2013 USACE NAO Topobathy Lidar DEM: Craney Island, VA

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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.

This graphic displays the footprint for this lidar data set.

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.

These data were collected using the CZMIL system. It is owned and operated by the U.S. Army Corps of Engineers (USACE). The system collects lidar data at 10 kHz and RGB imagery at 2 Hz. A CASI-1500 hyperspectral line scanner is integrated with the system as well. Aircraft position, velocity and acceleration information are collected through a combination of Novatel and POS A/V 510 equipment. All raw data streams are transferred to the office for downloading and processing in CZMIL’s Hydro Fusion software. Aircraft position data are processed using POSPac software and the results are combined with the lidar data to produce 3-D positions for each lidar shot. Upon inspection and QA/QC in the software packages Fledermaus and PFM_ABE, anomalous data are flagged as invalid. NOAA’s VDatum software then converts all valid data from ellipsoid to orthometric heights based on the NGS' GEOID12A model and exports valid lidar data as a series of unclassified ascii xyz files.

ASCII files are imported into GeoCue V2012.1.27.7, which is a geospatial workflow production and management software tool employed by JALBTCX to perform and monitor production of data products. Upon import into GeoCue, the ASCII files are converted to LAS format. A customized classification macro, built upon the TerraScan V13 module within Microstation V8i, classifies valid topographic data as ground points (2) and unclassified points (1). Upon completion the macro, the classification results undergo quality control and any misclassified points are manually edited. In areas of dense vegetation the bare earth ground points might be incorrectly classified due to the inability of the laser to penetrate the canopy and reach the bare ground. In these areas, JALBTCX defaults to the algorithm’s “ground” surface instead of manually reclassifying those points. The final classification results are comprised of individual lidar points with classifications of ground (2) or unclassified (1). They are partitioned into a series of 5km or quarter quad delivery boxes, one Classified LAS file per box. The format of the file is LAS version 1.2. Data are classified as 1 (valid non-ground topographic data), 2 (valid ground topographic data), 21 (valid topographic data acquired with the bathymetric sensor), 27 (invalid topographic and bathymetric data), 29 (valid bathymetric data).

Data classified as ground (2) and valid bathymetric data (29) in the las files are converted to a grid by generating a triangulated irregular network (TIN) and then extracting the grid node elevations from the TIN surface. The origin point of the grid is located at a horizontal position whose value is evenly divisible by the 1m grid resolution such that rasters from subsequent surveys have common cell boundaries. JALBTCX uses Quick Terrain Modeler V7.1.5 to perform this operation utilizing the following parameters; “Legacy Triangulation”, "Max Sample Excursion - 50”, “Max Triangle Side – 100”, “Radius - 4”, “Tolerance – 2”, and “Tiling Settings Snap to Grid (Expand)." The grid is exported from Quick Terrain Modeler as an ESRI ASCII Z grid file. Utilizing an in-house python script within ESRI ArcMap V10.0, the ASCII Z grid file is converted to a tiff-format raster file whose projection is defined as “The North American Datum of 1983.” The raster is then multiplied against a corresponding 1m Grid mask raster, a mask image produced from JALBTCX’s 1m Grid, in ESRI’s Raster Calculator to remove interpolated areas where data does not exist. Horizontal positions, provided in decimal degrees of latitude and longitude, are referenced to the North American Datum of 1983 National Adjustment of 2011 (NAD83 NA2011). Vertical positions are referenced to the NAD83 NA11 ellipsoid and provided in meters. The National Geodetic Survey's (NGS) GEOID12A model is used to transform the vertical positions from ellipsoid to orthometric heights referenced to the North American Vertical Datum of 1988 (NAVD88).

The NOAA Office for Coastal Management (OCM) received GeoTiff format files from USACE JALBTCX for the Craney Island project area. The bare earth raster files were at a 1 m grid spacing. The data were in geographic NAD83 (2011) coordinates and NAVD88 (Geoid12A) elevations in meters. OCM assigned the appropriate EPSG codes (Horiz - 6318, Vert - 5703) and copied the raster files to https for Digital Coast storage and provisioning purposes.