Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
gpsgridder - Interpolate GPS strain vectors using Green's functions for elastic deformation
SYNOPSIS
gpsgridder [ table ]
-Goutfile [ -Iincrement ] [ -Rregion ] [ -C[n|r|v]value[+ffile] ] [ -E[misfitfile] ] [
-F[d|f]fudge] [ -L ] [ -Nnodefile ] [ -Snu ] [ -Tmaskgrid ] [ -V[level] ] [ -W[w]] [ -bbinary ] [
-dnodata ] [ -eregexp ] [ -fflags ] [ -hheaders ] [ -oflags ] [ -x[[-]n] ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated arguments.
DESCRIPTION
gpsgridder grids 2-D vector data such as GPS velocities by using a coupled model based on 2-D elasticity.
The degree of coupling can be tuned by adjusting the effective Poisson’s ratio. The solution field can be
tuned to extremes such as incompressible (1), typical elastic (0.5) or even an unphysical value of -1
that basically removes the elastic coupling of vector interpolation. Smoothing is offered via the
optional elimination of small eigenvalues.
REQUIRED ARGUMENTS
table table with GPS strain rates at discrete locations. We expect the input format to be x y u v [ du
dv ] (see -W to specify data uncertainties or weights). If lon lat is given you must supply -fg
and we will use a flat Earth approximation in the calculation of distances.
-Goutfile
Name of resulting output file. (1) If options -R, -I, and possibly -r are set we produce two
equidistant output grids. In this case, outfile must be a name template containing the C format
specifier %s, which will be replaced with u and v, respectively. (2) If option -T is selected
then -R, -I cannot be given as the maskgrid determines the region and increments. Again, the
outfile must be a name template for the two output grids. (3) If -N is selected then the output
is a single ASCII (or binary; see -bo) table written to outfile; if -G is not given then this
table is written to standard output. The -G option is ignored if -C or -C0 is given.
OPTIONAL ARGUMENTS
-C[n|r|v]value[+ffile]
Find an approximate surface fit: Solve the linear system for the spline coefficients by SVD and
eliminate the contribution from all eigenvalues whose ratio to the largest eigenvalue is less than
value [Default uses Gauss-Jordan elimination to solve the linear system and fit the data exactly].
Optionally, append +ffile to save the eigenvalue ratios to the specified file for further
analysis. Finally, if a negative value is given then +ffile is required and execution will stop
after saving the eigenvalues, i.e., no surface output is produced. Specify -Cvvalue to use the
largest eigenvalues needed to explain value % of the data variance. Specify -Crvalue to use the
largest eigenvalues needed to leave approximately value as the model misfit. If value is not
given then -W is required and we compute value as the rms of the given data uncertainties.
Alternatively, use -Cnvalue to select the value largest eigenvalues. If a file is given with -Cv
then we save the eigenvalues instead of the ratios. Note: 1/4 of the total number of data
constraints is a good starting point for further experiments.
E[misfitfile]
Evaluate the spline exactly at the input data locations and report statistics of the misfit (mean,
standard deviation, and rms) for u and v separately and combined. Optionally, append a filename and
we will write the data table, augmented by two extra columns after each of the u and v columns holding
the spline estimates and misfits.
-F[d|f]fudge
The Green’s functions are proportional to terms like 1/r^2 and log(r) and thus blow up for r == 0.
To prevent that we offer two fudging schemes: -Fddel_radius lets you add a constant offset to all
radii and must be specified in the user units. Alternatively, use -Fffactor which will compute
del_radius from the product of the shortest inter-point distance and factor [0.01].
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
x_inc [and optionally y_inc] is the grid spacing. Optionally, append a suffix modifier.
Geographical (degrees) coordinates: Append m to indicate arc minutes or s to indicate arc seconds.
If one of the units e, f, k, M, n or u is appended instead, the increment is assumed to be given
in meter, foot, km, Mile, nautical mile or US survey foot, respectively, and 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 given but set to 0 it will be reset equal to x_inc; otherwise it will
be converted to degrees latitude. All coordinates: If +e is appended then the corresponding max x
(east) or y (north) may be slightly adjusted to fit exactly the given increment [by default the
increment may be adjusted slightly to fit the given domain]. Finally, instead of giving an
increment you may specify the number of nodes desired by appending +n to the supplied integer
argument; the increment is then recalculated from the number of nodes and the domain. The
resulting increment value depends on whether you have selected a gridline-registered or
pixel-registered grid; see App-file-formats for details. Note: if -Rgrdfile is used then the grid
spacing has already been initialized; use -I to override the values.
-L Leave trend alone. Do not remove a planer (2-D) trend from the data before fitting the spline.
[Default removes least squares plane, fits normalized residuals, and restores plane].
-Nnodefile
ASCII file with coordinates of desired output locations x in the first column(s). The resulting w
values are appended to each record and written to the file given in -G [or stdout if not
specified]; see -bo for binary output instead. This option eliminates the need to specify options
-R, -I, and -r.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
Specify the region of interest.
-Snu Specify Poisson’s ratio to use for this 2-D elastic sheet [0.5]. Note: 1.0 is incompressible in a
2-D formulation while -1 removes all coupling between the two directions.
-Tmaskgrid
Only evaluate the solutions at the nodes in the maskgrid that are not set to NaN. This option
eliminates the need to specify options -R, -I (and -r).
-W[w] One-sigma data uncertainties for u and v are provided in the last two columns. We then compute
weights that are inversely proportional to the uncertainties. Append w if weights are given
instead of uncertainties. This results in a weighted least squares fit. Note that -W only has an
effect if -C is used. [Default uses no weights or uncertainties]. Note: At present the -W option
is unstable. We do not yet know if it reflects a coding bug or a theoretical limitation. Users
beware, and make sure you compare the results with non-weighted output for basic sanity checking.
-V[level] (more …)
Select verbosity level [c].
-d[i|o]nodata (more …)
Replace input columns that equal nodata with NaN and do the reverse on output.
-e[~]”pattern” | -e[~]/regexp/[i] (more …)
Only accept data records that match the given pattern.
-fg Geographic grids (dimensions of longitude, latitude) will be converted to meters via a “Flat
Earth” approximation using the current ellipsoid parameters.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more …)
Skip or produce header record(s). Not used with binary data.
-icols[+l][+sscale][+ooffset][,…] (more …)
Select input columns and transformations (0 is first column).
-r (more …)
Set pixel node registration [gridline].
-:[i|o] (more …)
Swap 1st and 2nd column on input and/or output.
-^ 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.
UNITS
For map distance unit, append unit d for arc degree, m for arc minute, and s for arc second, or e for
meter [Default], f for foot, k for km, M for statute mile, n for nautical mile, and u for US survey foot.
By default we compute such distances using a spherical approximation with great circles. Prepend - to a
distance (or the unit is no distance is given) to perform “Flat Earth” calculations (quicker but less
accurate) or prepend + to perform exact geodesic calculations (slower but more accurate).
EXAMPLES
To compute the u and v strain rate grids from the GPS data set gps.txt, containing x y u v du dv, on a
2x2 arc minute grid for California, try
gmt gpsgridder gps.txt -R-125/-114/31/41 -I2m -fg -W -r -Ggps_strain_%s.nc -V
REFERENCES
Haines, A. J. et al., 2015, Enhanced Surface Imaging of Crustal Deformation, SpringerBriefs in Earth
Sciences, doi:10.1007/978-3-319-21578-5_2.
Sandwell, D. T. and P. Wessel, 2016, Interpolation of 2-D Vector Data Using Constraints from Elasticity,
Geophys. Res. Lett., 43, 10,703-10,709, http://dx.doi.org/10.1002/2016GL070340
SEE ALSO
gmt, greenspline nearneighbor, surface
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe