Single-beam bathymetry data along with SINS navigation data was collected as part of the U.S. Geological Survey cruise D-1-79-EG. The cruise was conducted in the Eastern Gulf of Alaska from May 24 to June 1, 1979. The chief scientists were Bruce Molnia from the USGS Coastal and Marine Geology office in Menlo Park, CA and Mark Wheeler.
The purpose of this cruise was to collect sediment samples and cores for a microfossil study. The geophysical source was a 3.5 kilohertz (kHz) bathymetry system.
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/d/d179eg/html/d-1-79-eg.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 collected using a Shipboard Integrated Navigation System (SINS). used Transit satellite navigation and a number of subsystems that were used in dead reckoning between satellite fixes. These included Doppler sonar, speed-of-sound velocimeter, water temperature sensor, gyrocompass, electromagnetic speed log, radionavigation and other subsystems. SINS was highly variable and at best there was a 50 m accuracy.
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 as analog records with a 3.5 kHz system and hand-digitized. 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.
