surfacegmt - Online in the Cloud

PROGRAM:

NAME

surface - Grid table data using adjustable tension continuous curvature splines

SYNOPSIS

surface [ table ] outputfile.nc increment region [ aspect_ratio ] [ convergence_limit ] [
llower ] [ -Luupper ] [ max_iterations ] [ ] [ search_radius[m|s] ] [ [i|b]tension_factor
] [ [level] ] [ over-relaxation_factor ] [ -a<flags> ] [ -bi<binary> ] [ -di<nodata> ] [
-f<flags> ] [ -h<headers> ] [ -i<flags> ] [ -:[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 Laplacian operator. T = 0
gives the "minimum curvature" 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 topography 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 minimum/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][=|+][/yinc[unit][=|+]]
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 = 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 + 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.

-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
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 convergence_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 intermediate (coarser) grids the effective convergence
limit is scaled 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 convergence_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 multiplier. [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 suppresses 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 surface 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 ...)
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 iterates the equations exactly, and will
always assure stable convergence. 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[...] (more ...)
Set aspatial column associations col=name.

-bi[ncols][t] (more ...)
Select native binary input. [Default is 3 input columns].

-dinodata (more ...)
Replace input columns that equal nodata with NaN.

-f[i|o]colinfo (more ...)
Specify data types of input and/or output columns.

-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 (0 is first column).

-:[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
use just -).

-+ 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 options, then
exits.

--version
Print GMT version and exit.

--show-datadir
Print full path to GMT share directory and exit.

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
precision once GMT operates on the grid or writes out new grids. To limit loss of
precision when processing data you should always consider normalizing 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

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