2009 USACE NCMP Topobathy Lidar DEM: North Carolina

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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 CHARTS system. It is owned by the Naval Oceanographic Office and operated through contract. The system collects topographic lidar data at 9 kHz, bathymetric lidar data at 1 kHz and RGB imagery at 1Hz. 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 410 equipment. All raw data streams are transferred to the office for downloading and processing in SHOALS GCS 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. PFM_ABE's chartsLAS module then converts all valid data from ellipsoid to orthometric heights based on the NGS' GEOID03 model and exports data as a series of first return topography (TF), last return topography (TL) and bathymetry (H) ASCII files. The bathymetry files contain all of the returns from the bathymetric sensor which include returns both above and below the water. The topographic data are also exported as a series of LAS files. There is one LAS file per flightline per 5-km NCMP box.

The flightline LAS files were imported into TerraScan V11 module within MicroStation V8.0. For each 5-km NCMP box, the points classified as first return are exported to an ASCII file with for all the flighline within the box are exported by running the macro Dump_Geo_TF.mac. This process is repeated for the points classified as last return by running the macro Dump_Geo_TL.mac.

The flightline LAS files are imported into the TerraScan V11 module within MicroStation V8.0. The resulting LAS files are thinned to eliminate duplicate points. A customized classification macro is used to distinguish ground points (2) and unclassified points (1). The classification results are QC'd and any misclassified points are manually edited. In areas of dense vegetation the bare 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 points are exported to an ASCII file containing the following fields: longitude, latitude, elevation (orthometric), intensity, classification, mirror angle, time (GPS time), number of echos, and echo number. The ASCII file is converted to LAS using txt2las code from LAStools (http://www.cs.unc.edu/~isenburg/lastools/).

The points classified as ground in the Classified LAS files are exported from TerraScan V11 and converted to a grid by generating a Triangulated Irregular Network (TIN) and the 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 V7.1.1 to perform this operation utilizing the following key parameters; “Max Sample Excursion 50,” “Max Triangle Side 100,” “Smoothing filter Level 4 with a Max Roughness 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, 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 NOAA Office for Coastal Management (OCM) received GeoTiff format files from USACE JALBTCX for the North Carolina coastline project area. The bare earth raster files were at a 1 m grid spacing. The data were in geographic NAD83 coordinates and NAVD88 (Geoid03) 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.