SURFACE

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NAME

surface - adjustable tension continuous curvature surface gridding algorithm  

SYNOPSIS

surface [ xyzfile ] -Goutputfile.grd -Ix_inc[m|c][/y_inc[m|c]] -Rwest/east/south/north [ -Aaspect_ratio ] [ -Cconvergence_limit ] [ -H ] [ -Lllower ] [ -Luupper ] [ -Nmax_iterations ] [ -Ssearch_radius[m] ] [ -Ttension_factor[ib] ] [ -V[l] ] [ -Zover-relaxation_factor ] [ -: ] [ -b[d] ]  

DESCRIPTION

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 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 manuscript), 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 or blockmedian 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.

xyzfile
3 column ASCII file [or binary, see -b] holding (x,y,z) data values. If no file is specified, surface will read from standard input.
-G
Output file name. Output is a binary 2-D .grd file.
-I
x_inc [and optionally y_inc] is the grid size. Append m to indicate minutes, or c to indicate seconds. #include "explain_-R.txt"
 

OPTIONS

-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 grid.]
-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 one percent of typical gradient in input data.] #include "explain_-H.txt" 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.]
-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 curvature solution.] #include "explain_-V.txt" -Vl will report the convergence after each iteration; -V will report only after each regional grid is converged.
-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.] #include "explain_-t.txt"
-b
Selects binary input mode [Default is ASCII]. Append d to indicate double precision [Default is single].
 

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.grd, and monitoring each iteration, try:

surface hawaii_5x5.xyg -R198/208/18/25 -I5m -Ghawaii_grd.grd -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 or blockmedian first. If you did run blockmean/median 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 blockmean/median. You may specify more decimal places by editing the parameter D_FORMAT in your .gmtdefaults file prior to running blockmean/median, or choose binary input and/or output using single or double precision storage.  

SEE ALSO

blockmean, blockmedian, gmt, nearneighbor, triangulate #include "refs.i"

Smith, W. H. F, and P. Wessel, 1990, Gridding with continuous curvature splines in tension, Geophysics, 55, 293-305.


 

Index

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
EXAMPLES
BUGS
SEE ALSO

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Time: 07:11:05 GMT, January 07, 2025