NGDC-ids

Can be one or more of five kinds of specifiers:

(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+ (".nc"), MGD77T (".m77t"), MGD77 (".mgd77" ) and plain text file (".dat"). 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:

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

Finally, the following computed quantities can be requested:

The following short-hand flags are also recognized:

As an option, logical tests may be added for any of the observations 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 operator (<, <=, =, !=, >=, >, |), and value is a constant used in the comparison. Floating point parameters are compared numerically; character parameters are compared lexically (after leading 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 characters 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 observations 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 systematic corrections along track; the latter are deactivated by -T.

-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 procedure 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 duration.

-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 "Assumed Sound Velocity" 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 (presumably) 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 "normal gravity") is required it is selected based on the MGD77 header parameter "Theoretical Gravity Formula Code"; if this code is not present or is invalid we default to 4. Alternatively, 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 ship's 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 ship's 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 aren't 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 directory]. 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 outside 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 [±]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 (Cartesian) 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 institution 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 ...)

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 ...)

Select native binary output. ignored if -bo is selected. Likewise, 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 ...)

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 explanation 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.