Finite rotation reconstruction of geographic grid
gmt grdrotater ingrdfile -Erot_file|lon/lat/angle -Goutgrdfile [ -Aregion ] [ -Drotoutline ] [ -Fpolygonfile ] [ -N ] [ -Rregion ] [ -S ] [ -Tages ] [ -V[level] ] [ -bbinary ] [ -dnodata ] [ -hheaders ] [ -nflags ] [ -:[i|o] ] [ --PAR=value ]
Note: No space is allowed between the option flag and the associated arguments.
grdrotater reads a geographical grid and reconstructs it given total reconstruction rotations. 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 reconstructed region. The outlines of the reconstructed region is also returned provided the rotated region is not the entire globe.
- Name of a grid file in geographical (lon, lat) coordinates.
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.
- Name of output grid. This is the grid with the data reconstructed according to the specified rotation. If more than one reconstruction time is implied then outgrdfile must contain a C-format specifier to format a floating point number (reconstruction time) to text.
- Specify directly the region of the rotated grid. By default, the output grid has a region that exactly matches the extent of the rotated domain, but -A can be used to crop or extend this region to that provided via region.
- Name of the grid polygon outline file. This represents the outline of the grid reconstructed to the specified time. If more than one reconstruction time is implied then rotoutline must contain a C-format specifier to format a floating point number (reconstruction time) to text. If only one time is implied and -D is not set then we write the polygon to stdout (but see -N).
- Specify a multisegment closed polygon file that describes the inside area of the grid that should be projected [Default projects entire grid].
- Do Not output the rotated polygon outline [Default will write it to stdout, or to a file via -D].
- 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). Set geographic regions by specifying ISO country codes from the Digital Chart of the World using -Rcode1,code2,…[+r|R[incs]] instead: Append one or more comma-separated countries using the 2-character ISO 3166-1 alpha-2 convention. To select a state of a country (if available), append .state, e.g, US.TX for Texas. To specify a whole continent, prepend = to any of the continent codes AF (Africa), AN (Antarctica), AS (Asia), EU (Europe), OC (Oceania), NA (North America), or SA (South America). Use +r to modify the bounding box coordinates from the polygon(s): Append inc, xinc/yinc, or winc/einc/sinc/ninc to adjust the region to be a multiple of these steps [no adjustment]. Alternatively, use +R to extend the region outward by adding these increments instead, or +e which is like +r but it ensures that the bounding box extends by at least 0.25 times the increment [no extension]. 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 and registration, 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.
- Skip the rotation of the grid, just rotate the polygon outline (requires -F if no grid is provided).
- Sets the desired reconstruction times. For a single time append the desired time. For an equidistant range of reconstruction times give -Tstart/stop/inc. Append +n if inc should be interpreted to mean npoints instead. For an non-equidistant set of reconstruction times please pass them via the first column in a file, e.g., -Tagefile. If no -T option is given and -E specified a rotation file then we equate the rotation file times with the reconstruction times.
- -V[level] (more …)
- Select verbosity level [w].
- -bi[ncols][t] (more …)
- Select native binary format for primary input. [Default is 2 input columns].
- -bo[ncols][type] (more …)
- Select native binary output. [Default is same as input].
- -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).
- -:[i|o] (more …)
- Swap 1st and 2nd column on input and/or output.
- -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more …)
- Select interpolation mode for grids.
- -^ 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.
Consequences of grid resampling¶
Resample or sampling of grids will use various algorithms (see -n) that may lead to possible distortions or unexpected results in the resampled values. One expected effect of resampling with splines is the tendency for the new resampled values to slightly exceed the global min/max limits of the original grid. If this is unacceptable, you can impose clipping of the resampled values values so they do not exceed the input min/max values by adding +c to your -n option.
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 rotate the data defined by grid topo.nc and the polygon outline clip_path.txt, using a total reconstruction rotation with pole at (135.5, -33.0) and a rotation angle of 37.3 degrees and bicubic interpolation, try
gmt grdrotater topo.nc -E135.5/-33/37.3 -V -Fclip_path.txt -Grot_topo.nc > rot_clip_path.txt
To rotate the entire grid faa.nc back to 32 Ma using the rotation file rotations.txt and a bilinear interpolation, try
gmt grdrotater faa.nc -Erotations.txt -T32 -V -Grot_faa.nc -nl > rot_faa_path.txt
To just see how the outline of the grid large.nc will plot after the same rotation, try
gmt grdrotater large.nc -Erotations.txt -T32 -V -S | gmt plot -Rg -JH180/6i -B30 -W0.5p \| gv -
To rotate the grid topo.nc back to 100 Ma using the rotation file rotations.txt and request a reconstruction every 10 Myr, saving both grids and outlines to filenames that derive from templates, try
gmt grdrotater topo.nc -Erotations.txt -T10/100/10 -V -Grot_topo_%g.nc -Drot_topo_path_%g.txt
Let say you have rotated gridA.nc and gridB.nc, restricting each rotation to nodes inside polygons polyA.txt and polyB.txt, respectively, using rotation A = (123W,22S,16,4) and rotation B = (108W, 16S, -14.5), yielding rotated grids rot_gridA.nc and rot_gridB.nc. To determine the region of overlap between the rotated grids, we use grdmath:
gmt grdmath 1 rot_gridA.nc ISNAN SUB 1 rot_gridB.nc ISNAN SUB 2 EQ = overlap.nc
The grid overlap.nc now has 1s in the regions of overlap and 0 elsewhere. You can use it as a mask or use grdcontour -D to extract a polygon (i.e., a contour).
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.