surface - adjustable tension continuous curvature surface gridding
surface [ xyzfile ] -Goutputfile.grd -Ix_inc[m|c][/y_inc[m|c]]
-Rwest/east/south/north[r] [ -Aaspect_ratio ] [ -Cconvergence_limit ] [
-H[nrec] ] [ -Lllower ] [ -Luupper ] [ -Nmax_iterations ] [ -Q ] [
-Ssearch_radius[m] ] [ -Ttension_factor[ib] ] [ -V[l] ] [ -Zover-
relaxation_factor ] [ -: ] [ -bi[s][n] ]
surface reads randomly-spaced (x,y,z) triples from standard input [or
xyzfile] and produces a binary grdfile of gridded values z(x,y) by
(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 grdfile with values, or simply
be the minimum/maximum input data values.
3 column ASCII file [or binary, see -b] holding (x,y,z) data
values. If no file is specified, surface will read from standard
-G Output file name. Output is a binary 2-D .grd file.
-I x_inc [and optionally y_inc] is the grid spacing. Append m to
indicate minutes or c to indicate seconds.
-R west, east, south, and north specify the Region of interest. To
specify boundaries in degrees and minutes [and seconds], use the
dd:mm[:ss] format. Append r if lower left and upper right map
coordinates are given instead of wesn.
-A 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
-C 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). [Default is
scaled to 0.1 percent of typical gradient in input data.]
-H Input file(s) has Header record(s). Number of header records can
be changed by editing your .gmtdefaults file. If used, GMT
default is 1 header record. Not used with binary data.
-L Impose limits on the output solution. llower sets the lower
bound. lower can be the name of a grdfile 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 grdfile with upper bound values, a fixed value, d
to set to maximum input value, or u for unconstrained [Default].
-N Number of iterations. Iteration will cease when
convergence_limit is reached or when number of iterations reaches
max_iterations. [Default is 250.]
-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.
-S Search radius. Enter search_radius in same units as x,y data;
append m to indicate minutes. 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 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
-Ttension_factori to set interior tension, and -Ttension_factorb
to set boundary tension. If you do not append i or b, both will
be set to the same value. [Default = 0 for both gives minimum
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"]. -Vl will report the convergence after
each iteration; -V will report only after each regional grid is
-Z 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.]
-: Toggles between (longitude,latitude) and (latitude,longitude)
input/output. [Default is (longitude,latitude)].
-bi Selects binary input. Append s for single precision [Default is
double]. Append n for the number of columns in the binary
file(s). [Default is 3 input columns].
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.grd, and
monitoring each iteration, try:
surface hawaii_5x5.xyg -R198/208/18/25 -I5m -Ghawaii_grd.grd -T0.25
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 blockm* 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 by blockm*. You may specify more decimal
places by editing the parameter D_FORMAT in your .gmtdefaults file
prior to running blockm*, or choose binary input and/or output using
single or double precision storage.
blockmean, blockmedian, blockmode, gmt, nearneighbor, triangulate
Smith, W. H. F, and P. Wessel, 1990, Gridding with continuous
curvature splines in tension, Geophysics, 55, 293-305.
Man(1) output converted with