Create CVA grid from a gravity or topography grid
gmt grdspotter ingrid -Erot_file|lon/lat/angle[+i] -Goutgrid -Iincrement -Rregion [ -Aagegrid ] [ -Di|pgrid ] [ -LIDgrid ] [ -M ] [ -Nupper_age ] [ -QIDinfo ] [ -S ] [ -Tt|ufixed_val ] [ [ -V[level] ] [ -Wn_try ]] [ -Zz_min[/z_max[/z_inc]] ] [ -rreg ] [ --PAR=value ]
Note: No space is allowed between the option flag and the associated arguments.
grdspotter reads a grid file with residual bathymetry or gravity and calculates flowlines from each node that exceeds a minimum value using the specified rotations file. These flowlines are then convolved with the volume of the prism represented by each grid node and added up to give a Cumulative Volcano Amplitude grid (CVA) [Wessel, 1999; 2008].
Data grid to be processed, typically residual bathymetry or free-air anomalies. Optionally, append =ID for reading a specific file format [Default is =nf] or ?varname for a specific netCDF variable [Default is the first 2-D grid found by GMT] (See full description). The following modifiers are supported:
+b - Select a band (for images only) [Default is 0].
+d - Divide data values by the given divisor [Default is 1].
+n - Replace data values matching invalid with NaN.
+o - Offset data values by the given offset [Default is 0].
+s - Scale data values by the given scale [Default is 1].
Note: Any offset is added after any scaling.
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. (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. Regardless of method, you may append +i to the argument to indicate you wish to invert the rotation(s).
Specify name for output CVA grid file. Optionally, append =ID for writing a specific file format (See full description). The following modifiers are supported:
+d - Divide data values by given divisor [Default is 1].
+n - Replace data values matching invalid with a NaN.
+o - Offset data values by the given offset, or append a for automatic range offset to preserve precision for integer grids [Default is 0].
+s - Scale data values by the given scale, or append a for automatic scaling to preserve precision for integer grids [Default is 1].
Note: Any offset is added before any scaling. +sa also sets +oa (unless overridden). To write specific formats via GDAL, use =gd and supply driver (and optionally dataType) and/or one or more concatenated GDAL -co options using +c. See the “Writing grids and images” cookbook section for more details.
Set the grid spacing as x_inc [and optionally y_inc].
Geographical (degrees) coordinates: Optionally, append an increment unit. Choose among:
m to indicate arc minutes
s to indicate arc seconds
e (meter), f (foot), k (km), M (mile), n (nautical mile) or u (US survey foot), in which case the increment will be converted to the equivalent degrees longitude at the middle latitude of the region (the conversion depends on PROJ_ELLIPSOID). If y_inc is not given or given but set to 0 it will be reset equal to x_inc; otherwise it will be converted to degrees latitude.
All coordinates: The following modifiers are supported:
+e to slightly adjust the max x (east) or y (north) to fit exactly the given increment if needed [Default is to slightly adjust the increment to fit the given domain].
+n to define the number of nodes rather than the increment, in which case the increment is recalculated from the number of nodes, the registration (see GMT File Formats), and the domain. Note: If -Rgrdfile is used then the grid spacing and the registration have already been initialized; use -I and -R to override these values.
Specify the region of interest.
The region may be specified in one of several ways:
-Rwest/east/south/north. This is the standard way to specify geographic regions when using map projections where meridians and parallels are rectilinear. The coordinates may be specified in decimal degrees or in [±]dd:mm[:ss.xxx][W|E|S|N] format.
-Rwest/south/east/north+r. This form is useful for map projections that are oblique, making meridians and parallels poor choices for map boundaries. Here, we instead specify the lower left corner and upper right corner geographic coordinates, followed by the modifier +r. This form guarantees a rectangular map even though lines of equal longitude and latitude are not straight lines.
-Rg or -Rd. These forms can be used to quickly specify the global domain (0/360 for -Rg and -180/+180 for -Rd in longitude, with -90/+90 in latitude).
-Rcode1,code2,…[+e|r|Rincs]. This indirectly supplies the region by consulting the DCW (Digital Chart of the World) database and derives the bounding regions for one or more countries given by the codes. Simply append one or more comma-separated countries using either the two-character ISO 3166-1 alpha-2 convention (e.g., NO) or the full country name (e.g., Norway). To select a state within a country (if available), append .state (e.g, US.TX), or the full state name (e.g., Texas). To specify a whole continent, spell out the full continent name (e.g., -RAfrica). Finally, append any DCW collection abbreviations or full names for the extent of the collection or named region. All names are case-insensitive. The following modifiers can be appended:
+r to adjust the region boundaries to be multiples of the steps indicated by inc, xinc/yinc, or winc/einc/sinc/ninc [default is no adjustment]. For example, -RFR+r1 will select the national bounding box of France rounded to nearest integer degree, where inc can be positive to expand the region or negative to shrink the region.
+R to adjust the region by adding the amounts specified by inc, xinc/yinc, or winc/einc/sinc/ninc [default is no extension], where inc can be positive to expand the region or negative to shrink the region.
+e to adjust the region boundaries to be multiples of the steps indicated by inc, xinc/yinc, or winc/einc/sinc/ninc, while ensuring that the bounding box is adjusted by at least 0.25 times the increment [default is no adjustment], where inc can be positive to expand the region or negative to shrink the region.
-Rxmin/xmax/ymin/ymax[+uunit] specifies a region in projected units (e.g., UTM meters) where xmin/xmax/ymin/ymax are Cartesian projected coordinates compatible with the chosen projection (-J) and unit is an allowable distance unit [e]; we inversely project to determine the actual rectangular geographic region. For projected regions centered on (0,0) you may use the short-hand -Rhalfwidth[/halfheight]+uunit, where halfheight defaults to halfwidth if not given. This short-hand requires the +u modifier.
-Rjustifylon0/lat0/nx/ny, where justify 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). The two character code justify indicates which point on a rectangular region region the lon0/lat0 coordinates refer to and the grid dimensions nx and ny are used with grid spacings given via -I to create the corresponding region. This method can be used when creating grids. For example, -RCM25/25/50/50 specifies a 50x50 grid centered on 25,25.
-Rgridfile. This will copy the domain settings found for the grid in specified file. Note that depending on the nature of the calling module, this mechanism will also set grid spacing and possibly the grid registration (see Grid registration: The -r option).
-Ra[uto] or -Re[xact]. Under modern mode, and for plotting modules only, you can automatically determine the region from the data used. You can either get the exact area using -Re [Default if no -R is given] or a slightly larger area sensibly rounded outwards to the next multiple of increments that depend on the data range using -Ra.
Supply a crustal age grid that is co-registered with the input data grid. These ages become the upper ages to use when constructing flowlines [Default extend flowlines back to oldest age found in the rotation file; but see -N].
For -Di, we use flowlines to determine the maximum CVA encountered along each flowline and create a Data Importance (DI) grid with these values at the originating nodes. For -Dp we will instead use flowlines to determine the flowline age at the CVA maximum for each node and create a Predicted Age (PA) grid with these values at the originating nodes. Repeatable.
Supply a co-registered grid with seamount chain IDs for each node. This option requires that you also use -Q.
Do not attempt to keep all flowlines in memory when using -D and/or -P. Should you run out of memory you can use this option to compute flowlines on-the-fly. It will be slower as we no longer can reuse the flowlines calculated for the CVA step. Cannot be used with -W or the multi-slice mode in -Z.
Set the upper age to assign to nodes whose crustal age is unknown (i.e., NaN) [no upper age]. Also see -A.
Either give (1) a single ID to use or (2) the name of a file with a list of IDs to use [Default uses all IDs]. Each line would be TAG ID [w e s n]. The w/e/s/n zoom box is optional; if specified it means we only trace the flowline if inside this region [Default uses region set by -R]. Requires -L.
Normalize the resulting CVA grid to percentages of the CVA maximum. This also normalizes the DI grid (if requested).
Selects ways to adjust ages; repeatable. Choose from -Tt to truncate crustal ages given via the -A option that exceed the upper age set with -N [no truncation], or -Tufixed_val which means that after a node passes the test implied by -Z, we use this fixed_val instead in the calculations. [Default uses individual node values].
Get n_try bootstrap estimates of the maximum CVA location; the longitude and latitude results are written to standard output [Default is no bootstrapping]. Cannot be used with -M.
Ignore nodes with z-values lower than z_min  and optionally larger than z_max [Inf]. Give z_min/z_max/z_inc to make separate CVA grids for each z-slice [Default makes one CVA grid]. Multi-slicing cannot be used with -M.
- -r[g|p] (more …)
Set node registration [gridline].
- -^ 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 create a CVA image from the Pacific topography grid Pac_res_topo.nc, using the DC85.txt Euler poles, and only output a grid for the specified domain, run
gmt grdspotter Pac_res_topo.nc -EDC85.txt -GCVA.nc -R190/220/15/25 -I2m -N145 -Tt -V
This file can then be plotted with grdimage.
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.
Wessel, P., 1999, “Hotspotting” tools released, EOS Trans. AGU, 80 (29), p. 319.
Wessel, P., 2008, Hotspotting: Principles and properties of a plate tectonic Hough transform, Geochem. Geophys. Geosyst. 9(Q08004): https://doi.org/10.1029/2008GC002058.