Ubuntu Manpage: surface - adjustable tension continuous curvature surface gridding algorithm

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[r]   [
       -Aaspect_ratio ] [ -Cconvergence_limit ] [ -H[nrec] ] [ -L ] [ -Lllower ] [ -Luupper ] [ -Nmax_iterations
       ]  [ -Q ] [ -Ssearch_radius[m] ] [ -Ttension_factor[ib] ] [ -V[l] ] [ -Zover-relaxation_factor ] [ -: ] [
       -bi[s][n] ]

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, 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.

       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 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.

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 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 Without any modifier, this option indicates that x is longitude and may be periodic in 360
degrees. With optional arguments it will instead 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 curvature solution.]

-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
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.]

-: Toggles between (longitude,latitude) and (latitude,longitude) input/output. [Default is
(longitude,latitude)]. Applies to geographic coordinates only.

-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].

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,  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.

REFERENCES

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

                                                   1 Jan 2004                                         SURFACE(l)