gpsgridder(1) GMT gpsgridder(1)
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 cou-
pled model based on 2-D elasticity. The degree of coupling can be
tuned by adjusting the effective Poissonas 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 calcula-
tion of distances.
-Goutfile
Name of resulting output file. (1) If options -R, -I, and possi-
bly -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 out-
file 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 ta-
ble 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 ei-
genvalues, 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 ei-
genvalues needed to leave approximately value as the model mis-
fit. If value is not given then -W is required and we compute
value as the rms of the given data uncertainties. Alterna-
tively, 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 con-
straints 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 Greenas 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 off-
set 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, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude 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 coordi-
nates: 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 recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-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 elim-
inates the need to specify options -R, -I, and -r.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
Specify the region of interest.
-Snu Specify Poissonas 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 a|)
Select verbosity level [c].
-d[i|o]nodata (more a|)
Replace input columns that equal nodata with NaN and do the
reverse on output.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-fg Geographic grids (dimensions of longitude, latitude) will be
converted to meters via a aFlat Eartha approximation using the
current ellipsoid parameters.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s). Not used with binary data.
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-r (more a|)
Set pixel node registration [gridline].
-:[i|o] (more a|)
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 explana-
tion 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 aFlat Eartha 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 Defor-
mation, 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(1), greenspline(1), nearneighbor(1), surface(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 gpsgridder(1)
gmt5 5.4.2 - Generated Wed Jun 28 18:06:58 CDT 2017