Provided by: gmt-common_5.4.3+dfsg-1_all 
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 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][+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.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (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 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 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.
-e[~]”pattern” | -e[~]/regexp/[i] (more …)
Only accept data records that match the given pattern.
-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 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.
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
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, blockmedian, blockmode, gmt, grdcut, grdsample, greenspline, nearneighbor, triangulate,
sphtriangulate
REFERENCES
Smith, W. H. F, and P. Wessel, 1990, Gridding with continuous curvature splines in tension, Geophysics,
55, 293-305.
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe