2012 USACE Post Sandy Topographic LiDAR: Virginia and Maryland

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This graphic shows the post Sandy LiDAR coverage for Maryland and Virginia.

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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 an Optech Gemini LiDAR Sensor, 32 flight lines of high density data, at a nominal pulse spacing (NPS) of 1 meter, were collected along the Virginia coastal area separated into areas of interest: Assateague Island (Maryland),

Assateague Island (Virginia), Tangier Island (Virginia), and Cape Henry to Willoughby Point (Virginia) (approximately 80 square miles). Data Acquisition Height = 5,500 feet AGL - Aircraft Speed = 125 Knots. Multiple returns were

recorded for each laser pulse along with an intensity value for each return. A total of three (3) missions were flown during a period from November 9, 2012 through November 11, 2012. Two airborne global positioning system (GPS)

base stations were used in support of the LiDAR data acquisition. Sixteen (16) ground control points were surveyed through static methods. The geoid used to reduce satellite derived elevations to orthometric heights was Geoid12A.

Data for the task order is referenced to the UTM Zone 18N, North American Datum of 1983 (NAD83), and NAVD88, in Meters. Airborne GPS data was differentially processed and integrated with the post processed IMU data to derive a

smoothed best estimate of trajectory (SBET). 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. In addition to the LAS deliverables, one layer of coverage was delivered in the ArcGrid Format: bare-earth.

The LiDAR system 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.

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 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 homogenous

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), Model Key Points (Class 8), and Overlap (Class 12) classifications. The bare-earth (Class 2 - Ground) LiDAR points underwent a manual QA/QC step to verify that

artifacts have been removed from the bare-earth surface. The surveyed ground control points are used to perform the accuracy checks and statistical analysis of the LiDAR dataset.

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 projected coordinates of NAD83 UTM18N, meters

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 Geoid12a.

2. The topographic las files were converted from Projected Coordinates (NAD83 UTM18N, meters) to Geographic Coordinates (NAD83, decimal degrees).