p192sc.m77t and p192sc.h77t: MGD77T data and header files for single-beam bathymetry data for field activity P-1-92-SC in Santa Monica Basin, Southern California from 01/30/1992 to 02/04/1992

The navigation data are digitally collected using two Northstar Global Positioning System (GPS) 8000X receivers and Northstar 800 Loran-C receivers. 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. The advantage of this coupled system provided a better control for the survey area since data gathered from the GPS system automatically calibrates the accuracy of the coordinates derived from the Loran-C receiver and automatically selects either Loran-C or GPS for ship positioning depending on which system provides the better signals at the given time.. 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 a Raytheon 12 kHz PTR sounder and an ORE 3.5 kHz sounder. 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.