The Tenix LADS Corporation (TLI) acquired bathymetric LIDAR for NOAA from 4/07/2006 to 5/15/2006. Data was acquired with a LADS (Laser Airborne Depth Sounder) Mk II Airborne System from altitudes between 1,200 and 2,200ft at ground speeds between 140 and 175 knots. The 900 Hertz Nd: YAG (neodymium-doped yttrium aluminum garnet) laser (1064 nm) acquired 4x4 meter spot spacing and 200% seabed coverage. In total, 265 square nautical miles of LiDAR were collected between -50 m (topographic) and up to 70 m (depth), requiring a total of 102 flight hours (134 hours, including flight time to and from San Juan airport). Environmental factors such as wind strength and direction, cloud cover, and water clarity influenced the area of data acquisition on a daily basis. The data was processed using the LADS Mk II Ground System and data visualization, quality control and final products were created using CARIS HIPS and SIPS 6.1 and CARIS BASE Editor 2.1 The project was conducted to meet the IHO (International Hydrograph Organization) Order 1 accuracy standards, dependant on the project area and depth. All users should individually evaluate the suitability of this data according to their own needs and standards.
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. The LADS ground system includes a reflectivity algorithm in which reflectivity is calculated for each sounding as the ratio of returned energy to transmitted energy, normalized for losses. A comprehensive description of the GS is provided in the Data Acquisition and Processing Report delivered for project number OPR-I305-KRL-06.
The reflectance XYZ data was imported into CARIS HIPS and SIPS as Singlebeam data. The reflectance data was treated much the same as ordinary XYZ data (XY horizontal position and Z as reflectance value). The procedure used is as follows: - Modified XYZ file to add timestamp by using the CARIS XYZ File Manipulator utility - Created an import script using the CARIS Generic Data Parser - Imported modified XYZ file into CARIS HIPS and SIPS - Computed Total Propagated Error (TPE) using zero values - Applied tide corrections using a zero value file - Merged TPE and tide correction with data - Created fieldsheet to compute BASE surface of reflectance data
The XYZ had to be modified to add timestamp to each data point. The XYZ file was divided into smaller files containing a maximum of 1,000,000 points per file. Each modified XYZ file was imported using the CARIS Generic Data Parser, creating an import script to recognize each field attribute. Two scripts were created to import timestamps 1000000 to 9999999 and the other 10000000 to 138000000. Once each XYZ file was imported into CARIS a generic zero tide value was applied to each modified XYZ file followed by a zero TPE value computation. The TPE and tide calculations were then merged with the reflectance data. A single fieldsheet was then created to incorporate six 'mean' BASE surfaces, gridded at a 5m resolution, which covered the entire survey area. Using CARIS BASE Editor, these six BASE surfaces were combined into one surface to produce a final 'mean' reflectance BASE surface. The necessity to divide the area into six initially was purely due to processing limitations. Also, the grid resolution does not change relative to depth, as the laser pulse footprint stays relatively constant regardless of depth and the laser spot spacing is consistent irrespective of aircraft altitude. The 5m grid provides the largest amount of detail that can be supported by the LiDAR data density.
CARIS HIPS and SIPS 6.1 and CARIS BASE Editor 2.1 were used for data visualization, quality control and final product creation.
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. If it were known that these laser dropout areas would affect the quality of the reflectivity data then the lines would have been reflown.
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. Significant variations, temporally or spatially in turbidity may impact the reflectivity algorithm.