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mgd77list(1)                          GMT                         mgd77list(1)




NAME

       mgd77list - Extract data from MGD77 files


SYNOPSIS

       mgd77list  NGDC-ids  -Fcolumns[,logic][:bittests] [  -A[+]c|d|f|m|tcode
       ] [  -Cf|g|e ] [  -DA|astartdate ] [  -DB|bstopdate ] [  -E ] [   -Gas-
       tartrec ] [  -Gbstoprec ] [  -Iignore ] [  -L[corrtable] ] [  -Nd|sunit
       ]  [   -Qa|c|vmin/max  ]  [   -Rregion  ]  [   -Sastartdist[unit]  ]  [
       -Sbstopdist[unit]  ] [  -T[m|e] ] [  -V[level] ] [  -Wweight ] [  -Z+|-
       ] [ -bobinary ] [ -hheaders ]

       Note: No space is allowed between the option flag  and  the  associated
       arguments.


DESCRIPTION

       mgd77list  reads  MGD77  files and produces an ASCII [or binary] table.
       The MGD77 files contain track information  such  as  leg-id,  time  and
       position,  geophysical  observables such as gravity, magnetics, and ba-
       thymetry, and control codes and corrections such as Eotvos and  diurnal
       corrections.  The  MGD77+  extended netCDF files may also contain addi-
       tional user columns (for a listing of available columns, use  mgd77info
       -C,  and to learn how to add your own custom columns, see mgd77manage).
       The user may extract any combination of these parameters, any of 8 com-
       puted  quantities  (distance,  heading, course-change, velocity, Carter
       correction, Eotvos correction and gravity and magnetic global reference
       fields), calendar sub-units of time (year, month, day, hour, min, sec),
       the NGDC id, and finally a preset weight (see -W). A sub-section can be
       specified  by  passing  time-  or  distance-intervals along track or by
       selecting a geographical region. Finally, each  output  record  may  be
       required  to  pass any number of logical tests involving data values or
       bit flags. If multiple cruises are requested then they are separated by
       segment headers.


REQUIRED ARGUMENTS

       NGDC-ids
              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).  If  no  file
              extension is given then we search for files with one of the four
              known extensions.  The search order (and the  extensions)  tried
              is MGD77+ (a.nca), MGD77T (a.m77ta), MGD77 (a.mgd77a ) and plain
              text file (a.data).  Use -I to ignore one or more of these  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].

       -Fcolumns[,logic][:bittests]
              The required columns string must be a  comma-separated  list  of
              parameter  abbreviations  given in the desired output order. Any
              parameters given in UPPER case must not be NaN in a  record  for
              output  to occur. Unless specified separately, the output format
              (if ASCII) is controlled by the GMT parameter  FORMAT_FLOAT_OUT.
              The available column abbreviations for information stored in the
              files (some columns may be NaN) are:

              recno  The record number counter.

              drt    The digital record type, usually 3 or 5 (for  Y2K-compli-
                     ant cruises).

              id     The survey ID string (leg name).

              ngdcid The  8-character  NGDC cruise ID string (usually the file
                     prefix).

              time   Choose  between  Absolute  calendar  time   (atime,   the
                     default)  in  the  format  dictated by the GMT parameters
                     FORMAT_DATE_OUT  and  FORMAT_CLOCK_OUT,   Relative   time
                     (rtime) in the format dictated by the GMT parameters FOR-
                     MAT_FLOAT_OUT  and   TIME_SYSTEM   (or   TIME_EPOCH   and
                     TIME_UNIT)),  or  Fractional  year  (ytime) in the format
                     dictated by FORMAT_FLOAT_OUT.

              lon    Longitude in the format dictated  by  the  GMT  parameter
                     FORMAT_GEO_OUT.

              lat    Longitude  in  the  format  dictated by the GMT parameter
                     FORMAT_GEO_OUT.

              twt    Two-Way Travel time (in s).

              depth  Corrected bathymetry (in m, positive below sea level).

              mtf1   Magnetic Total Field intensity from sensor 1 (in nTesla).

              mtf2   Magnetic Total Field intensity from sensor 2 (in nTesla).

              mag    Residual magnetic anomaly (in nTesla).

              gobs   Observed gravity (in mGal).

              faa    Free-air gravity anomaly (in mGal).

              ptc    Position Type Code (1  =  fix,  3  =  interpolated,  9  =
                     unspecified).

              bcc    Bathymetric  Correction  Code,  indicating  the procedure
                     used to convert travel time to depth. (01-55 =  Matthewsa
                     zone  used  to  correct the depth, 59 = Matthewsa correc-
                     tions used but the  zones  is  unspecified  in  the  data
                     record,  60  =  S.  Kuwahara formula for T-S, 61 = Wilson
                     formula for T-S, 62 = Del Grosso formula for  T-S,  63  =
                     Carteras  tables,  88  =  Other, described in header sec-
                     tions, 99 = unspecified).

              btc    Bathymetric Type  Code,  indicating  how  the  bathymetry
                     value  was  obtained (1 = observed, 3 = interpolated, 9 =
                     unspecified).

              msens  Magnetic sensor for used to evaluate the  residual  field
                     (1 = 1st or leading sensor, 2 = 2nd or trailing sensor, 9
                     = unspecified).

              msd    Depth (or altitude) of the magnetic sensor (in  m,  posi-
                     tive below sealevel).

              diur   Magnetic diurnal correction (in nTesla).

              eot    Eotvos correction (in mGal).

              sln    Seismic Line Number string.

              sspn   Seismic Shot Point Number string.

              nqc    Navigation  Quality Code (5 = suspected, by source insti-
                     tution, 6 = suspected, by NGDC, 9 = no  problems  identi-
                     fied).

              In  addition,  the following derived navigational quantities can
              be requested:

              year   The year of each record.

              month  The month of each record.

              day    The day of the month of each record.

              hour   The hour of each record.

              min    The minutes of each record.

              sec    The decimal seconds of each record.

              dist   Along-track distance from start of  leg.  For  method  of
                     calculation,  see  -C [spherical great circle distances],
                     and for distance units, see -N [km].

              az     Ship azimuth (heading) measured clockwise from north  (in
                     degrees).

              cc     Ship course change (change in heading) measured clockwise
                     from north (in degrees).

              vel    Ship speed; see -N for units [m/s].

              Finally, the following computed quantities can be requested:

              weight Weight assigned to this data set (see -W).

              carter Carter depth correction, if twt is present  in  file  (in
                     m). Sign: Correction is to be added to uncorrected depths
                     to yield a corrected depth.

              igrf   International geomagnetic reference field  (total  field)
                     (in nTesla).

              ngrav  International  Gravity reference Field (anormal gravitya)
                     (in mGal).  Field is  selected  based  on  the  parameter
                     Gravity  Theoretical  Formula  Code in the cruiseas MGD77
                     header. If this is not set or is invalid  we  default  to
                     the  IGF  1980. Alternatively, specify the field directly
                     using -Af (see that option for more details).

              ceot   Calculated Eotvos correction from navigation, using  E  =
                     7.5038  *  V  *  cos(lat)  * sin(az) + 0.004154 * V^2 (in
                     mGal). Sign: Correction is to be added to uncorrected faa
                     to yield a corrected faa.

              The following short-hand flags are also recognized:

              mgd77  This  results in all 27 MGD77 fields being written out in
                     the official MGD77 order.

              mgd77t The full set of all 26 columns in the  MGD77T  specifica-
                     tion.

              geo    This  limits the output to 10 fields (time, lon, lat plus
                     the seven  geophysical  observations  twt,  depth,  mtf1,
                     mtf2,  mag,  gobs,  and faa). By appending + to either of
                     these set we will also append dist, azim,  cc,  vel,  and
                     weight as listed above.

              all    This returns all data columns in the file.

              allt   As  mgd77t  but  with  time  items written as a date-time
                     string.

              As an option, logical tests may be added for any of the observa-
              tions  by  appending  ,logic, which is itself composed of one or
              more comma-separated instructions of the form parOPvalue,  where
              par is one of the parameters listed above, OP is a logical oper-
              ator (<, <=, =, !=, >=, >, |), and value is a constant  used  in
              the  comparison.  Floating point parameters are compared numeri-
              cally; character parameters are compared lexically (after  lead-
              ing  and  trailing blanks have been removed). The bit comparison
              (|) means that at least one of the bits in value must be  turned
              on in par. At least one of the tests must be true for the record
              to be output, except for tests using UPPER case parameters which
              all  must  be  true for output to occur. Notes: (1) Specifying a
              test does not  imply  that  the  corresponding  column  will  be
              included in the output stream; it must be present in columns for
              that to occur. (2) Some of the operators are special UNIX  char-
              acters  and  you  are  advised to place quotes around the entire
              argument to -F.  (3) The logical tests only  apply  to  observed
              data; derived data (such as distances, velocities, etc.) must be
              limited using program options such as -D, -Q, -S, etc.

              Finally, for MGD77+ files you may  optionally  append  :bittests
              which  is  :  (a  colon) followed by one or more comma-separated
              +-col terms. This  compares  specific  bitflags  only  for  each
              listed  column.   Here,  +  means  the chosen bit must be 1 (ON)
              whereas - means it must be 0 (OFF). All bit tests given must  be
              passed.   By   default,  MGD77+  files  that  have  the  special
              MGD77_flags column present will use those  flags,  and  observa-
              tions  associated with ON-bits (meaning they are flagged as bad)
              will be set to NaN; append : with  no  trailing  information  to
              turn  this  behavior off (i.e., no bit flags will be consulted).
              Note that these record-based flags are different from  any  sys-
              tematic  corrections  along track; the latter are deactivated by
              -T.


OPTIONAL ARGUMENTS

       -A[+]c|d|f|m|tcode
              By default, corrected depth (depth), magnetic  residual  anomaly
              (mag),  free-air gravity anomaly (faa), and the derived quantity
              Carter depth correction  (carter)  are  all  output  as  is  (if
              selected  in -F); this option adjusts that behavior. For each of
              these columns there are 2-4 ways  to  adjust  the  data.  Append
              c(arter),  d(epth),  f(aa), or m(ag) and select the code for the
              procedure you want applied. You may select more than one  proce-
              dure  for  a data column by summing their numerical codes (1, 2,
              4, and 8). E.g., -Ac3 will first try method -Ac1 to  estimate  a
              Carter  correction  but  if  depth  is NaN we will next try -Ac2
              which only uses twt. In all cases, if any of the values required
              by  an  adjustment procedure is NaN then the result will be NaN.
              This is also true if the original anomaly is NaN. Specify -A+ to
              recalculate  anomalies  even  if the anomaly in the file is NaN.
              Additionally, you can use -At to create fake times  for  cruises
              that  has no time; these are based on distances and cruise dura-
              tion.

       -Ac    Determines how the carter correction term is calculated.  Below,
              C(twt) stands for the Carter-corrected depth (it also depends on
              lon, lat), U(twt, v) is the uncorrected depth (= twt *  v  /  2)
              using  as  v the aAssumed Sound Velocitya parameter in the MGD77
              header (if it is a valid velocity, otherwise we default to  1500
              m/s);  alternatively,  append  your preferred velocity v in m/s,
              TU(depth, v) is the 2-way travel time estimated from  the  (pre-
              sumably)  uncorrected  depth,  and TC(depth) is the 2-way travel
              time obtained by  inverting  the  (presumably)  corrected  depth
              using the Carter correction formula. Select from

              -Ac1[,v]   returns   difference  between  U(twt,  v)  and  depth
              [Default].

              -Ac2[,v] returns difference between U(twt, v) and Carter  (twt).

              -Ac4[,v]  returns difference between (assumed uncorrected) depth
              and Carter (TU(depth)).

              -Ac8[,v] returns difference between U(TC(depth), v) and depth.

       -Ad    Determines how the depth column output is obtained:

              -Ad1 returns depth as stored in the data set [Default].

              -Ad2[,v] returns calculated uncorrected depth U(twt, v).

              -Ad4 returns calculated corrected depth C(twt).

       -Af    Determines how the faa  column  output  is  obtained.  If  ngrav
              (i.e., the International Gravity reference Field (IGF), or anor-
              mal gravitya) is required it is  selected  based  on  the  MGD77
              header  parameter  aTheoretical  Gravity  Formula Codea; if this
              code is not present or is invalid  we  default  to  4.  Alterna-
              tively,  append the preferred field (1-4) to select 1 (Heiskanen
              1924), 2 (IGF 1930), 3 (IGF 1967) or 4 (IGF 1980). Select from

              -Af1[,field] returns faa as stored in the  data  set  [Default].
              Optionally, sets the IGF field to use if you also have requested
              ngrav as an output column in -F.

              -Af2[,field] returns the difference between gobs and ngrav (with
              optional field directive).

              -Af4[,field] returns the combination of gobs + eot - ngrav (with
              optional field directive).

              -Af8[,field] returns the combination of gobs + pred_eot -  ngrav
              (with optional field directive).

       -Am    Determines  how  the mag column output is obtained. There may be
              one or two total field measurements in the file (mtf1 and mtf2),
              and  the  column msens may state which one is the leading sensor
              (1 or 2; it may also be undefined). Select from

              -Am1 returns mag as stored in the data set [Default].

              -Am2 returns the difference between mgfx and igrf,  where  x  is
              the  leading  sensor  (1 or 2) indicated by the msens data field
              (defaults to 1 if unspecified).

              -Am4 returns the difference between mgfx and igrf,  where  x  is
              the  sensor  (2  or  1)  not  indicated  by the msens data field
              (defaults to 2 if unspecified).

              -Amcoffset[unit] applies a correction to compensate for the fact
              that the magnetic field was not acquired at the same position as
              the shipas position [i.e., the navigation]. This is accomplished
              by  re-interpolating  the  total magnetic field to what it would
              have been if it were measured at the shipas position  (remember,
              it  probably  was  measured  offset  meters behind). Due to this
              interpolation step, bad navigation,  namely  too  many  repeated
              points,  may cause trouble.  Measures are taken to minimize this
              effect but they arenat 100% fool proof. The interpolation method
              is  controlled  by the GMT default GMT_INTERPOLANT. Append e for
              meter, f for feet, k for km, M for miles, n for nautical  miles,
              or u for survey feet [Default is e (meters)].

       -Cf|g|e
              Append a one-letter code to select the procedure for along-track
              distance calculation (see -N for selecting units):

              f Flat Earth distances.

              g Great circle distances [Default].

              e Geodesic distances on current GMT ellipsoid.

       -Dastartdate
              Do    not    list    data     collected     before     startdate
              (yyyy-mm-ddT[hh:mm:ss]) [Default is start of cruise]. Use -DA to
              exclude records whose time is undefined (i.e.,  NaN).   [Default
              reports those records].

       -Dbstopdate
              Do    not   list   data   collected   on   or   after   stopdate
              (yyyy-mm-ddT[hh:mm:ss]). [Default is end of cruise]. Use -DB  to
              exclude  records  whose  time is undefined (i.e., NaN). [Default
              reports those records].

       -E     Exact match: Only output records that match  all  the  requested
              geophysical  columns  [Default  outputs  records that matches at
              least one of the observed columns].

       -Gastartrec
              Do not list records before startrec [Default  is  0,  the  first
              record].

       -Gbstoprec
              Do not list data after stoprec. [Default is the last record].

       -Iignore
              Ignore  certain  data  file  formats  from consideration. Append
              a|c|m|t to ignore MGD77 ASCII, MGD77+ netCDF, MGD77T  ASCII,  or
              plain  tab-separated ASCII table files, respectively. The option
              may be repeated to ignore more than one format. [Default ignores
              none].

       -L[corrtable]
              Apply  optimal corrections to columns where such corrections are
              available. Append the correction table to use [Default uses  the
              correction table mgd77_corrections.txt in the $MGD77_HOME direc-
              tory]. For the format of this file, see CORRECTIONS below.

       -n     Issue a segment header record with cruise ID for each cruise.

       -Nd|sunit
              Append d for distance or s for speed, then give the desired unit
              as  e  (meter  or  m/s),  f (feet or feet/s), k (km or km/hr), m
              (miles or miles/hr), n (nautical miles or knots), or  u  (survey
              feet or sfeet/s). [Default is -Ndk -Nse (km and m/s)].

       -Qamin/max
              Specify  an  accepted range (min/max) of azimuths. Records whose
              track azimuth falls outside this range are ignored [0/360].

       -Qcmin/max
              Specify an accepted range (min/max) of course  changes.  Records
              whose  track  course change falls outside this range are ignored
              [-360/+360]. Use -QC to take the absolute value  of  the  course
              change before the test [Default uses signed course changes].

       -Qvmin/max
              Specify  an  accepted range (min/max; or just min if there is no
              upper limit) of velocities. Records whose track speed falls out-
              side this range are ignored [0/infinity].

       -Rwest/east/south/north[/zmin/zmax][+r][+uunit]
              west, east, south, and north specify the region of interest, and
              you   may   specify   them   in   decimal    degrees    or    in
              [A+-]dd:mm[:ss.xxx][W|E|S|N]  format Append +r if lower left and
              upper right map coordinates are given instead  of  w/e/s/n.  The
              two  shorthands  -Rg  and -Rd stand for global domain (0/360 and
              -180/+180 in longitude respectively, with -90/+90 in  latitude).
              Alternatively  for grid creation, give Rcodelon/lat/nx/ny, where
              code is a 2-character combination of L, C, R (for left,  center,
              or  right)  and T, M, B for top, middle, or bottom. e.g., BL for
              lower left.  This indicates which point on a rectangular  region
              the lon/lat coordinate refers to, and the grid dimensions nx and
              ny with grid spacings via -I is used to create the corresponding
              region.   Alternatively,  specify  the  name of an existing grid
              file and the -R settings (and grid spacing, if  applicable)  are
              copied from the grid. Appending +uunit expects projected (Carte-
              sian) coordinates compatible with chosen  -J  and  we  inversely
              project  to determine actual rectangular geographic region.  For
              perspective view (-p), optionally append /zmin/zmax.  In case of
              perspective view (-p), a z-range (zmin, zmax) can be appended to
              indicate the third dimension. This needs to be  done  only  when
              using  the -Jz option, not when using only the -p option. In the
              latter case a perspective view of the plane is plotted, with  no
              third dimension.

       -Sastartdist[unit]
              Do  not list data that are less than startdist meter along track
              from port of departure. Append e for meter, f for  feet,  k  for
              km,  m  for  miles,  n  for nautical miles, or u for survey feet
              [Default is 0e (meters)].

       -Sbstopdist[unit]
              Do not list data that are stopdist or more  meters  along  track
              from  port  of  departure. Append e for meter, f for feet, k for
              km, m for miles, n for nautical miles,  or  u  for  survey  feet
              [Default is end of track].

       -T[m|e]
              Turns  OFF the otherwise automatic adjustment of values based on
              correction terms that are stored in the MGD77+ file and used  to
              counteract  such things as wrong units used by the source insti-
              tution when creating the original  MGD77  file  from  which  the
              MGD77+  file  derives  (the  option has no effect on plain MGD77
              ASCII files).  Append m or e to limit the option to the MGD77 or
              extended  columns set only [Default applies to both].  Note that
              record-based E77 flags (MGD77+ format only) are  not  considered
              systematic  corrections.   Instead,  the  application  of  these
              bit-flags can be  controlled  via  the  :  (colon)  modifier  to
              OPT(F).

       -V[level] (more a|)
              Select verbosity level [c].

       -Wweight
              Set  the weight for these data. Weight output option must be set
              in -F. This is useful if the data are to be processed  with  the
              weighted averaging techniques offered by blockmean, blockmedian,
              and blockmode [1].

       -Z+|-  Append the sign you want for depth, carter, and msd values below
              sea  level  (-Z- gives negative bathymetry) [Default is positive
              down].

       -bo[ncols][type] (more a|)
              Select native binary output. ignored if -bo is  selected.  Like-
              wise, string-fields cannot be selected. Note that if time is one
              of the binary output columns it  will  be  stored  as  Unix-time
              (seconds  since 1970). To read this information in GMT to obtain
              absolute calendar time will require you to use  --TIME_SYSTEM=1.

       -h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
              Skip or produce header record(s).

       -^ or just -
              Print  a  short  message  about  the syntax of the command, then
              exits (NOTE: on Windows just use -).

       -+ or just +
              Print an extensive usage (help) message, including the  explana-
              tion  of  any  module-specific  option  (but  not the GMT common
              options), then exits.

       -? or no arguments
              Print a complete usage (help) message, including the explanation
              of all options, then exits.


EXAMPLES

       To   get  a  (distance,  heading,  gravity,  bathymetry)  listing  from
       01010047.mgd77, starting at June 3 1971 20:45 and ending at distance  =
       5000 km, use the following command:

              gmt mgd77list 01010047 -Da1971-06-03T20:45 -Sb5000 -Fdist,azim,faa,depth > myfile.d

       To  make  input for blockmean and surface using free-air anomalies from
       all the cruises listed in the file cruises.lis, but only the data  that
       are inside the specified area, and make the output binary:

              gmt mgd77list `cat cruises.lis` -Flon,lat,faa -R-40/-30/25/35 -bo > allgrav.b

       To  extract  the locations of depths exceeding 9000 meter that were not
       interpolated (btc != 1)  from  all  the  cruises  listed  in  the  file
       cruises.lis:

              gmt mgd77list `cat cruises.lis` -F"depth,DEPTH>9000,BTC!=1" > really_deep.d

       To  extract dist, faa, and grav12_2 from records whose depths are shal-
       lower than 3 km and where none of the requested fields  are  NaN,  from
       all  the  MGD77+  netCDF  files whose cruise ids are listed in the file
       cruises.lis, we try

              gmt mgd77list `cat cruises.lis` -E -Ia -F"dist,faa,grav12_2,depth<3000" > \
                  shallow_grav.d

       To extract dist, faa, and grav12_2 from all  the  MGD77+  netCDF  files
       whose  cruise ids are listed in the file cruises.lis, but only retrieve
       records whose bitflag for faa indicates BAD values, we try

              gmt mgd77list `cat cruises.lis` -E -Ia -F"dist,faa,grav12_2:+faa" > bad_grav.d

       To output lon, lat, mag, and faa from all the  cruises  listed  in  the
       file cruises.lis, but recalculate the two residuals based on the latest
       reference fields, try:

              gmt mgd77list `cat cruises.lis` -Flon,lat,mag,faa -Af2,4 -Am2 > data.d


RECALCULATED ANOMALIES

       When recalculated anomalies are requested (either explicitly via the -A
       option or implicitly via E77 metadata in the MGD77+ file) we only do so
       for the records whose original anomaly was not a NaN. This  restriction
       is implemented since many anomaly columns contains corrections, usually
       in the form of hand-edited changes, that cannot be duplicated from  the
       corresponding observation.


IGRF

       The  IGRF  calculations are based on a Fortran program written by Susan
       Macmillan, British Geological  Survey,  translated  to  C  via  f2c  by
       Joaquim Luis, U Algarve, and adapted to GMT-style by Paul Wessel.


IGF

       The  equations  used  are  reproduced here using coefficients extracted
       directly from the source code (let us know if you find errors):

       (1) g = 978052.0 * [1 + 0.005285 * sin^2(lat) - 7e-6 *  sin^2(2*lat)  +
       27e-6 * cos^2(lat) * cos^2(lon-18)]

       (2)  g  =  978049.0  *  [1  +  0.0052884  *  sin^2(lat)  -  0.0000059 *
       sin^2(2*lat)]

       (3) g = 978031.846  *  [1  +  0.0053024  *  sin^2(lat)  -  0.0000058  *
       sin^2(2*lat)]

       (4) g = 978032.67714 * [(1 + 0.00193185138639 * sin^2(lat)) / sqrt (1 -
       0.00669437999013 * sin^2(lat))]


CORRECTIONS

       The correction table is an ASCII file with coefficients and  parameters
       needed  to  carry out corrections. Comment records beginning with # are
       allowed. All correction records are of the form

       cruiseID observation correction

       where cruiseID is a NGDC prefix, observation is one  of  the  abbrevia-
       tions  for  geophysical observations listed under -F above, and correc-
       tion consists of one or more terms that will be summed up and then sub-
       tracted  from  the  observation before output. Each term must have this
       exact syntax:

       factor[*[function]([scale](abbrev[-origin]))[^power]]

       where terms in brackets are optional (the brackets themselves  are  not
       used  but regular parentheses must be used as indicated). No spaces are
       allowed except between terms. The factor is the amplitude of the  basis
       function,  while  the optional function can be one of sin, cos, or exp.
       The optional scale and origin can be used  to  translate  the  argument
       (before giving it to the optional function). The argument abbrev is one
       of the abbreviations for observations listed above. If origin is  given
       as  T  it  means that we should replace it with the value of abbrev for
       the very first record in the file (this is usually only done for time).
       If  the first record entry is NaN we revert origin to zero. Optionally,
       raise the entire expression to the given power, before  multiplying  by
       the amplitude. The following is an example of fictitious corrections to
       the cruise 99999999, implying the depth should have the Carter  correc-
       tion  removed,  faa  should  have  a linear trend removed, the magnetic
       anomaly (mag) should be corrected by a strange dependency on ship head-
       ing  and latitude, and gobs needs to have 10 mGal added (hence given as
       -10):

       99999999 depth 1.0*((carter))

       99999999 faa 14.1 1e-5*((time-T))

       99999999 mag 0.5*cos(0.5*(azim-19))^2 1.0*exp(-1e-3(lat))^1.5

       99999999 gobs -10


SEE ALSO

       blockmean(1), blockmedian(1), blockmode(1), mgd77convert(1),
       mgd77info(1), mgd77manage(1), mgd77track(1), surface(1)


REFERENCES

       The   Marine   Geophysical   Data   Exchange   Format   -   MGD77,  see
       http://www.ngdc.noaa.gov/mgg/dat/geodas/docs/mgd77.txt

       IGRF, see http://www.ngdc.noaa.gov/IAGA/vmod/igrf/


COPYRIGHT

       2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe



5.4.2                            Jun 24, 2017                     mgd77list(1)

gmt5 5.4.2 - Generated Thu Jun 29 14:30:27 CDT 2017
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