2012 USGS Lidar: Central Virginia Seismic (Louisa County)

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The project area addressed by this report falls within the Virginia county of Louisa with actual data edges falling within Fluvanna, Goochland, and Spotsylvania.

Date that the source FGDC record was last modified.

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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Report completed: August 03, 2012; used as process date below.

LiDAR mass points were produced to LAS 1.2 specifications, including the following LAS classification codes: Class 1 = Unclassified, and used for all other features that do not fit into the Classes 2, 7, 9, or 10, including vegetation, buildings, etc.; Class 2 = Ground, includes accurate LiDAR points in overlapping flight lines; Class 7 = Noise, low and high points; Class 9 = Water, points located within collected breaklines; Class 10 = Ignored Ground due to breakline proximity.

The data was processed using GeoCue and TerraScan software. The initial step is the setup of the GeoCue project, which is done by importing a project defined tile boundary index encompassing the entire project area. The acquired 3D laser point clouds, in LAS binary format, were imported

into the GeoCue project and tiled according to the project tile grid. Once tiled, the laser points were classified using a proprietary routine in TerraScan. This routine classifies any obvious outliers in the dataset to class 7. After points that could negatively affect the ground are removed from class 1, the ground layer is extracted from this remaining point cloud. The ground extraction process encompassed in this routine takes place by building an iterative surface model.

This surface model is generated using three main parameters: building size, iteration angle and iteration distance. The initial model is based on low points being selected by a roaming window with the assumption that these are the ground points. The size of this roaming window is determined by the building size parameter. The low points are triangulated and the remaining points are evaluated and subsequently added to the model if they meet the iteration angle and distance constraints. This process is repeated until no additional points are added within iterations. A second critical parameter is the maximum terrain angle constraint, which determines the maximum terrain angle allowed within the classification model.

The following fields within the LAS files are populated to the following precision: GPS Time (0.000001 second precision), Easting (0.003 meter precision), Northing (0.003 meter precision), Elevation (0.003 meter precision), Intensity (integer value - 12 bit dynamic range), Number of Returns (integer - range of 1-4), Return number (integer range of 1-4), Scan Direction Flag (integer - range 0-1), Classification (integer), Scan Angle Rank (integer), Edge of flight line (integer, range 0-1), User bit field (integer - flight line information encoded). The LAS file also contains a Variable length record in the file header that defines the projection, datums, and units.

Once the initial ground routine has been performed on the data, Dewberry creates Delta Z (DZ) orthos to check the relative accuracy of the LiDAR data. These orthos compare the elevations of LiDAR points from overlapping flight lines on a 1 meter pixel cell size basis. If the elevations of points within each pixel are within 5 cm of each other, the pixel is colored green. If the elevations of points within each pixel are between 5 cm and 10 cm of each other, the pixel is colored yellow, and if the elevations of points within each pixel are greater than 10 cm in difference, the pixel is colored red. Pixels that do not contain points from overlapping flight

lines are colored according to their intensity values. DZ orthos can be created using the full point cloud or ground only points and are used to review and verify the calibration of the data is acceptable. Some areas are expected to show sections or portions of red, including terrain variations, slope changes, and vegetated areas or buildings if the full point cloud is used.

However, large or continuous sections of yellow or red pixels can indicate the data was not calibrated correctly or that there were issues during acquisition that could affect the usability of the data. The DZ orthos for Louisa, Virginia showed that the data was calibrated correctly with no issues that would affect its usability. The figure below shows an example of the DZ orthos.

Dewberry utilized a variety of software suites for data processing. The LAS dataset was received and imported into GeoCue task management software for processing in Terrascan. Each tile was imported into Terrascan and a surface model was created to examine the ground classification. Dewberry analysts visually reviewed the ground surface model and corrected errors in the ground classification such as vegetation, buildings, and bridges that were present following the initial processing conducted by Dewberry. Dewberry analysts employ 3D visualization techniques to view the point cloud at multiple angles and in profile to ensure that non-ground points are removed from the ground classification. After the ground classification corrections were completed, the dataset was processed through a water classification routine that utilizes breaklines compiled by dewberry to automatically classify hydro features. The water classification routine selects ground points within the breakline polygons and automatically classifies them as class 9, water. The final classification routine applied to the dataset selects ground points within a specified distance of the water breaklines and classifies them as class 10, ignored ground due to breakline proximity.

The NOAA Office for Coastal Management (OCM) received the topographic/bathymetric files in LAS format from the University of William and Mary's Center for Geospatial Analysis. A number of LAS files were found to have corrupt GPS times and other unknown factors affecting header information. The files contained lidar easting, northing, elevation, intensity, return number, etc. The data was received in UTM coordinates, zone 18 North, referenced to the NAVD88 for vertical using the Geoid09 model. OCM performed the following processing for data storage and Digital Coast provisioning purposes:

1. The LAS files were converted to geographic horizontal coordinates and ellipsoidal vertical coordinates.

2. The LAS files were cleared of error points and variable length records removed.