# 7. GMT Supplemental Packages

These packages are for the most part written and supported by us, but there are some exceptions. They provide extensions of GMT that are needed for particular rather than general applications. Questions or bug reports for this software should be addressed to the person(s) listed in the `README` file associated with the particular program. It is not guaranteed that these programs are fully ANSI-C, Y2K, or POSIX compliant, or that they necessarily will install smoothly on all platforms, but most do. Note that the data sets some of these programs work on are not distributed with these packages; they must be obtained separately. The contents of the supplemental archive may change without notice; at this writing it contains these directories:

## 7.1. gshhg: GSHHG data extractor

This package contains gshhg which you can use to extract shoreline polygons from the Global Self-consistent Hierarchical High-resolution Shorelines (GSHHG) available separately from or the (GSHHG is the polygon data base from which the GMT coastlines derive). The package is maintained by Paul Wessel.

## 7.2. gsfml: Global seafloor fabric and magnetic lineations analysis

This package contains three modules and five bash scripts used to model the vertical gravity gradient signal across oceanic fracture zones. The various tools allow modeling of the signal as a blend of the typical “Atlantic”-FZ and “Pacific-FZ” shapes, modeled by Gaussian-derived building blocks [29]. The package is maintained by Paul Wessel.

## 7.3. img: gridded altimetry extractor

This package consists of the program img2grd to extract subsets of the global gravity and predicted topography solutions derived from satellite altimetry [23]. The package is maintained by Walter Smith and Paul Wessel.

## 7.4. geodesy: Geodesy

This package contains the programs earthtide for computing solid Earth tides, gpsgridder for gridding GPS velocity components, and velo for plotting error ellipses, velocity arrows, and rotational wedges. The velo program was developed by Kurt Feigl and Genevieve Patau and is now maintained by the GMT team. The earthtide program is a translation to C of a program written by Dennis Milbert and distributed with his permission.

## 7.5. mgd77: MGD77 extractor and plotting tools

This package currently holds the programs mgd77convert, mgd77header, mgd77info, mgd77list, mgd77magref, mgd77manage, mgd77path, mgd77sniffer, and mgd77track which can be used to extract information or data values from or plot marine geophysical data files in the ASCII MGD77 or netCDF MGD77+ formats [24]). This package has replaced the old mgg package. The package is maintained by Paul Wessel and Mike Chandler.

## 7.6. potential: Geopotential tools

At the moment, this package contains the programs gravfft, which performs gravity, isostasy, and admittance calculation for grids, flexure and grdflexure which calculates flexural deformation for profiles and grids, respectively, grdredpol, which compute the continuous reduction to the pole, AKA differential RTP for magnetic data, grdseamount, which computes synthetic bathymetry over various seamount shapes, and gravmag3d and grdgravmag3d, which computes the gravity or magnetic anomaly of a body by the method of Okabe [25], and talwani2d and talwani3d and which uses the methods of Talwani to compute various geopotential components from 2-D [26] or 3-D [27] bodies. The package is maintained by Joaquim Luis and Paul Wessel.

## 7.7. seis: Seismology

This package contains the programs coupe, meca, polar, sac, grdvs30, and grdshake, which are used by seismologists for plotting focal mechanisms (including cross-sections and polarities), compute Vs30 velocities, intensity maps and SAC files. The coupe, meca, and polar were developed by Kurt Feigl and Genevieve Patau, while Dongdong Tian added sac; the package is now maintained by the GMT team.

## 7.8. segy: plotting SEGY seismic data

This package contains programs to plot SEGY seismic data files using the GMT mapping transformations and postscript library. segy generates a 2-D plot (x:location and y:time/depth) while segyz generates a 3-D plot (x and y: location coordinates, z: time/depth). Locations may be read from predefined or arbitrary portions of each trace header. Finally, segy2grd can convert SEGY data to a GMT grid file. The package is maintained by Tim Henstock [28].

## 7.9. spotter: backtracking and hotspotting

This package contains the plate tectonic programs backtracker, which you can use to move geologic markers forward or backward in time, grdpmodeler which evaluates predictions of a plate motion model on a grid, grdrotater which rotates entire grids using a finite rotation, hotspotter which generates CVA grids based on seamount locations and a set of absolute plate motion stage poles (grdspotter does the same using a bathymetry grid instead of seamount locations), originater, which associates seamounts with the most likely hotspot origins, polespotter, which determines likely stage pole locations from seafloor fabric, and rotconverter which does various operations involving finite rotations on a sphere. The package is maintained by Paul Wessel.

## 7.10. x2sys: track crossover error estimation

This package contains the tools x2sys_datalist, which allows you to extract data from almost any binary or ASCII data file, and x2sys_cross which determines crossover locations and errors generated by one or several geospatial tracks. Newly added are the tools x2sys_init, x2sys_binlist, x2sys_get, x2sys_list, x2sys_put, x2sys_report, x2sys_solve and x2sys_merge which extends the track-management system employed by the mgg supplement to generic track data of any format. This package represents a new generation of tools and replaces the old x_system package. The package is maintained by Paul Wessel.

## 7.11. Footnotes

[ 23 ]

For data bases, see http://topex.ucsd.edu/marine_grav/mar_grav.html.

[ 24 ]

The ASCII MGD77 data are available on CD-ROM from NCEI (http://www.ncei.noaa.gov/).

[ 25 ]

Okabe, M., 1979, Analytical expressions for gravity anomalies due to polyhedral bodies and translation into magnetic anomalies, Geophysics, 44, 730–741.

[ 26 ]

Talwani, M., J. L. Worzel, and M. Landisman (1959), Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone, J. Geophys. Res., 64, 49–59.

[ 27 ]

Talwani, M., and M. Ewing (1960), Rapid computation of gravitational attraction of three-dimensional bodies of arbitrary shape, Geophysics, 25, 203–225.

[ 28 ]

Timothy J. Henstock, University of Southampton

[29]

Wessel, P., Matthews, K. J., Müller, R. D., Mazzoni, A., Whittaker, J. M., Myhill, R., Chandler, M. T., 2015, “Semiautomatic fracture zone tracking”, Geochem. Geophys. Geosyst., 16 (7), 2462–2472. https://doi.org/10.1002/2015GC005853.