$Id: README.mgg,v 1.6 2009/09/09 23:27:04 guru Exp $ README file for GMT supplemental mgg programs Distributed under the GNU Public License (see file ../LICENSE.TXT) AUTHORS: Paul Wessel and Walter H. F. Smith DATE: 15-MAR-1999 INSTALL INFO: The mgg directory should install under GMT/src These programs are intended for use with the marine geophysical data base files known as GMT-files. These GMT-files are written using a native binary data format. These data files are NOT supplied with GMT, but we provide you with a program (mgd77togmt) that allows you to create your own GMT-files from the MGD-77 standard exchange format files provided by the National Geophysical Data Center*. The following is a list of the programs and a short description of what they do: dat2gmt Convert an ASCII listing to a binary .gmt file gmt2dat Convert a binary .gmt file to ASCII listing gmtinfo Information about selected cruises gmtlegs Find cruises in a given area gmtlist A data extractor for .gmt files gmtpath Get full pathname for .gmt files gmttrack Plot .gmt file tracklines on maps mgd77togmt Convert a MGD-77 file to gmtfile format In addition, two "system-level" programs are supplied that creates and maintains the indexfiles needed by most of the above programs: binlegs Add info about new gmtfiles to the index files gmt2bin Create bin-index files from gmtfiles (for binlegs) man-files for all the above programs can be found in the manl subdirectory. The support files carter.d and gmtfile_paths are put in the GMT/share/mgg subdirectory directory. The ASCII file carter.d is used to convert two-way travel-time to depth using the Carter correction tables. The gmtfile_paths file gives lists of directories where the *.gmt datafile will be stored. To install these programs, simply type make install To install the manual pages, type make install-man The gmtfiles are unformatted, binary, direct access files with a fixed record length of 18 bytes. Each file has one header record and one or more data records. The compact file format makes these files fast to read. This is the structure of the files (number in parentheses indicates size of elements and records in bytes): The first (header) record (18): long int (4) year Year of first data point in file long int (4) nrecords Number of data records to follow char[10] (10) info Currently holds the leg id, e.g. c2104 Data records (18): long int (4) time Seconds since 00:00:00 1-Jan-year long int (4) latitude Stored in micro degrees long int (4) longitude Stored in micro degrees short int (2) grav FAA, in mGal * 10 (g.u.), on 1980 ellipsoid short int (2) mag Regional anomaly in nTesla short int (2) top Carter corrected topography in m NB! If a particular data field is empty, the value is set to -32000. Note that these files are not portable across platforms unless you take care of swabbing data to account for big- and little-endian problems. * Available on CD-ROM from NGDC (www.ngdc.noaa.gov).