Data collected for the mapping cooperative have been released in a series of USGS Open-File Reports<http://woodshole.er.usgs.gov/project-pages/coastal_mass/html/current_map.html>. This spatial dataset is from the Cape Ann and Salisbury Beach Massachusetts project area. They were collected in two separate surveys in 2004 and 2005 and cover approximately 325 square kilometers of the inner continental shelf. High resolution bathymetry and backscatter intensity were collected in 2004 and 2005. Seismic profile data, sediment samples and bottom photography were also collected in 2005.
A patch test was conducted at the beginning of the survey in order to measure roll angle offsets. Roll angle offsets can have a strong effect on the accuracy of depth measurements. (The roll angle offset is the difference between the motion sensor vertical measurement and the nominal mount angle of the transducers (30 degrees)). The calculated roll angle offsets were stored within the SEA Swath Processor acquisition software.
Navigation: The USGS established a Real-Time Kinematic Global Positioning System (RTK-GPS) that provided horizontal (x and y) and vertical (z) positioning during the survey. The RTK-corrected GPS signal was sent to the ship once every second from a base station positioned on the roof of the Air Force Antenna Station (USAF/AFRC/SNHA) in Ipswich, Massachusetts. Shipboard navigation data (x, y, and z) were stored in Hypack, Inc. Hydrographic Survey Software (<http://www.hypack.com>) raw data files and used for positioning of geophysical instrumentation.
The offset between orthometric (NAVD88) and local chart datum (MLLW) as reported at the NOAA Tidal Station #8440452, Parker River National Wildlife Refuge, was applied to the RTK signal. This allows for elevations (z) received at the shipboard RTK antenna to be referenced to Mean Lower Low Water (MLLW) (that is, elevations are stored as distance of the RTK antenna above MLLW).
During post-processing, the distance of the RTK antenna above the water-line is removed from the stored RTK elevations. The resulting value is then applied to the bathymetric soundings, yielding data referenced to MLLW.