2012 USACE NCMP Post-Sandy Topobathy Lidar DEM: Long Island, NY

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Information on the NOAA Office for Coastal Management (OCM)

The Data Access Viewer (DAV) allows a user to search for and download elevation, imagery, and land cover data for the coastal U.S. and its territories. The data, hosted by the NOAA Office for Coastal Management, can be customized and requested for free download through a checkout interface. An email provides a link to the customized data, while the original data set is available through a link within the viewer.

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

This graphic shows the post Sandy LiDAR coverage for eastern Long Island, New York

Using an Optech Gemini LiDAR Sensor, 21 flight lines of high density data, at a nominal pulse spacing (NPS) of 1 meter, were collected along the Eastern Long Island, New York (approximately one hundred sixteen (116) 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 two (2) missions were flown during a period from November 14, 2012 through November 15, 2012. One airborne global positioning system (GPS) base station was used in support of the LiDAR data acquisition. Eighteen (18) 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 GeoTiff format files from USACE JALBTCX for the Long Island, NY project area. The bare earth raster files were at a 1 m grid spacing. The data were in geographic NAD83 coordinates and NAVD88 (Geoid12A) elevations in meters. OCM assigned the appropriate EPSG codes (Horiz - 4269, Vert - 5703) and copied the raster files to https for Digital Coast storage and provisioning purposes. No metadata record was provided with this data. NOAA OCM created this metadata based on the metadata record from the related 2012 Post-Sandy NY data collection.