 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. The geoid used to reduce satellite derived elevations to orthometric heights was GEOID12A. Data for the task order is referenced to the UTM Zone 19N, North American Datum of 1983 (2011), and NAVD88, in meters. 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 ALS70 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.

Ground control and QAQC control point survey was performed by Shyka, Sheppard & Garster Land Surveyors,(SSG), that supported the 2013 USGS NRCS lidar project. All surveys were performed in such a way as to achieve ground control that supports lidar data at 9.25 cm RMSE accuracy and satisfy a local network accuracy of 5 cm at a 95% confidence level. All control points were observed using GPS and consistent with the second order horizontal and third order vertical as defined by the NGS. All ground control survey field activities took place from December 3, 2013 to March 21, 2014. SSG collected 35 calibration control points as well as 158 supplemental QAQC points. The supplemental QAQC points were collect to be used in independent accuracy testing. The survey was performed using two (2) Leica GS15 Series GPS receivers in conjunction with simultaneous data collected across nineteen (19) Continuously Operating Reference Stations (CORS) GPS receivers. The survey was conducted utilizing Rapid Static observations made using 5-second sync rats and observations of typically 15 to 20 minutes. All GPS ground control observations were processed using Trimble Navigation's Trimble Business Center. All horizontal GPS control was based on UTM Zone 19N, NAD83(2011) expressed in meters. The vertical datum used for this project was based on the North American Vertical Datum of 1988 (NAVD88), GEOID12A, also expressed in meters.

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 NOAA Office for Coastal Management (OCM) received topographic files in LAS format. The files contained lidar

elevation and intensity measurements. The data were received in UTM Zone 19N (NAD83) coordinates

and were vertically referenced to NAVD88 using the Geoid12a model. The vertical units of the data were

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

1. The topographic las files were converted from orthometric (NAVD88) heights to ellipsoidal heights using Geoid03.

2. The topographic las files were converted from a Projected Coordinate System (UTM 19N) to a Geographic Coordinate system (GCS).

3. The topographic las files' horizontal units were converted from meters to decimal degrees.