1MGD77LIST(1) GMT MGD77LIST(1)
2
6 mgd77list - Extract data from MGD77 files
7
9 mgd77list NGDC-ids -Fcolumns[,logic][:bittests] [ -A[+]c|d|f|m|tcode
10 ] [ -Cf|g|e ] [ -DA|astartdate ] [ -DB|bstopdate ] [ -E ] [ -Gas‐
11 tartrec ] [ -Gbstoprec ] [ -Iignore ] [ -L[corrtable] ] [ -Nd|sunit
12 ] [ -Qa|c|vmin/max ] [ -Rregion ] [ -Sastartdist[unit] ] [
13 -Sbstopdist[unit] ] [ -T[m|e] ] [ -V[level] ] [ -Wweight ] [ -Z+|-
14 ] [ -bobinary ] [ -hheaders ]
15 Note: No space is allowed between the option flag and the associated
17 arguments.
18
20 mgd77list reads MGD77 files and produces an ASCII [or binary] table.
21 The MGD77 files contain track information such as leg-id, time and
22 position, geophysical observables such as gravity, magnetics, and ba‐
23 thymetry, and control codes and corrections such as Eotvos and diurnal
24 corrections. The MGD77+ extended netCDF files may also contain addi‐
25 tional user columns (for a listing of available columns, use mgd77info
26 -C, and to learn how to add your own custom columns, see mgd77manage).
27 The user may extract any combination of these parameters, any of 8 com‐
28 puted quantities (distance, heading, course-change, velocity, Carter
29 correction, Eotvos correction and gravity and magnetic global reference
30 fields), calendar sub-units of time (year, month, day, hour, min, sec),
31 the NGDC id, and finally a preset weight (see -W). A sub-section can be
32 specified by passing time- or distance-intervals along track or by
33 selecting a geographical region. Finally, each output record may be
34 required to pass any number of logical tests involving data values or
35 bit flags. If multiple cruises are requested then they are separated by
36 segment headers.
37
39 NGDC-ids
40 Can be one or more of five kinds of specifiers:
41 1. 8-character NGDC IDs, e.g., 01010083, JA010010etc., etc.
43 2. 2-character agency codes which will return all cruises from
45 each agency.
46 3. 4-character <agency><vessel> codes, which will return all
48 cruises from those vessels.
49 4. =list, where list is a table with NGDC IDs, one per line.
51 5. If nothing is specified we return all cruises in the data
53 base.
54 (See mgd77info -L for agency and vessel codes). If no file
56 extension is given then we search for files with one of the four
57 known extensions. The search order (and the extensions) tried
58 is MGD77+ (".nc"), MGD77T (".m77t"), MGD77 (".mgd77" ) and plain
59 text file (".dat"). Use -I to ignore one or more of these file
60 types). Cruise files will be looked for first in the current
61 directory and second in all directories listed in
62 $MGD77_HOME/mgd77_paths.txt [If $MGD77_HOME is not set it will
63 default to $GMT_SHAREDIR/mgd77].
64 -Fcolumns[,logic][:bittests]
66 The required columns string must be a comma-separated list of
67 parameter abbreviations given in the desired output order. Any
68 parameters given in UPPER case must not be NaN in a record for
69 output to occur. Unless specified separately, the output format
70 (if ASCII) is controlled by the GMT parameter FORMAT_FLOAT_OUT.
71 The available column abbreviations for information stored in the
72 files (some columns may be NaN) are:
73 recno The record number counter.
75 drt The digital record type, usually 3 or 5 (for Y2K-compli‐
77 ant cruises).
78 id The survey ID string (leg name).
80 ngdcid The 8-character NGDC cruise ID string (usually the file
82 prefix).
83 time Choose between Absolute calendar time (atime, the
85 default) in the format dictated by the GMT parameters
86 FORMAT_DATE_OUT and FORMAT_CLOCK_OUT, Relative time
87 (rtime) in the format dictated by the GMT parameters FOR‐
88 MAT_FLOAT_OUT and TIME_SYSTEM (or TIME_EPOCH and
89 TIME_UNIT)), or Fractional year (ytime) in the format
90 dictated by FORMAT_FLOAT_OUT.
91 lon Longitude in the format dictated by the GMT parameter
93 FORMAT_GEO_OUT.
94 lat Longitude in the format dictated by the GMT parameter
96 FORMAT_GEO_OUT.
97 twt Two-Way Travel time (in s).
99 depth Corrected bathymetry (in m, positive below sea level).
101 mtf1 Magnetic Total Field intensity from sensor 1 (in nTesla).
103 mtf2 Magnetic Total Field intensity from sensor 2 (in nTesla).
105 mag Residual magnetic anomaly (in nTesla).
107 gobs Observed gravity (in mGal).
109 faa Free-air gravity anomaly (in mGal).
111 ptc Position Type Code (1 = fix, 3 = interpolated, 9 =
113 unspecified).
114 bcc Bathymetric Correction Code, indicating the procedure
116 used to convert travel time to depth. (01-55 = Matthews'
117 zone used to correct the depth, 59 = Matthews' correc‐
118 tions used but the zones is unspecified in the data
119 record, 60 = S. Kuwahara formula for T-S, 61 = Wilson
120 formula for T-S, 62 = Del Grosso formula for T-S, 63 =
121 Carter's tables, 88 = Other, described in header sec‐
122 tions, 99 = unspecified).
123 btc Bathymetric Type Code, indicating how the bathymetry
125 value was obtained (1 = observed, 3 = interpolated, 9 =
126 unspecified).
127 msens Magnetic sensor for used to evaluate the residual field
129 (1 = 1st or leading sensor, 2 = 2nd or trailing sensor, 9
130 = unspecified).
131 msd Depth (or altitude) of the magnetic sensor (in m, posi‐
133 tive below sealevel).
134 diur Magnetic diurnal correction (in nTesla).
136 eot Eotvos correction (in mGal).
138 sln Seismic Line Number string.
140 sspn Seismic Shot Point Number string.
142 nqc Navigation Quality Code (5 = suspected, by source insti‐
144 tution, 6 = suspected, by NGDC, 9 = no problems identi‐
145 fied).
146 In addition, the following derived navigational quantities can
148 be requested:
149 year The year of each record.
151 month The month of each record.
153 day The day of the month of each record.
155 hour The hour of each record.
157 min The minutes of each record.
159 sec The decimal seconds of each record.
161 dist Along-track distance from start of leg. For method of
163 calculation, see -C [spherical great circle distances],
164 and for distance units, see -N [km].
165 az Ship azimuth (heading) measured clockwise from north (in
167 degrees).
168 cc Ship course change (change in heading) measured clockwise
170 from north (in degrees).
171 vel Ship speed; see -N for units [m/s].
173 Finally, the following computed quantities can be requested:
175 weight Weight assigned to this data set (see -W).
177 carter Carter depth correction, if twt is present in file (in
179 m). Sign: Correction is to be added to uncorrected depths
180 to yield a corrected depth.
181 igrf International geomagnetic reference field (total field)
183 (in nTesla).
184 ngrav International Gravity reference Field ("normal gravity")
186 (in mGal). Field is selected based on the parameter
187 Gravity Theoretical Formula Code in the cruise's MGD77
188 header. If this is not set or is invalid we default to
189 the IGF 1980. Alternatively, specify the field directly
190 using -Af (see that option for more details).
191 ceot Calculated Eotvos correction from navigation, using E =
193 7.5038 * V * cos(lat) * sin(az) + 0.004154 * V^2 (in
194 mGal). Sign: Correction is to be added to uncorrected faa
195 to yield a corrected faa.
196 The following short-hand flags are also recognized:
198 mgd77 This results in all 27 MGD77 fields being written out in
200 the official MGD77 order.
201 mgd77t The full set of all 26 columns in the MGD77T specifica‐
203 tion.
204 geo This limits the output to 10 fields (time, lon, lat plus
206 the seven geophysical observations twt, depth, mtf1,
207 mtf2, mag, gobs, and faa). By appending + to either of
208 these set we will also append dist, azim, cc, vel, and
209 weight as listed above.
210 all This returns all data columns in the file.
212 allt As mgd77t but with time items written as a date-time
214 string.
215 As an option, logical tests may be added for any of the observa‐
217 tions by appending ,logic, which is itself composed of one or
218 more comma-separated instructions of the form parOPvalue, where
219 par is one of the parameters listed above, OP is a logical oper‐
220 ator (<, <=, =, !=, >=, >, |), and value is a constant used in
221 the comparison. Floating point parameters are compared numeri‐
222 cally; character parameters are compared lexically (after lead‐
223 ing and trailing blanks have been removed). The bit comparison
224 (|) means that at least one of the bits in value must be turned
225 on in par. At least one of the tests must be true for the record
226 to be output, except for tests using UPPER case parameters which
227 all must be true for output to occur. Notes: (1) Specifying a
228 test does not imply that the corresponding column will be
229 included in the output stream; it must be present in columns for
230 that to occur. (2) Some of the operators are special UNIX char‐
231 acters and you are advised to place quotes around the entire
232 argument to -F. (3) The logical tests only apply to observed
233 data; derived data (such as distances, velocities, etc.) must be
234 limited using program options such as -D, -Q, -S, etc.
235 Finally, for MGD77+ files you may optionally append :bittests
237 which is : (a colon) followed by one or more comma-separated
238 +-col terms. This compares specific bitflags only for each
239 listed column. Here, + means the chosen bit must be 1 (ON)
240 whereas - means it must be 0 (OFF). All bit tests given must be
241 passed. By default, MGD77+ files that have the special
242 MGD77_flags column present will use those flags, and observa‐
243 tions associated with ON-bits (meaning they are flagged as bad)
244 will be set to NaN; append : with no trailing information to
245 turn this behavior off (i.e., no bit flags will be consulted).
246 Note that these record-based flags are different from any sys‐
247 tematic corrections along track; the latter are deactivated by
248 -T.
249
251 -A[+]c|d|f|m|tcode
252 By default, corrected depth (depth), magnetic residual anomaly
253 (mag), free-air gravity anomaly (faa), and the derived quantity
254 Carter depth correction (carter) are all output as is (if
255 selected in -F); this option adjusts that behavior. For each of
256 these columns there are 2-4 ways to adjust the data. Append
257 c(arter), d(epth), f(aa), or m(ag) and select the code for the
258 procedure you want applied. You may select more than one proce‐
259 dure for a data column by summing their numerical codes (1, 2,
260 4, and 8). E.g., -Ac3 will first try method -Ac1 to estimate a
261 Carter correction but if depth is NaN we will next try -Ac2
262 which only uses twt. In all cases, if any of the values required
263 by an adjustment procedure is NaN then the result will be NaN.
264 This is also true if the original anomaly is NaN. Specify -A+ to
265 recalculate anomalies even if the anomaly in the file is NaN.
266 Additionally, you can use -At to create fake times for cruises
267 that has no time; these are based on distances and cruise dura‐
268 tion.
269 -Ac Determines how the carter correction term is calculated. Below,
271 C(twt) stands for the Carter-corrected depth (it also depends on
272 lon, lat), U(twt, v) is the uncorrected depth (= twt * v / 2)
273 using as v the "Assumed Sound Velocity" parameter in the MGD77
274 header (if it is a valid velocity, otherwise we default to 1500
275 m/s); alternatively, append your preferred velocity v in m/s,
276 TU(depth, v) is the 2-way travel time estimated from the (pre‐
277 sumably) uncorrected depth, and TC(depth) is the 2-way travel
278 time obtained by inverting the (presumably) corrected depth
279 using the Carter correction formula. Select from
280 -Ac1[,v] returns difference between U(twt, v) and depth
282 [Default].
283 -Ac2[,v] returns difference between U(twt, v) and Carter (twt).
285 -Ac4[,v] returns difference between (assumed uncorrected) depth
287 and Carter (TU(depth)).
288 -Ac8[,v] returns difference between U(TC(depth), v) and depth.
290 -Ad Determines how the depth column output is obtained:
292 -Ad1 returns depth as stored in the data set [Default].
294 -Ad2[,v] returns calculated uncorrected depth U(twt, v).
296 -Ad4 returns calculated corrected depth C(twt).
298 -Af Determines how the faa column output is obtained. If ngrav
300 (i.e., the International Gravity reference Field (IGF), or "nor‐
301 mal gravity") is required it is selected based on the MGD77
302 header parameter "Theoretical Gravity Formula Code"; if this
303 code is not present or is invalid we default to 4. Alterna‐
304 tively, append the preferred field (1-4) to select 1 (Heiskanen
305 1924), 2 (IGF 1930), 3 (IGF 1967) or 4 (IGF 1980). Select from
306 -Af1[,field] returns faa as stored in the data set [Default].
308 Optionally, sets the IGF field to use if you also have requested
309 ngrav as an output column in -F.
310 -Af2[,field] returns the difference between gobs and ngrav (with
312 optional field directive).
313 -Af4[,field] returns the combination of gobs + eot - ngrav (with
315 optional field directive).
316 -Af8[,field] returns the combination of gobs + pred_eot - ngrav
318 (with optional field directive).
319 -Am Determines how the mag column output is obtained. There may be
321 one or two total field measurements in the file (mtf1 and mtf2),
322 and the column msens may state which one is the leading sensor
323 (1 or 2; it may also be undefined). Select from
324 -Am1 returns mag as stored in the data set [Default].
326 -Am2 returns the difference between mgfx and igrf, where x is
328 the leading sensor (1 or 2) indicated by the msens data field
329 (defaults to 1 if unspecified).
330 -Am4 returns the difference between mgfx and igrf, where x is
332 the sensor (2 or 1) not indicated by the msens data field
333 (defaults to 2 if unspecified).
334 -Amcoffset[unit] applies a correction to compensate for the fact
336 that the magnetic field was not acquired at the same position as
337 the ship's position [i.e., the navigation]. This is accomplished
338 by re-interpolating the total magnetic field to what it would
339 have been if it were measured at the ship's position (remember,
340 it probably was measured offset meters behind). Due to this
341 interpolation step, bad navigation, namely too many repeated
342 points, may cause trouble. Measures are taken to minimize this
343 effect but they aren't 100% fool proof. The interpolation method
344 is controlled by the GMT default GMT_INTERPOLANT. Append e for
345 meter, f for feet, k for km, M for miles, n for nautical miles,
346 or u for survey feet [Default is e (meters)].
347 -Cf|g|e
349 Append a one-letter code to select the procedure for along-track
350 distance calculation (see -N for selecting units):
351 f Flat Earth distances.
353 g Great circle distances [Default].
355 e Geodesic distances on current GMT ellipsoid.
357 -Dastartdate
359 Do not list data collected before startdate
360 (yyyy-mm-ddT[hh:mm:ss]) [Default is start of cruise]. Use -DA to
361 exclude records whose time is undefined (i.e., NaN). [Default
362 reports those records].
363 -Dbstopdate
365 Do not list data collected on or after stopdate
366 (yyyy-mm-ddT[hh:mm:ss]). [Default is end of cruise]. Use -DB to
367 exclude records whose time is undefined (i.e., NaN). [Default
368 reports those records].
369 -E Exact match: Only output records that match all the requested
371 geophysical columns [Default outputs records that matches at
372 least one of the observed columns].
373 -Gastartrec
375 Do not list records before startrec [Default is 0, the first
376 record].
377 -Gbstoprec
379 Do not list data after stoprec. [Default is the last record].
380 -Iignore
382 Ignore certain data file formats from consideration. Append
383 a|c|m|t to ignore MGD77 ASCII, MGD77+ netCDF, MGD77T ASCII, or
384 plain tab-separated ASCII table files, respectively. The option
385 may be repeated to ignore more than one format. [Default ignores
386 none].
387 -L[corrtable]
389 Apply optimal corrections to columns where such corrections are
390 available. Append the correction table to use [Default uses the
391 correction table mgd77_corrections.txt in the $MGD77_HOME direc‐
392 tory]. For the format of this file, see CORRECTIONS below.
393 -n Issue a segment header record with cruise ID for each cruise.
395 -Nd|sunit
397 Append d for distance or s for speed, then give the desired unit
398 as e (meter or m/s), f (feet or feet/s), k (km or km/hr), m
399 (miles or miles/hr), n (nautical miles or knots), or u (survey
400 feet or sfeet/s). [Default is -Ndk -Nse (km and m/s)].
401 -Qamin/max
403 Specify an accepted range (min/max) of azimuths. Records whose
404 track azimuth falls outside this range are ignored [0/360].
405 -Qcmin/max
407 Specify an accepted range (min/max) of course changes. Records
408 whose track course change falls outside this range are ignored
409 [-360/+360]. Use -QC to take the absolute value of the course
410 change before the test [Default uses signed course changes].
411 -Qvmin/max
413 Specify an accepted range (min/max; or just min if there is no
414 upper limit) of velocities. Records whose track speed falls out‐
415 side this range are ignored [0/infinity].
416 -Rwest/east/south/north[/zmin/zmax][+r][+uunit]
418 west, east, south, and north specify the region of interest, and
419 you may specify them in decimal degrees or in
420 [±]dd:mm[:ss.xxx][W|E|S|N] format Append +r if lower left and
421 upper right map coordinates are given instead of w/e/s/n. The
422 two shorthands -Rg and -Rd stand for global domain (0/360 and
423 -180/+180 in longitude respectively, with -90/+90 in latitude).
424 Alternatively for grid creation, give Rcodelon/lat/nx/ny, where
425 code is a 2-character combination of L, C, R (for left, center,
426 or right) and T, M, B for top, middle, or bottom. e.g., BL for
427 lower left. This indicates which point on a rectangular region
428 the lon/lat coordinate refers to, and the grid dimensions nx and
429 ny with grid spacings via -I is used to create the corresponding
430 region. Alternatively, specify the name of an existing grid
431 file and the -R settings (and grid spacing, if applicable) are
432 copied from the grid. Appending +uunit expects projected (Carte‐
433 sian) coordinates compatible with chosen -J and we inversely
434 project to determine actual rectangular geographic region. For
435 perspective view (-p), optionally append /zmin/zmax. In case of
436 perspective view (-p), a z-range (zmin, zmax) can be appended to
437 indicate the third dimension. This needs to be done only when
438 using the -Jz option, not when using only the -p option. In the
439 latter case a perspective view of the plane is plotted, with no
440 third dimension.
441 -Sastartdist[unit]
443 Do not list data that are less than startdist meter along track
444 from port of departure. Append e for meter, f for feet, k for
445 km, m for miles, n for nautical miles, or u for survey feet
446 [Default is 0e (meters)].
447 -Sbstopdist[unit]
449 Do not list data that are stopdist or more meters along track
450 from port of departure. Append e for meter, f for feet, k for
451 km, m for miles, n for nautical miles, or u for survey feet
452 [Default is end of track].
453 -T[m|e]
455 Turns OFF the otherwise automatic adjustment of values based on
456 correction terms that are stored in the MGD77+ file and used to
457 counteract such things as wrong units used by the source insti‐
458 tution when creating the original MGD77 file from which the
459 MGD77+ file derives (the option has no effect on plain MGD77
460 ASCII files). Append m or e to limit the option to the MGD77 or
461 extended columns set only [Default applies to both]. Note that
462 record-based E77 flags (MGD77+ format only) are not considered
463 systematic corrections. Instead, the application of these
464 bit-flags can be controlled via the : (colon) modifier to
465 OPT(F).
466 -V[level] (more ...)
468 Select verbosity level [c].
469 -Wweight
471 Set the weight for these data. Weight output option must be set
472 in -F. This is useful if the data are to be processed with the
473 weighted averaging techniques offered by blockmean, blockmedian,
474 and blockmode [1].
475 -Z+|- Append the sign you want for depth, carter, and msd values below
477 sea level (-Z- gives negative bathymetry) [Default is positive
478 down].
479 -bo[ncols][type] (more ...)
481 Select native binary output. ignored if -bo is selected. Like‐
482 wise, string-fields cannot be selected. Note that if time is one
483 of the binary output columns it will be stored as Unix-time
484 (seconds since 1970). To read this information in GMT to obtain
485 absolute calendar time will require you to use --TIME_SYSTEM=1.
486 -h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
488 Skip or produce header record(s).
489 -^ or just -
491 Print a short message about the syntax of the command, then
492 exits (NOTE: on Windows just use -).
493 -+ or just +
495 Print an extensive usage (help) message, including the explana‐
496 tion of any module-specific option (but not the GMT common
497 options), then exits.
498 -? or no arguments
500 Print a complete usage (help) message, including the explanation
501 of all options, then exits.
502
504 To get a (distance, heading, gravity, bathymetry) listing from
505 01010047.mgd77, starting at June 3 1971 20:45 and ending at distance =
506 5000 km, use the following command:
507 gmt mgd77list 01010047 -Da1971-06-03T20:45 -Sb5000 -Fdist,azim,faa,depth > myfile.d
509 To make input for blockmean and surface using free-air anomalies from
511 all the cruises listed in the file cruises.lis, but only the data that
512 are inside the specified area, and make the output binary:
513 gmt mgd77list `cat cruises.lis` -Flon,lat,faa -R-40/-30/25/35 -bo > allgrav.b
515 To extract the locations of depths exceeding 9000 meter that were not
517 interpolated (btc != 1) from all the cruises listed in the file
518 cruises.lis:
519 gmt mgd77list `cat cruises.lis` -F"depth,DEPTH>9000,BTC!=1" > really_deep.d
521 To extract dist, faa, and grav12_2 from records whose depths are shal‐
523 lower than 3 km and where none of the requested fields are NaN, from
524 all the MGD77+ netCDF files whose cruise ids are listed in the file
525 cruises.lis, we try
526 gmt mgd77list `cat cruises.lis` -E -Ia -F"dist,faa,grav12_2,depth<3000" > \
528 shallow_grav.d
529 To extract dist, faa, and grav12_2 from all the MGD77+ netCDF files
531 whose cruise ids are listed in the file cruises.lis, but only retrieve
532 records whose bitflag for faa indicates BAD values, we try
533 gmt mgd77list `cat cruises.lis` -E -Ia -F"dist,faa,grav12_2:+faa" > bad_grav.d
535 To output lon, lat, mag, and faa from all the cruises listed in the
537 file cruises.lis, but recalculate the two residuals based on the latest
538 reference fields, try:
539 gmt mgd77list `cat cruises.lis` -Flon,lat,mag,faa -Af2,4 -Am2 > data.d
541
543 When recalculated anomalies are requested (either explicitly via the -A
544 option or implicitly via E77 metadata in the MGD77+ file) we only do so
545 for the records whose original anomaly was not a NaN. This restriction
546 is implemented since many anomaly columns contains corrections, usually
547 in the form of hand-edited changes, that cannot be duplicated from the
548 corresponding observation.
549
551 The IGRF calculations are based on a Fortran program written by Susan
552 Macmillan, British Geological Survey, translated to C via f2c by
553 Joaquim Luis, U Algarve, and adapted to GMT-style by Paul Wessel.
554
556 The equations used are reproduced here using coefficients extracted
557 directly from the source code (let us know if you find errors):
558 (1) g = 978052.0 * [1 + 0.005285 * sin^2(lat) - 7e-6 * sin^2(2*lat) +
560 27e-6 * cos^2(lat) * cos^2(lon-18)]
561 (2) g = 978049.0 * [1 + 0.0052884 * sin^2(lat) - 0.0000059 *
563 sin^2(2*lat)]
564 (3) g = 978031.846 * [1 + 0.0053024 * sin^2(lat) - 0.0000058 *
566 sin^2(2*lat)]
567 (4) g = 978032.67714 * [(1 + 0.00193185138639 * sin^2(lat)) / sqrt (1 -
569 0.00669437999013 * sin^2(lat))]
570
572 The correction table is an ASCII file with coefficients and parameters
573 needed to carry out corrections. Comment records beginning with # are
574 allowed. All correction records are of the form
575 cruiseID observation correction
577 where cruiseID is a NGDC prefix, observation is one of the abbrevia‐
579 tions for geophysical observations listed under -F above, and correc‐
580 tion consists of one or more terms that will be summed up and then sub‐
581 tracted from the observation before output. Each term must have this
582 exact syntax:
583 factor[*[function]([scale](abbrev[-origin]))[^power]]
585 where terms in brackets are optional (the brackets themselves are not
587 used but regular parentheses must be used as indicated). No spaces are
588 allowed except between terms. The factor is the amplitude of the basis
589 function, while the optional function can be one of sin, cos, or exp.
590 The optional scale and origin can be used to translate the argument
591 (before giving it to the optional function). The argument abbrev is one
592 of the abbreviations for observations listed above. If origin is given
593 as T it means that we should replace it with the value of abbrev for
594 the very first record in the file (this is usually only done for time).
595 If the first record entry is NaN we revert origin to zero. Optionally,
596 raise the entire expression to the given power, before multiplying by
597 the amplitude. The following is an example of fictitious corrections to
598 the cruise 99999999, implying the depth should have the Carter correc‐
599 tion removed, faa should have a linear trend removed, the magnetic
600 anomaly (mag) should be corrected by a strange dependency on ship head‐
601 ing and latitude, and gobs needs to have 10 mGal added (hence given as
602 -10):
603 99999999 depth 1.0*((carter))
605 99999999 faa 14.1 1e-5*((time-T))
607 99999999 mag 0.5*cos(0.5*(azim-19))^2 1.0*exp(-1e-3(lat))^1.5
609 99999999 gobs -10
611
613 blockmean, blockmedian, blockmode, mgd77convert, mgd77info, mgd77man‐
614 age, mgd77track, surface,
615
617 The Marine Geophysical Data Exchange Format - MGD77, see
618 http://www.ngdc.noaa.gov/mgg/dat/geodas/docs/mgd77.txt
619 IGRF, see http://www.ngdc.noaa.gov/IAGA/vmod/igrf/
621
623 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
6245.4.5 Feb 24, 2019 MGD77LIST(1)