Evaluate a plate motion model on a geographic grid
Note: No space is allowed between the option flag and the associated arguments.
grdpmodeler reads a geographical age grid and a plate motion model and evaluates one of several model predictions. Optionally, the user may supply a clipping polygon in multiple-segment format; then, only the part of the grid inside the polygon is used to determine the model prediction; the remainder of the grid is set to NaN.
Rotations can be specified in one of three ways: (1): Give file with rotation parameters. This file must contain one record for each rotation; each record must be of the following format:
lon lat tstart [tstop] angle [ khat a b c d e f g df ]
where tstart and tstop are in Myr and lon lat angle are in degrees. tstart and tstop are the ages of the old and young ends of a stage. If tstop is not present in the record then a total reconstruction rotation is expected and tstop is implicitly set to 0 and should not be specified for any of the records in the file. If a covariance matrix C for the rotation is available it must be specified in a format using the nine optional terms listed in brackets. Here, C = (g/khat)*[ a b d; b c e; d e f ] which shows C made up of three row vectors. If the degrees of freedom (df) in fitting the rotation is 0 or not given it is set to 10000. Blank lines and records whose first column contains # will be ignored. You may append +i to the filename to indicate you wish to invert the rotations. (2): Give the filename composed of two plate IDs separated by a hyphen (e.g., PAC-MBL) and we will instead extract that rotation from the GPlates rotation database. We return an error if the rotation cannot be found. (3): Specify lon/lat/angle, i.e., the longitude, latitude, and opening angle (all in degrees and separated by /) for a single total reconstruction rotation.
- Type of model prediction(s). Append one or more items: choose from a for plate motion azimuth, d for great-circle distance between current location and its origin at the ridge (in km), s for plate motion model stage ID (1 is youngest), v for plate motion rate (in mm/yr), w for plate rotation rate (degree/Myr), x for change in longitude relative to location of crust formation, y for change in latitude relative to location of crust formation, X for longitude of crust formation, and Y for latitude of crust formation. If no arguments are given we default to all [adsvwxyXY].
- Name of a grid file in geographical (lon, lat) coordinates with ages in Myr. If no grid is provided then you may define the domain via -R, -I, and optionally -r.
- Specify a multisegment closed polygon file that describes the inside area of the grid where the model should be evaluated; the outside will be set to NaN [Default evaluates model on the entire grid].
- Name of output grid. This is the grid with the model predictions given the specified rotations. Note: If you specified more than one model prediction in -S then the filename must be a template that contains the format %s; this will be replaced with the corresponding tags az, dist, stage, vel, omega, dlon, dlat, lon, lat. If the -G option is not used then we create no grids and instead write lon, lat, age, predictions records to standard output.
- Use a fixed age for model evaluation (i.e., override the ages in the age grid). This lets you evaluate the model at a snapshot in time. Required if no age grid was provided.
- -V[level] (more …)
- Select verbosity level [w].
- -bi[ncols][t] (more …)
- Select native binary format for primary input. [Default is 2 input columns].
- -d[i|o]nodata (more …)
- Replace input columns that equal nodata with NaN and do the reverse on output.
- -h[i|o][n][+c][+d][+msegheader][+rremark][+ttitle] (more …)
- Skip or produce header record(s).
- -^ or just -
- Print a short message about the syntax of the command, then exit (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 exit.
- -? or no arguments
- Print a complete usage (help) message, including the explanation of all options, then exit.
- Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.
Geodetic versus Geocentric Coordinates¶
All spherical rotations are applied to geocentric coordinates. This means that incoming data points and grids are considered to represent geodetic coordinates and must first be converted to geocentric coordinates. Rotations are then applied, and the final reconstructed points are converted back to geodetic coordinates. This default behavior can be bypassed if the ellipsoid setting PROJ_ELLIPSOID is changed to Sphere.
To determine if a point is inside, outside, or exactly on the boundary of a polygon we need to balance the complexity (and execution time) of the algorithm with the type of data and shape of the polygons. For any Cartesian data we use a non-zero winding algorithm, which is quite fast. For geographic data we will also use this algorithm as long as (1) the polygons do not include a geographic pole, and (2) the longitude extent of the polygons is less than 360. If this is the situation we also carefully adjust the test point longitude for any 360 degree offsets, if appropriate. Otherwise, we employ a full spherical ray-shooting method to determine a points status.
We will use a grid with Pacific crust ages (pac_age.nc), a plate motion model (Pac_APM.txt), and a polygon that contains the outline of the present Pacific plate (pac_clip_path.txt). To evaluate the plate motion azimuths at the present time for the Pacific, try
gmt grdpmodeler pac_age.nc -EPac_APM.txt -V -Fpac_clip_path.txt \ -Gpac_dir_0.nc -Sa -T0
To determine the changes in latitude since crust formation for the entire Pacific, try
gmt grdpmodeler pac_age.nc -EPac_APM.txt -V -Fpac_clip_path.txt \ -Gpac_dlat.nc -Sy
To determine the plate motion velocities in effect when the Pacific crust was formed, try
gmt grdpmodeler pac_age.nc -EPac_APM.txt -V -Fpac_clip_path.txt \ -Gpac_vel.nc -Sv
To determine how far the crust has moved since formation, try
gmt grdpmodeler pac_age.nc -EPac_APM.txt -V -Fpac_clip_path.txt \ -Gpac_dist.nc -Sd
To save the coordinates of the crust’s formation to separate grids, try
gmt grdpmodeler pac_age.nc -EPac_APM.txt -V -Fpac_clip_path.txt \ -Gpac_origin_%s.nc -SXY
To repeat the same exercise but save output lon,lat,age,xorigin,yorigin to a table, use
gmt grdpmodeler pac_age.nc -EPac_APM.txt -V -Fpac_clip_path.txt -SXY > origin.txt
GMT distributes the EarthByte rotation model Global_EarthByte_230-0Ma_GK07_AREPS.rot. To use an alternate rotation file, create an environmental parameters named GPLATES_ROTATIONS that points to an alternate rotation file.