SURFACE(1) Generic Mapping Tools SURFACE(1)NAMEsurface - adjustable tension continuous curvature surface gridding
algorithm
SYNOPSISsurface [ xyzfile ] -Goutputfile.grd
-Ixinc[unit][=|+][/yinc[unit][=|+]] -Rwest/east/south/north[r] [
-Aaspect_ratio ] [ -Cconvergence_limit ] [ -H[i][nrec] ] [ -Lllower ] [
-Luupper ] [ -Nmax_iterations ] [ -Q ] [ -Ssearch_radius[m] ] [ -Tten‐
sion_factor[i|b] ] [ -V[l] ] [ -Zover-relaxation_factor ] [ -:[i|o] ] [
-bi[s|S|d|D[ncol]|c[var1/...]] ] [ -fcolinfo ]
DESCRIPTIONsurface reads randomly-spaced (x,y,z) triples from standard input [or
xyzfile] 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 Lapla‐
cian 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 topog‐
raphy 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 mini‐
mum/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 stan‐
dard input.
-G Output file name. Output is a binary 2-D .grd file. Note that
the smallest grid dimension must be at least 4.
-I 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 c to indicate arc seconds.
If one of the units e, k, i, or n is appended instead, the
increment is assumed to be given in meter, km, miles, or nauti‐
cal miles, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the con‐
version depends on 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 Appendix B for details. Note: if -Rgrdfile is used then
grid spacing has already been initialized; use -I to override
the values.
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of w/e/s/n.
The two shorthands -Rg and -Rd stand for global domain (0/360
and -180/+180 in longitude respectively, with -90/+90 in lati‐
tude). Alternatively, specify the name of an existing grid file
and the -R settings (and grid spacing, if applicable) are copied
from the grid. For calendar time coordinates you may either
give (a) relative time (relative to the selected TIME_EPOCH and
in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
time of the form [date]T[clock] (append T to -JX|x). At least
one of date and clock must be present; the T is always required.
The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
string must be of the form hh:mm:ss[.xxx]. The use of delim‐
iters and their type and positions must be exactly as indicated
(however, input, output and plot formats are customizable; see
gmtdefaults).
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 con‐
vergence_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). If used, the default number
of header records is N_HEADER_RECS. Use -Hi if only input data
should have header records [Default will write out header
records if the input data have them]. Blank lines and lines
starting with # are always skipped. Not used with binary data.
-L 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].
-N Number of iterations. Iteration will cease when conver‐
gence_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 sup‐
presses 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 sur‐
face 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 and/or output. [Default is (longitude,latitude)]. Append
i to select input only or o to select output only. [Default
affects both].
-bi Selects binary input. Append s for single precision [Default is
d (double)]. Uppercase S or D will force byte-swapping.
Optionally, append ncol, the number of columns in your binary
input file if it exceeds the columns needed by the program. Or
append c if the input file is netCDF. Optionally, append
var1/var2/... to specify the variables to be read. [Default is
3 input columns].
-f Special formatting of input and/or output columns (time or geo‐
graphical data). Specify i or o to make this apply only to
input or output [Default applies to both]. Give one or more
columns (or column ranges) separated by commas. Append T (abso‐
lute calendar time), t (relative time in chosen TIME_UNIT since
TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
to each column or column range item. Shorthand -f[i|o]g means
-f[i|o]0x,1y (geographic coordinates).
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.grd, and
monitoring each iteration, try:
surface hawaii_5x5.xyg -R 198/208/18/25 -I 5m -G hawaii_grd.grd -T 0.25
-C 0.1 -Vl
BUGSsurface 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 .gmtdefaults4 file prior to
running blockm*, 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 ALSOblockmean(1), blockmedian(1), blockmode(1), GMT(1), nearneighbor(1),
triangulate(1)REFERENCES
Smith, W. H. F, and P. Wessel, 1990, Gridding with continuous curvature
splines in tension, Geophysics, 55, 293-305.
GMT 4.5.14 1 Nov 2015 SURFACE(1)
