surface(1) GMT surface(1)
NAME
surface - Grid table data using adjustable tension continuous curvature
splines
SYNOPSIS
surface [ table ] -Goutputfile.nc
-Iincrement
-Rregion [ -Aaspect_ratio ] [ -Cconvergence_limit[%] ] [ -Lllower ]
[ -Luupper ] [ -Nmax_iterations ] [ -Q ] [ -Ssearch_radius[m|s] ] [
-T[i|b]tension_factor ] [ -V[level] ] [ -Zover-relaxation_factor ] [
-aflags ] [ -bibinary ] [ -dinodata ] [ -eregexp ] [ -fflags ] [
-hheaders ] [ -iflags ] [ -r ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
surface reads randomly-spaced (x,y,z) triples from standard input [or
table] and produces a binary grid file of gridded values z(x,y) by
solving:
(1 - T) * L (L (z)) + T * L (z) = 0
where T is a tension factor between 0 and 1, and L indicates the Lapla-
cian operator. T = 0 gives the aminimum curvaturea solution which is
equivalent to SuperMISP and the ISM packages. Minimum curvature can
cause undesired oscillations and false local maxima or minima (See
Smith and Wessel, 1990), and you may wish to use T > 0 to suppress
these effects. Experience suggests T ~ 0.25 usually looks good for
potential field data and T should be larger (T ~ 0.35) for steep topog-
raphy data. T = 1 gives a harmonic surface (no maxima or minima are
possible except at control data points). It is recommended that the
user pre-process the data with blockmean, blockmedian, or blockmode to
avoid spatial aliasing and eliminate redundant data. You may impose
lower and/or upper bounds on the solution. These may be entered in the
form of a fixed value, a grid with values, or simply be the mini-
mum/maximum input data values. Natural boundary conditions are applied
at the edges, except for geographic data with 360-degree range where we
apply periodic boundary conditions in the longitude direction.
REQUIRED ARGUMENTS
-Goutputfile.nc
Output file name. Output is a binary 2-D .nc file. Note that the
smallest grid dimension must be at least 4.
-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.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
Specify the region of interest.
OPTIONAL ARGUMENTS
table One or more ASCII (or binary, see -bi[ncols][type]) data table
file(s) holding a number of data columns. If no tables are given
then we read from standard input.
-Aaspect_ratio
Aspect ratio. If desired, grid anisotropy can be added to the
equations. Enter aspect_ratio, where dy = dx / aspect_ratio
relates the grid dimensions. [Default = 1 assumes isotropic
grid.]
-Cconvergence_limit[%]
Convergence limit. Iteration is assumed to have converged when
the maximum absolute change in any grid value is less than con-
vergence_limit. (Units same as data z units). Alternatively,
give limit in percentage of rms deviation by appending %.
[Default is scaled to 1e-4 of the root-mean-square deviation of
the data from a best-fit (least-squares) plane.]. This is the
final convergence limit at the desired grid spacing; for inter-
mediate (coarser) grids the effective convergence limit is
divided by the grid spacing multiplier.
-Lllower and -Luupper
Impose limits on the output solution. llower sets the lower
bound. lower can be the name of a grid file with lower bound
values, a fixed value, d to set to minimum input value, or u for
unconstrained [Default]. uupper sets the upper bound and can be
the name of a grid file with upper bound values, a fixed value,
d to set to maximum input value, or u for unconstrained
[Default]. Grid files used to set the limits may contain NaNs.
In the presence of NaNs, the limit of a node masked with NaN is
unconstrained.
-Nmax_iterations
Number of iterations. Iteration will cease when conver-
gence_limit is reached or when number of iterations reaches
max_iterations. This is the final iteration limit at the
desired grid spacing; for intermediate (coarser) grids the
effective iteration limit is scaled by the grid spacing multi-
plier. [Default is 500.]
-Q Suggest grid dimensions which have a highly composite greatest
common factor. This allows surface to use several intermediate
steps in the solution, yielding faster run times and better
results. The sizes suggested by -Q can be achieved by altering
-R and/or -I. You can recover the -R and -I you want later by
using grdsample or grdcut on the output of surface.
-Ssearch_radius[m|s]
Search radius. Enter search_radius in same units as x,y data;
append m to indicate arc minutes or s for arc seconds. This is
used to initialize the grid before the first iteration; it is
not worth the time unless the grid lattice is prime and cannot
have regional stages. [Default = 0.0 and no search is made.]
-T[i|b]tension_factor
Tension factor[s]. These must be between 0 and 1. Tension may be
used in the interior solution (above equation, where it sup-
presses spurious oscillations) and in the boundary conditions
(where it tends to flatten the solution approaching the edges).
Using zero for both values results in a minimum curvature sur-
face with free edges, i.e., a natural bicubic spline. Use
-Titension_factor to set interior tension, and -Tbtension_factor
to set boundary tension. If you do not prepend i or b, both will
be set to the same value. [Default = 0 for both gives minimum
curvature solution.]
-V[level] (more a|)
Select verbosity level [c]. -V3 will report the convergence
after each iteration; -V will report only after each regional
grid is converged.
-Zover-relaxation_factor
Over-relaxation factor. This parameter is used to accelerate the
convergence; it is a number between 1 and 2. A value of 1 iter-
ates the equations exactly, and will always assure stable con-
vergence. Larger values overestimate the incremental changes
during convergence, and will reach a solution more rapidly but
may become unstable. If you use a large value for this factor,
it is a good idea to monitor each iteration with the -Vl option.
[Default = 1.4 converges quickly and is almost always stable.]
-acol=name[^<i>a|] (more a|)
Set aspatial column associations col=name.
-bi[ncols][t] (more a|)
Select native binary input. [Default is 3 input columns].
-dinodata (more a|)
Replace input columns that equal nodata with NaN.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-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.
GRID VALUES PRECISION
Regardless of the precision of the input data, GMT programs that create
grid files will internally hold the grids in 4-byte floating point
arrays. This is done to conserve memory and furthermore most if not all
real data can be stored using 4-byte floating point values. Data with
higher precision (i.e., double precision values) will lose that preci-
sion once GMT operates on the grid or writes out new grids. To limit
loss of precision when processing data you should always consider nor-
malizing the data prior to processing.
EXAMPLES
To grid 5 by 5 minute gravity block means from the ASCII data in
hawaii_5x5.xyg, using a tension_factor = 0.25, a convergence_limit =
0.1 milligal, writing the result to a file called hawaii_grd.nc, and
monitoring each iteration, try:
gmt surface hawaii_5x5.xyg -R198/208/18/25 -I5m -Ghawaii_grd.nc -T0.25 -C0.1 -Vl
BUGS
surface will complain when more than one data point is found for any
node and suggest that you run blockmean, blockmedian, or blockmode
first. If you did run these decimators and still get this message it
usually means that your grid spacing is so small that you need more
decimals in the output format used. You may specify more decimal places
by editing the parameter FORMAT_FLOAT_OUT in your gmt.conf file prior
to running the decimators or choose binary input and/or output using
single or double precision storage.
Note that only gridline registration is possible with surface. If you
need a pixel-registered grid you can resample a gridline registered
grid using grdsample -T.
SEE ALSO
blockmean(1), blockmedian(1), blockmode(1), gmt(1), grdcut(1),
grdsample(1), greenspline(1), nearneighbor(1), triangulate(1),
sphtriangulate(1)
REFERENCES
Smith, W. H. F, and P. Wessel, 1990, Gridding with continuous curvature
splines in tension, Geophysics, 55, 293-305.
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 surface(1)
gmt5 5.4.2 - Generated Thu Jun 29 16:34:48 CDT 2017