Access Data Acquisition and Processing Report (OPR-I305-KRL-06) at <http://ccma.nos.noaa.gov/products/biogeography/lidar_pr/DAPR_OPR-I305-KRL-06.pdf>
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For this project (OPR-I305-KRL-06), the Chief of Party was TLI's Darren Stephenson and Hydrographer was TLI's Mark Sinclair. Data was collected between 4/7/2006 & 5/15/2006 using the LADS Mk II Airborne System. The LADS Mk II Airborne System (AS) consists of a Dash 8-200 series aircraft, which has a transit speed of 250 knots at altitudes of up to 25,000ft and an endurance of up to eight hours. Survey operations are conducted from heights between 1,200 and 2,200ft at ground speeds between 140 and 175 knots. The aircraft was fitted with an Nd: YAG laser, which operates at 900 Hertz from a stabilized platform to provide a number of different spot spacings. The survey area was sounded at 4x4m laser spot spacing with main lines of sounding spaced at 80m, which provided the required 200% coverage.
Green laser pulses are scanned beneath the aircraft in a rectilinear pattern. The pulses are reflected from the land, sea surface, within the water column and from the seabed. The height of the aircraft is determined by the infrared laser return, which is supplemented by the inertial height from the Attitude and Heading Reference System and GPS height. Real-time positioning is obtained by an Ashtech GG24 GPS receiver combined with Wide Area DGPS (Differential Global Positioning System) provided by the Fugro Omnistar to provide a differentially corrected position. Ashtech Z12 GPS receivers are also provided as part of the Airborne System and Ground Systems to log KGPS (Kinetic Global Positioning System) data on the aircraft and at a locally established GPS (Global Positioning System) base station. For more details on the airborne system, refer to the DAPR (Data Acquisition and Processing Report).
The data was processed using the LADS Mk II Ground System. It consists of a portable Compaq Alpha ES40 Series 3 processor server with 1 GB EEC RAM, 764 GB disk space, digital linear tape (DLT) drives and magazines, digital audio tape (DAT) drive, CD ROM drive and is networked to up to 12 Compaq 1.5 GHz PCs and a HP 800ps Design Jet Plotter, printers and QC workstations. The GS supports survey planning, data processing, quality control and data export. The GS component also includes a KGPS base station, which provides independent post-processed position and height data. A comprehensive description of the GS is provided in the Data Acquisition and Processing Report delivered for project number OPR-I305-KRL-06.
Corrections to Soundings:
The optics and electronics for laser transmission and reflected waveform collection for all soundings is done by equipment mounted on a stabilized platform within the aircraft. This platform is stabilized by an Attitude and Heading Reference System (AHRS) that minimizes the motion effect (roll and pitch) of the aircraft and all residuals from the local horizontal are logged by the Airborne System for correctional processing by the Ground System. Sounding depths and positions are determined in the Ground System from the raw waveform, aircraft height and platform attitude parameters as logged by the Airborne System. The Ground System automatically corrects soundings for aircraft height and heading, offsets between sensors, latency, mirror and platform angles, sea surface model errors, refraction of the laser beam at the sea surface, the effects of scattering of the beam in the water column and reduction for tide. Correct operation of the system is verified by static and dynamic position checks, benchmark lines and analysis of overlaps, redundancy from the 200% coverage of the seabed and crossline comparison results.
CARIS HIPS and SIPS 6.1, Terramodel, Generic Mapping Tool, Visualization Tool Kit and Olex were used for data visualization, quality control and final product creation. Validation proceeds through the following steps: 1. Examining the Depth Profile for the correct processing of each expected Survey Run. 2. Examining the Position Confidence (C3) profile to verify that adequate position accuracy is maintained during the Survey Run. Note: Other profiles of supporting data such as EHE, number of satellites, and latency may also be examined as run profiles. 3. Examining the Coverage Confidence (C6) profile to verify that no coverage gaps exist in the Survey Run.
Resolving anomalous soundings by examining data points in the Survey Run by checking: a. The Primary Depth Display b. The Waterfall Display c. The Waveform Display d. The Local Area Display
Editing operations include selection of the alternate depth, assignment of NBA or deletion of the sounding as appropriate. Based on assessments made in the above steps the operator segments the line classifying each segment as: a. Accepted b. Anomalous (data not to be used) or c. Rejected (for refly) All operator interactions during the validation phase are logged so that complete traceability is maintained.
Data Visualization - All validated and checked data is exported from the GS in a defined ASCII format for spatial presentation and checking. The position, depth, run and other relevant information are extracted from the line-based data for use in the generation of Triangulated Irregular Networks (TINs) and gridded data sets. Both of these are used to produce contour plots, sun-illuminated color banded images and coverage check plots. Anomalies found in these plots are reported back to the checkers for remedial action in the GS.
Data Gaps -The survey area was sounded at 4x4m laser spot spacing with main lines of sounding spaced at 80m, which provided the required 200% coverage. It should be noted that at 4x4m laser spot spacing, there is a gap of 1 to 1.5m between the illuminated areas of adjacent soundings at the sea surface. There is a possibility that small objects in shallow water along the coastline may fall between consecutive 4x4m soundings and not be detected. There are also some gaps in the data due to turbidity and very shallow water, as well as an intermittent laser problem on the last survey sortie. This has resulted in some along track and cross track anomalies and at the time this satisfied the requirement of the survey.
Position Checks - Two independent positioning systems were used during the survey. Real-time positions were aided by WADGPS (Wide Area Differential Global Positioning System). A post-processed KGPS position was also determined relative to a local GPS base station that was established on the rooftop of the Courtyard Marriott Hotel in San Juan. The post-processed KGPS position solutions were applied to each sounding during post-processing and the height used in the datum filter.
Horizontal Control - Data collection and processing were conducted on the Airborne and Ground Systems in World Geodetic System (WGS84) on Universal Transverse Mercator (Northern Hemisphere) projection UTM (N) in Zone 19, Central Meridian 69 W. All units are in meters. This data was post-processed and all soundings are relative to the North American Datum 1983 (NAD83). For more details, please see the Vertical and Horizontal Control Report.
Water Clarity - The water clarity in the survey area was ideal for laser bathymetry as the water was very clear. Coverage was obtained for the majority of the survey area. The only area where coverage was not achieved was due to turbidity or very shallow water. Water depths to 50m were achieved at the extent of the predominant reef structure in SW Puerto Rico. The majority of the survey area is less than 20m deep. There are a number of areas throughout the survey area where no depths were achieved due to turbidity or very shallow water. The water clarity in some areas did vary on a daily basis, which required careful management. Additional survey lines were planned and flown to minimize the data gaps due to turbidity.