2013 NOAA Topographic Lidar: US Virgin Islands Digital Elevation Models (DEMs)

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LiDAR points were used to produce the deliverables. | Source Geospatial Form: digital data | Type of Source Media: Hard Drive

Applanix + POSPac Mobile Mapping Suite software was used for post-processing of airborne GPS and inertial data (IMU), which is critical to the positioning and orientation of the LiDAR sensor during all flights. POSPac combines aircraft raw trajectory data with stationary GPS base station data yielding a "Smoothed Best Estimate Trajectory (SBET) necessary for additional post processing software to develop the resulting geo-referenced point cloud from the LiDAR missions.

During the sensor trajectory processing (combining GPS and IMU datasets) certain statistical graphs and tables are generated within the Applanix POSPac processing environment which are commonly used as indicators of processing stability and accuracy. This data for analysis include: Max horizontal / vertical GPS variance, separation plot, altitude plot, PDOP plot, base station baseline length, processing mode, number of satellite vehicles, and mission trajectory.

The generated point cloud is the mathematical three dimensional composite of all returns from all laser pulses as determined from the aerial mission. Point clouds were created using the Leica ALS Post Processor software. Laser point data are imported into TerraScan and a manual calibration is performed to assess the system offsets for pitch, roll, heading and scale. At this point this data is ready for analysis, classification, and filtering to generate a bare earth surface model in which the above-ground features are removed from the data set. GeoCue distributive processing software was used in the creation of some files needed in downstream processing, as well as in the tiling of the dataset into more manageable file sizes.

TerraScan and TerraModeler software packages were used for the automated data classification, manual cleanup, and bare earth generation. Project specific macros were developed to classify the ground and remove side overlap between parallel flight lines.

The classes used in the dataset are as follows and have the following descriptions:

Class 1 - Processed, but Unclassified - These points would be the catch all for points that do not fit any of the other deliverable classes. This would cover things like vegetation, buildings, cars, bridges, etc.

Class 2 - Bare earth ground - This is the bare earth surface

Class 7 - Noise - Low or high points, manually and/or automatically identified above or below the surface that could be noise points in point cloud.

Class 9 - Water - Points found inside of inland lake/ponds, rivers or points on the ocean side of any shoreline feature.

Class 10 - Ignored Ground - Points found to be close to breakline features. Points are typically moved to this class from Class 2. This class is ignored during the DEM creation.

Class 17 - Overlap Default (Unclassified) - Points found in the overlap between flight lines. These points are created through automated processing methods and are not cleaned up during manual classification.

Class 18 - Overlap Bare-earth ground - Points found in the overlap between flight lines. These points are created through automated processing, matching the specifications determined during the automated process, that are close to the Class 2 dataset (when analyzed using height from ground analysis)

Class 25 - Overlap Water - Points found in the overlap between flight lines that are located inside hydro features. These points are created through automated processing methods and are not cleaned up during manual classification.

All overlap data was processed through automated functionality provided by TerraScan to classify the overlapping flight line data to approved classes by USGS. The overlap data was classified to Class 17 (Overlap Default) and Class 18 (Overlap Ground). These classes were created through automated processes only and were not verified for classification accuracy. Due to software limitations within TerraScan, these classes were used to trip the withheld bit within various software packages.

The bare earth surface is then manually reviewed to ensure correct classification on the Class 2 (Ground) points. After the bare earth surface is finalized; it is then used to generate all hydro-breaklines through heads-up digitization.

Class 2 LIDAR was used to create a bare earth surface model. The surface model and LAS intensity were then used to heads-up digitize 2D breaklines of inland streams and rivers as well as the ocean shoreline. Inland Ponds and Lakes of 2 acres or greater were also collected.

Elevation values were assigned to all Inland Ponds and Lakes using TerraModeler functionality.

Elevation values were assigned to all Inland Streams and Rivers using Photo Science proprietary software.

Elevation values were assigned to all Coastal Shoreline features using TerraModeler functionality. Z values for the shorelines were assigned based on tidal conditions at the time of acquisition.

All ground (ASPRS Class 2) LiDAR data inside of the Lake Pond and Double Line Drain hydro flattening breaklines were then classified to water (ASPRS Class 9) using TerraScan macro functionality. A buffer of 1 meter was also used around each hydro flattened feature to classify these ground (ASPRS Class 2) points to Ignored ground (ASPRS Class 10). All Lake Pond Island and Double Line Drain Island features were checked to ensure that the ground (ASPRS Class 2) points were reclassified to the correct classification after the automated classification was completed.

LAS data was then run throu

NOAA OCM received the Digital Elevation Models (DEMs) along with the las points and the hydro-flattened breaklines. Overall the DEMs were acceptable, but appear oversampled as a result of low ground point density (heavy vegetation), and is not a contractor issue as a 1 m DEM was specified. Incremental improvements to the DEMs were earned through breakline adjustments. Another characteristic of the USVI lidar data that was observed on all islands is the appearance of divots where there is a single tree or a narrow line of trees that lie in areas of open, bare terrain. The cause appeared to be sub-canopy points that were lower than the surrounding land. In most cases these low points were lower than the true ground surface at those locations, but were classified as ground and retained in the point cloud for use in DEM generation. This issue was not addressed during revisions. For further information, the QA Review Report may be accessed here: https://noaa-nos-coastal-lidar-pds.s3.amazonaws.com/laz/geoid12a/3669/supplemental/usvi2013_stc_stj_stt_m3669_qa_report.pdf