2020 NOAA NGS Topobathy Lidar DEM: Alamagan, CNMI

Woolpert, Inc. was contracted to acquire and process topographic-bathymetric lidar for the islands of Anatahan, Alamagan, Guguan, and Sarigan in response to Hurricane Yutu for Quantum Spatial, Inc. (QSI).  Woolpert collected lidar using their Leica HawkEye 4X (HE4X) topo-bathy lidar sensor that consists of a Chiroptera 4X (CH4X) sensor, with an additional Leica 40kHz deep bathymetric channel  to provide high density topo lidar.  The HE4X is a latest generation topographic and bathymetric lidar sensor. The system provides denser data than previous traditional bathymetric lidar systems. It is unique in its ability to acquire bathymetric lidar, topographic lidar and 4-band digital camera imagery simultaneously. The HE4X provided 300 kHz topographic data, an effective 140 kHz shallow bathymetric data and 40 kHz deep bathymetric data.  4-band 80 MP digital camera imagery was also collected simultaneously with the sensorâs RCD-30 camera.

The bathymetric and topographic lasers are independent and do not share an optical chain or receivers, so they are optimized for their specific function. As with any bathymetric lidar, maximum depth penetration is a function of water clarity and seabed reflectivity.  The HE4X is designed to penetrate to 3 times the secchi depth.  This is also represented as Dmax = 4/K, where K is the diffuse attenuation coefficient, and assuming K is between 0.1 and 0.3, a normal sea state and 15% seabed reflectance.

Both the topographic and bathymetric sub-systems use a palmer scanner to produce an elliptical scan pattern of laser points with a degree of incidence ranging from +/-14 degrees (front and back) to +/-20 degrees (sides), providing a 40 degrees field of view.  This has the benefit of providing multiple look angles on a single pass and helps to eliminate shadowing effects. This can be of particular use in urban areas, where all sides of a building are illuminated, or for bathymetric features such as the sides of narrow water channels or features on the seafloor such as smaller objects and wrecks.  It also assists with penetration in the surf zone where the back scan passes the same ground location a couple of seconds after the front scan, allowing the areas of whitewater to shift.

All topo lidar data for this project were collected simultaneous to meet United States Geological Survey, Quality Level 1 (USGS QL1) with a minimum of 8 pts per square meter at an accuracy of 10cm RMSEz.  A minimum of 2 points per square meter were acquired for bathymetric lidar data.    For practical purposes the survey area was divided into survey blocks in each island, allowing acquisition to be conducted in the most efficient and consistent manner possible.

The data includes topobathy data in an LAS 1.4 format file along with associated bare earth digital elevation models (DEM). This file is the project specified 1 meter bare earth DEM dataset. The dataset was derived from topobathymetric data in a LAS format 1.4, point data record format 6, with the following classifications in accordance with project specifications and the American Society for Photogrammetry and Remote Sensing (ASPRS) classification standards:
			1 - unclassified
			2 - ground
			7 - noise
			40 - bathymetric bottom or submerged topography
			41 - water surface
			42 - derived water surface
			43 - manmade submerged feature
			45 - water column
			1 Overlap - edge clip 
			1 Withheld - bathy land

User data values differentiates between NIR and green lasers. A value of of 1 indicates the point is from the NIR laser, and a values of 2-5 indicate the green laser.

This dataset is the 1m orthometric NAVD88 (using Geoid12b) DEM.

Tags: airplane, bathymetry, bathymetry/seafloor topography, beach, coastal bathymetry, coastal elevation, coastal processes, dem, doc/noaa/nos/ngs, earth science, erosion, land surface, lidar, light detection and ranging, national geodetic survey, national ocean service, ngs lidar, noaa, oceans, sea floor, terrain elevation, topography, u.s. department of commerce, vertical location
Organization: National Oceanic and Atmospheric Administration, Department of Commerce