2021 NOAA NGS Topobathy Lidar DEM: Niihau, Hawaii

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This graphic shows the lidar footprint for this dataset.

Woolpert acquired, calibrated, and performed the refraction correction of the lidar data. 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 correction process corrects for this difference by adjusting the depth (distance traveled) and horizontal position (change of angle/direction) of the lidar data acquired within water.

Relative accuracy of the swaths compared to overlapping and adjacent swaths was verified through the use of Delta-Z (DZ) orthos created in LASTools software. Profiles of elevated planar features, such as roofs, were used to verify horizontal alignment between overlapping swaths.

The NovAtel software was utilized for GPS and inertial processing, Data were processed to an initial LAS format using the Leica Lidar Survey Studio software.

Algorithms in TerraScan was used to create the initial ground surface.

All lidar data was peer-reviewed. QAQC also included creating void polygons for use during review. All necessary edits were applied to the dataset. LASTools was used to update LAS header information, including all projection and coordinate reference system information. The final lidar data is in LAS format 1.4 and point data record format 6.

Data were processed in NAD83 (PA11) horizontal and vertical datum. The National Oceanic and Atmospheric Administration (NOAA) received files in laz format.

Data for Niihau, HI was delivered in UTM Zone 4 coordinates and ellipsoid elevations in meters.

Horizontal Datum-NAD83 (PA11) epoch: 2010.0

Projection-UTM Zone 4 North

Horizontal Units-meters

Vertical Datum-NAD83 (PA11) Ellipsoid height

Vertical Units-meters

The final classification scheme for all data delivered to NOAA is as follows:

1-Unclassified

2-Ground

7-Low Noise

9-Topo Water Surface

18-High Noise

40-Bathymetric Point

41-Bathymetric Water Surface

42-Synthetic Derived Water Surface

43-Submerged Object

44-International Hydrographic Organization (IHO) S-57 Object

64-Submerged Aquatic Vegetation

65-Bathymetric Bottom Temporal Changes

The lidar data is referenced to the NAD83 (PA11) ellipsoid. The lidar data in ellipsoid heights was used to create the final topobathymetric DEMs in ellipsoid heights.

Lidar data were used to generate a raster in geotiff format with 1 meter pixel resolution for the entire area. The raster was then clipped to the tile grid and named according to project specifications to result in tiled topobathymetric DEMs.

Bathy void polygons represent bathymetric areas with no bathymetric bottom returns and accompany the DEMs. Void polygon creation is described in the final project report and the void polygon metadata.

A confidence layer has been created and provided to NOAA as part of the deliverables. Except where there are not enough lidar data points in a pixel to calculate a standard deviation value, the confidence layer is a raster product in geotiff format with 1 meter square pixels. The confidence layer provides a standard deviation value for every pixel by calculating the standard deviation of all ground and/or submerged topography lidar points that are located within a single pixel. The confidence layer pixels align to the pixels in the topobathy DEMs. The confidence layer can be displayed by unique values or classified into desired bins/ranges for analysis over larger areas.