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Manual Reference Pages  -  MGD77CONVERT (1)


mgd77convert - Translate between different formats of MGD77 files


File Formats
Other Tools


mgd77convert NGDC-ids -Fa|c|m|t -T[+]a|b|m|t [ -L[w][e][+] ] [ -V ] [ -4 ]


mgd77convert reads versions of MGD77 files and writes the same data in (probably) another format to a new file in the current directory. Both pre- and post-Y2K MGD77 formats can be processed.
  Can be one or more of five kinds of specifiers:
1) 8-character NGDC IDs, e.g., 01010083, JA010010etc., etc.
2) 2-character <agency> codes which will return all cruises from each agency.
3) 4-character <agency><vessel> codes, which will return all cruises from those vessels.
4) =<list>, where <list> is a table with NGDC IDs, one per line.
5) If nothing is specified we return all cruises in the data base.
(See mgd77info -L for agency and vessel codes). The ".mgd77" or ".nc" extensions will automatically be appended, if needed (use -I to ignore certain file types). Cruise files will be looked for first in the current directory and second in all directories listed in $MGD77_HOME/mgd77_paths.txt [If $MGD77_HOME is not set it will default to $GMT_SHAREDIR/mgd77].
-F Specifies the format of the input (From) files. Choose from a for standard MGD77 ASCII table (with extension .mgd77), c for the new MGD77+ netCDF format (with extension .nc), revised MGD77T (tab-delimited; with extension .m77t) and t for a plain ASCII tab-separated table dump (with extension .dat). Use -FC to recover the original MGD77 setting from the MGD77+ file [Default will apply any E77 corrections encoded in the file].
-T Specifies the format of the output (To) files. Choose from a for standard MGD77 ASCII table (with extension .mgd77), c for the new MGD77+ netCDF format (with extension .nc), revised MGD77T (tab-delimited; with extension .m77t) and t for a plain ASCII tab-separated table dump (with extension .dat). We will refuse to create the file(s) if they already exist in the current directory. Prepend + to override this policy.


No space between the option flag and the associated arguments.
-L Set the level of verification reporting [none] and where to send such reports [stderr]. Append a combination of w for warnings, e for errors, and + to send such log information to stdout.
-V Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
-4 By default, the storage types used in a MGD77+ netCDF file greatly exceed the precision imposed by the ASCII MGD77 format. However, for the five items faa, eot, mag, diur and msd we use 2-byte integers with implied precisions of 0.1 mGal, 0.1 nTesla, and 1 m as in the MGD77 format. It is possible that at some point these items will need to be stored as 4-byte ints which would allow precisions of 1 fTesla, 1 nGal, and 0.01 mm, respectively. This option activates such storage [Default uses 2-byte integers].


To convert 01010047.mgd77 and 01010008.mgd77 to new netCDF .nc files, and capture all verification messages, try

mgd77convert 01010047 01010008 -Fa -Tc -V -Lew+ > log.lis

To convert back to MGD77 ASCII and make sure it is identical to the original file, try (Bourne shell syntax)

orig=‘mgd77path 01010047 -Ic
mgd77convert 01010047 -Fc -Ta -V
diff $orig 01010047.mgd77

To convert to a plain ASCII table for manual editing, overwriting any existing table, try

mgd77convert 01010047 -Fc -T+t -V

To recover the original NGDC MGD77 version of and ignore any E77 corrections, use
mgd77convert 01020051 -FC -Ta -V


mgd77convert handles four different formats. (1) The MGD77 ASCII tables are the conventional standard for distribution of underway geophysical data to and from the NGDC data center. Normally, only the ship-operations people and the cruise PI might be involved in making an MGD77 ASCII file for transmission to NGDC; users are more interested in reading such files. (2) The MGD77+ netCDF format was developed to fascilitate the use of MGD77 data by scientists. It contains all the information of the original MGD77 file and if you convert back and forth you end up with the original. However, file sizes are typically ~30% of the original ASCII format and is much faster to operate on. (3) NGDC has now started to use a new tab-delimited version of the MGD77 data, called MGD77T. In addition to all the info in old MGD77 files it contains a few more quality flags for grav, mag, and bathymetry. (4) The plain ASCII tab-separated dump is available for users who need to manually edit the content of a MGD77 file. This is usually easier to do when the columns are tab-separated than when they are all crunched together in the MGD77 punch-card format. It differs from format MGD77T in that missing items are written as NaNs.


The MGD77+ netCDF files are CF-1.0 and COARDS compliant and can be examined with general-purpose tools such as ncBrowse and ncView.


mgd77manage(1), mgd77list(1), mgd77sample(1), mgd77track(1) x2sys_init(1)


Wessel, P., and W. H. F. Smith, 2015, The Generic Mapping Tools (GMT) version 4.5.14 Technical Reference & Cookbook, SOEST/NOAA.
Wessel, P., and W. H. F. Smith, 1998, New, Improved Version of Generic Mapping Tools Released, EOS Trans., AGU, 79 (47), p. 579.
Wessel, P., and W. H. F. Smith, 1995, New Version of the Generic Mapping Tools Released, EOS Trans., AGU, 76 (33), p. 329.
Wessel, P., and W. H. F. Smith, 1995, New Version of the Generic Mapping Tools Released,, Copyright 1995 by the American Geophysical Union.
Wessel, P., and W. H. F. Smith, 1991, Free Software Helps Map and Display Data, EOS Trans., AGU, 72 (41), p. 441.
ncBrowse, see
ncView, see
The Marine Geophysical Data Exchange Format - "MGD77", see
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GMT 4.5.14 MGD77CONVERT (1) 1 Nov 2015

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