2013 Cecil County Lidar

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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.

Using a Trimble Harrier 68i High Definition LiDAR sensor on board a Cessna 206, 191 flight lines of high-density data were collected during 15 separate missions, flown as close together as the weather conditions permitted, to ensure consistent ground conditions across the project area, at a nominal pulse spacing (NPS) of 0.46 m achieved a point density of 4.73 ppm2, were collected by Axis Geospatial, LLC covering Cecil County, MD (approximately 473 square miles). Data Acquisition Height = 2,034 feet Above Ground Level (AGL) - Aircraft Speed = 116 Knots. Multiple returns were recorded for each laser pulse along with an intensity value for each return. A total of 15 missions were flown from December 20, 2013 to January 20, 2014. The presence of a strong CORS (Continuously Operating Reference Station) configuration allowed for the LiDAR to be acquired with Global Navigation Satellite System (GNSS) techniques and procedures. 58 ground control points were surveyed through RTK and static methods. The GEOID used to reduce satellite-derived elevations to orthometric heights was GEOID12A. Data for the task order is referenced to the Maryland State Plane, North American Datum of 1983 (NAD83), and North American Vertical Datum of 1988 (NAVD88), in US Survey Feet. Airborne GPS data was differentially processed and integrated with the post-processed inertial measurement unit (IMU) data to derive a smoothed best estimate of trajectory (SBET). The SBET was used to reduce the lidar slant range measurements to a raw reflective surface for each flight line. The coverage was classified to extract a bare earth digital elevation model (DEM) and separate last returns.

During the project data acquisition by Axis Geospatial, LLC, East-West cross flights were flown over each of the 6 delivery block areas in order to tie the data together from multi-day North-South collections.

Once the data acquisition and GPS processing phases are complete, the lidar data were processed immediately by Axis Geospatial, LLC to verify the coverage had no voids. The GPS and IMU data were post-processed using ApplanixPOSPac v.6.1 software to derive a best estimate of trajectory. The quality of the solution was verified to be consistent with the accuracy requirements of the project.

The individual flight lines were inspected by Axis Geospatial, LLC to ensure the systematic and residual errors have been identified and removed. Then, the flight lines were compared to adjacent flight lines for any mismatches to obtain a homogenous coverage throughout the project area. Axis Geospatial, LLC compared LiDAR intensity values against known control point locations that were on defineable features such as the end of a paint line and measured the apparent offsets; these values were then averaged to come up with the X and Y values used for the horizontal correction. Additionally Axis Geospatial, LLC compared a TIN (Triangulated Irregular Network) of LiDAR points within 25 feet of each surveyed control point and measured the apparent vertical offsets; these values were then averaged to come up with the Z value used for the vertical correction. Next the point cloud underwent a classification process to determine bare-earth points and non-ground points utilizing TerraSolid, TerraScanLiDAR processing and modeling software. v. 014.013. This process determined Default (Class 1), Ground (Class 2), and Noise (Class 7), classifications. The bare-earth (Class 2 - Ground) lidar points underwent a manual QA/QC step to verify that artifacts have been removed from the bare-earth surface. The surveyed ground control points are used to perform the accuracy checks and statistical analysis of the lidar dataset.

Applanix software (PosPAC MMS) was used in the post processing of the airborne GPS and inertial data, which are critical to the positioning and orientation of the sensor during all flights. POSPac MMS provides the smoothed best estimate of trajectory (SBET) that is necessary for the post processor to develop the point cloud from the lidar missions. The point cloud is the mathematical three-dimensional collection of all returns from all laser pulses as determined from the aerial mission. The GEOID used to reduce satellite derived elevations to orthometric heights was GEOID12A. Data for the task order is referenced to the Maryland State Plane, NAD83, and NAVD88, in US Survey Feet. At this point the data are 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. The point cloud was manipulated by the Trimble LPMaster software; GeoCue, TerraScan, and TerraModeler software were used for the automated data classification, manual cleanup, and bare-earth generation from the data. Project specific macros were used to classify the ground.

All data were manually reviewed and any remaining artifacts removed using functionality provided by TerraScan and TerraModeler.

Data were then run through additional macros to ensure deliverable classification levels matching the ASPRS LAS Version 1.2 Classification structure. GeoCue functionality was then used to ensure correct LAS Versioning. In-house software was used as a final QA/QC check to provide LAS Analysis of the delivered tiles. QA/QC checks were performed on a per tile level to verify final classification metrics and full LAS header information.

Axis Geospatial, LLC located a total of 58 check points for use in the analysis of the lidar data quality. The points were located on relatively flat terrain on surfaces that generally consisted of grass, gravel, or bare earth. The final vertical accuracy (RMSEz x 1.96) of the following landcover types were tested; Bare Earth-FVA (Fundamental Vertical Accuracy), CVA (Consolidated Vertical Accuracy), Crop/Weeds SVA (Supplemental Vertical Accuracy), and Wooded SVA (Supplemental Vertical Accuracy). The final results for FVA (based on 24 points) was found to be 0.17 feet; SVA Crop/Weeds (based on 10 points) was found to be 0.27 feet; and SVA Woods (based on 24 points) was found to be 0.52 feet. The CVA was found to be .37 feet.

NOAA OCM downloaded the data from Maryland iMap (imap.maryland.gov) in October 2017. Data were converted from Maryland State Plane (1900) with NAVD88 (geoid12a) vertical feet to geographic coordinates with height above the ellipsoid in vertical meters. Data were ingested into the Digital Coast Data Access Viewer for custom distribution.