Beach Topography—Fire Island, New York, Pre-Hurricane Sandy, January 2012: Ground Based Lidar (1-Meter Digital Elevation Model)

USACE-FRF Processing - Data for the survey extents was collected along with a boresight to resolve subtle differences between the scanner and the Inertial Measurement Unit (IMU). After collection, the data were post-processed using multiple software packages to produce the final digital elevation model product.

Terrestrial global position system (GPS) data were post-processed with National Geodetic Survey's Continuously Operating Reference Stations (CORS) and integrated with post-processed IMU data through Applanix's POSPac global navigation satellite system (GNSS) software to derive a smoothed, best estimate of trajectory (SBET). After processing, quantitative quality-metrics were analyzed to ensure the trajectory accuracy met the manufacturer's specified values.

The SBET is imported into Riegl's RiProcess lidar software, where the lidar point cloud is rectified and geo-registered to a particular coordinate system, projection, and geoid model as well as applying boresight values for roll, pitch and yaw. Control monuments and previously validated datasets were used for comparison to determine the accuracy of the survey. These were measured by cutting cross-sections of the current survey overlayed on previously validated data within the software through stable structures such as houses, monuments, and road centerlines.

The processed point cloud was manipulated within RiProcess to delineate the shoreline, filter structures and vegetation, and classify points as water, vegetation, structures, and ground. The classified point cloud was exported as a .las (1.1) file.

The .las file was imported into Applied Imagery's QTModeler software where a mean-z (elevation) algorithm was applied to the point cloud data to grid the data at 0.5-meter resolution. Anti-aliasing was also applied to increase the precision of the triangulation by subdividing each grid cell into 16 equal segments and recording which segment contained the point that was used to represent the elevation value of the grid cell. When triangulation occurs, this increased level of precision will enhance the accuracy of the triangles and the subsequent sampling that occurs. The sub-grid was not retained after the model was created. During the model triangulation, any grid cells with no data were assigned a value of "null" - for example, no data exists. After gridding was completed, XYZ point data were exported as a space delimitated .txt file, projected in the North American Datum of 1983 (NAD83) UTM Zone 18N (North American Vertical Datum of 1988 [NAVD88] GEOID12B meters).

SPCMSC Processing - The XYZ .txt file was converted to an ArcGIS multipoint feature using the 'Create Feature Class from XYZ Table’ tool within ArcCatalog 10.2.2. The output feature class was assigned as NAD83 UTM Zone 18N, NAVD88 GEOID12B. XYZ point features are [0]QA/QC’ed to remove any outlier points.

An ArcGIS (version 10.2.2) Digital Terrain Model was built from the XYZ feature class using the 'Create Terrain' tool. A 1-meter (m) raster grid was created from the terrain surface using the 'Terrain to Raster' tool.

Added keywords section with USGS persistent identifier as theme keyword.