2018-2019 NOAA NGS Topobathy Lidar DEM: Trappe to Toddville, MD

Create custom data files by choosing data area, product type, map projection, file format, datum, etc. A new metadata will be produced to reflect your request using this record as a base. Change to an orthometric vertical datum is one of the many options.

Bulk download of data files in LAZ format, geographic coordinates, orthometric heights. Note that the vertical datum (hence elevations) of the files here are different than described in this document. They will be in an orthometric datum.

This graphic displays the footprint for this lidar data set.

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.

Link to lidar report

Data for the NOAA Chesapeake Bay MD1804 Option 1 East 4 Topobathymetric lidar project area was acquired by NV5 Geospatial (NV5) using Riegl VQ-880-G, VQ-880-GII, and VQ-880-GH Topobathymetric lidar systems. All derived DEM data is referenced to:

Horizontal Datum-NAD83(2011) epoch: 2010.00

Projection-UTM Zone 18N

Horizontal Units-meters

Vertical Datum-NAVD88 (Geoid12b)

Vertical Units-meters

The NOAA Chesapeake Bay MD1804 Option 1 East 4 dataset encompasses 36 5000m x 5000m tiles in an area encompassing a portion of the Chesapeake Bay and the nearby census designated places of Taylors Island, Cambridge, Madison, Woolford, and Algonquin, Maryland. Lidar data were acquired with VQ-880-G, VQ-880-GII, and VQ-880-GH Riegl sensors.

The collected lidar data were immediately processed in the field by NV5 to a level that will allow QA\QC measures to determine if the sensor is functioning properly and assess the coverage of submerged topography. An initial SBET was created in POSPAC MMS 8.3 SP3 and loaded into RiProcess which applies pre-calibrated angular misalignment corrections of scanner position to extract the raw point cloud into geo-referenced LAS files. These files were inspected for sensor malfunctions and then passed through automated raster generation using LAStools to develop an initial assessment of bathymetric coverage. NV5 reviewed all acquired flight lines to ensure complete coverage and positional accuracy of the laser points. These rasters were also used to create an initial product in Quick Look Coverage Maps. These Quick Look files are not fully processed data or final products but provide rapid assessment of approximate coverage and depth penetration.

NV5 resolved kinematic corrections for aircraft position data using aircraft GNSS and Applanix's proprietary PP-RTX solution. When PP-RTX was not used NV5 conducted static Global Navigation Satellite System (GNSS) ground surveys (1 Hz recording frequency) using base stations over known monument locations during flights. After the airborne survey, static GPS data were triangulated with nearby Continuously Operating Reference Stations (CORS) using the Online Positioning User Service (OPUS) for precise positioning. Multiple independent sessions over the same base station were performed to confirm antenna height measurements and to refine position accuracy. This data was used to correct the continuous on board measurements of the aircraft position recorded throughout the flight. A final smoothed best estimate trajectory (SBET) was developed that blends post-processed aircraft position with attitude data. Using the SBETs, sensor head position and attitude were then calculated throughout the survey. Trimble Business Center v.3.90, Blue Marble Geographic Calculator 2019, and PosPac MMS 8.3 SP3 were used for these processes.

Following final SBET creation, NV5 used RiProcess 1.8.5 to calculate laser point positioning by associating SBET positions to each laser point return time, scan angle, and intensity. Terra 19 and LasTools were used to classify water surface and create a water surface model. They are created for single swaths to ensure temporal differences and wave or water surface height variations between flight lines do not impact the refraction of the bathymetric data. These models are used in NV5's LasMonkey refraction tool to determine the accurate positioning of bathymetric points. All lidar data below water surface models were classified as water column to be refracted. Light travels at different speeds in air versus water and its direction of travel or angle is changed or refracted when entering the water column. The refraction tool corrects for this difference by adjusting the depth (distance traveled) and horizontal positioning (change of angle/direction) of the lidar data. Using raster-based QC methods, the output data is verified to ensure the refraction tool functioned properly.

Once all data was refracted by flight line data was exported to LAS 1.4 format and combined into 500 m x 500 m tiles. Data were then further calibrated using TerraMatch. NV5 used custom algorithms in TerraScan to classify the initial ground/submerged topography surface points. Relative accuracy of overlapping swaths was compared and verified through the use Delta-Z (DZ) orthos created using NV5's Las Product Creator. Absolute vertical accuracy of the calibrated data was assessed using ground survey data and complete coverage was again verified.

Post automated classification NV5 then performed manual editing to review all classification and improve the final Topobathymetric surface. NV5's LasMonkey was used to update LAS header information, including all projection and coordinate reference system information. The final lidar data are in LAS format 1.4 and point data record format 6.

The final classification scheme is as follows:

1 - unclassified

2 - ground

7 - noise

40 - bathymetric bottom or submerged topography

43 - submerged feature

45 - water column

46 - overlap bathymetric bottom

71 - adjacent lift unclassified

72 - adjacent lift ground

85 - adjacent lift water column

1 Overlap - edge clip

NV5 transformed the final lidar data from ellipsoid heights to orthometric heights referenced to NAVD88, Geoid 12b to create the final topobathymetric void clipped DEMs. The topobathymetric bare earth DEMs were output at 1 meter resolution in GeoTIFF format into 36 - 5,000 m x 5,000 m tiles. The NOAA Chesapeake Bay MD1804 Option 1 East 4 rasters are clipped to the extent of the project boundary and named according to project specifications.

A bathymetric void shapefile was created to indicate areas where there was a lack of bathymetric returns. This shape was created by triangulating bathymetric bottom points with an edge length maximum of 4.56m to identify all areas greater then 9 square meters without bathymetric returns. This shapefile was used to clip and exclude interpolated elevation data from these areas in the bathymetric void clipped topobathymetric bare earth model.