Single-beam bathymetry data along with Loran-C RHO-RHO and GPS navigation data was collected as part of the U.S. Geological Survey cruise F-7-90-NC. The cruise was conducted in the Gulf of Farallones, Northern California from July 19 to August 3, 1990. The chief scientists were Herman Karl and Dave Drake from the USGS Coastal and Marine Geology office in Menlo Park, CA and Bill Schwab from the USGS Coastal and Marine Geology office in Woods Hole, Mass.
The purpose of this cruise was a slope stability survey of the Farallones Escarpment. The geophysical sources were 10 kilohertz (kHz), 4.5 kHz, and 3.5 kHz systems.
These data are reformatted from space-delimited ASCII text files located in the Coastal and Marine Geology Program (CMGP) InfoBank field activity catalog at
http://walrus.wr.usgs.gov/infobank/f/f790nc/html/f-7-90-nc.meta.html into MGD77T format provided by the NOAA's National Geophysical Data Center(NGDC). The MGD77T format includes a header (documentation) file (.h77t) and a data file (.m77t). More information regarding this format can be found in the publication listed in the Cross_reference section of this metadata file.
These data and information are intended for science researchers,students, policy makers, and general public. These data are not to be used for navigation purposes. The data are reformatted into MGD77T standard format for inclusion into NOAA's NGDC geophysical database. From there the data will be made available to GeoMapApp (
http://www.geomapapp.org/) and Virtual Ocean (
http://www.virtualocean.org/) Earth-browsing software. Access to the data is also provided via the USGS Coastal and Marine Geoscience Data System (
http://cmgds.marine.usgs.gov/).
The navigation data are digitally collected using Loran-C RHO-RHO and a Global Positioning System (GPS). The GPS is a space-based satellite navigation system maintained by the United States government and is accessible using a GPS receiver. Data accuracy is based on individual receiver models, and most GPS data are collected in the WGS84 horizontal datum. Loran-C was a federally provided radionavigation service managed by the U.S. Coast Guard. Low frequency radio signals transmitted by fixed land based radio beacons served the 48 continental states, their coastal areas, and parts of Alaska. Loran-C could be utilized in different modes of operation; one of these was circular mode, also known as, range-range or RHO-RHO. It only needed a fix from two stations in order to establish a position and an accuracy of 20 to 40 m was achievable.
The Data Acquisition, Processing, and Storage (DAPS) group performs a number of processing steps to the navigation data once it is collected which can vary by cruise. These steps are outlined within the header information in each individual digital navigation (.0xx, .6xx) space-delimited ASCII data file. The header information rows are indicated by lines beginning with the character "!". The single-beam bathymetry data are collected both digitally and as analog records with 10 kHz, 4.5 kHz, and 3.5 kHz systems. The bathymetry datasets were commonly processed using three programs: (A) Batred, which calculates depth from travel times; (B) nosamebathy, which eliminates identical adjacent bathymetry values; and (C) nozerobathy, which removes records with a depth value of "0.0". However, not all of the datasets included in this report were processed with one or more of these programs. If the data was not run through these programs it was reformatted using the most recent "best file" on InfoBank, often these were categorized by InfoBank as raw bathymetry files. Details about which processing steps were taken by the DAPS group can be found in the header information of the bathymetry and navigation ASCII files downloaded from InfoBank.
Prior to reformatting the data into MGD77T format the "best" bathymetry and navigation files from InfoBank were edited to remove the header information and reformatted using Excel 2010 to separate the data into columns based on tabs. Next the bathymetry and navigation files were merged based on time and latitude and longitude. The combined navigation and bathymetry was saved as a .txt file. This combined bathymetry and navigation file was used as an input file into the GEODAS ReFormat to MDG77T program developed by NOAA's NGDC. The program outputs the reformatted data as a tab-delimited .m77t file. The program is available for use at:
http://www.ngdc.noaa.gov/mgg/gdas/gx_announce.Html. For information regarding fields found within the MGD77T files, please refer to the publication, Hittelman and others (2010), found within the Cross-reference section of this metadata file.
Each MGD77T data file also has an accompanying header file (.h77t). The fields within these header files are also defined in the aforementioned publication. Fields are populated based on InfoBank metadata records, digital data file headers, and accompanying publications (often cruise reports) for the survey. If a field is unknown or did not apply to the dataset it is intentionally left blank.
