GRDSAMPLE(1) Generic Mapping Tools GRDSAMPLE(1)NAMEgrdsample - Resample a grid file onto a new grid
SYNOPSISgrdsample in_grdfile -Gout_grdfile [ -F ] [
-Ixinc[unit][=|+][/yinc[unit][=|+]] ] [ -Lflag ] [
-Q[b|c|l|n][[/]threshold] ] [ -Rwest/east/south/north[r] ] [ -T ] [ -V
] [ -fcolinfo ]
DESCRIPTIONgrdsample reads a grid file and interpolates it to create a new grid
file with either: a different registration (-F or -T); or, a new grid-
spacing or number of nodes (-I), and perhaps also a new sub-region
(-R). A bicubic [Default], bilinear, B-spline or nearest-neighbor
interpolation (-Q) is used, requiring boundary conditions (-L). Note
that using -R only is equivalent to grdcut or grdedit -S.grdsample
safely creates a fine mesh from a coarse one; the converse may suffer
aliasing unless the data are filtered using grdfft or grdfilter.
When -R is omitted, the output grid will cover the same region as the
input grid. When -I is omitted, the grid spacing of the output grid
will be the same as the input grid. Either -F or -T can be used to
change the grid registration. When omitted, the output grid will have
the same registration as the input grid.
in_grdfile
The name of the input 2-D binary grid file. (See GRID FILE FOR‐
MAT below.)
-G The name of the output grid file. (See GRID FILE FORMAT below.)
OPTIONS-F Force pixel node registration on output grid. [Default is same
registration as input grid].
-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.
-L Boundary condition flag may be x or y or xy indicating data is
periodic in range of x or y or both set by -R, or flag may be g
indicating geographical conditions (x and y are lon and lat).
[Default uses "natural" conditions (second partial derivative
normal to edge is zero) unless the grid is automatically recog‐
nised as periodic.]
-Q Quick mode, use bilinear rather than bicubic interpolation
[Default]. Alternatively, select the interpolation mode by
adding b for B-spline smoothing, c for bicubic interpolation, l
for bilinear interpolation or n for nearest-neighbor value.
Optionally, append threshold in the range [0,1]. This parameter
controls how close to nodes with NaN values the interpolation
will go. E.g., a threshold of 0.5 will interpolate about half
way from a non-NaN to a NaN node, whereas 0.1 will go about 90%
of the way, etc. [Default is 1, which means none of the (4 or
16) nearby nodes may be NaN]. -Q0 will just return the value of
the nearest node instead of interpolating. This is the same as
using -Qn.
-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).
-T Translate between grid and pixel registration; if the input is
grid-registered, the output will be pixel-registered and vice-
versa.
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-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.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a COARDS-
complaint netCDF file format. However, GMT is able to produce grid
files in many other commonly used grid file formats and also facili‐
tates so called "packing" of grids, writing out floating point data as
2- or 4-byte integers. To specify the precision, scale and offset, the
user should add the suffix =id[/scale/offset[/nan]], where id is a two-
letter identifier of the grid type and precision, and scale and offset
are optional scale factor and offset to be applied to all grid values,
and nan is the value used to indicate missing data. When reading
grids, the format is generally automatically recognized. If not, the
same suffix can be added to input grid file names. See grdreformat(1)
and Section 4.17 of the GMT Technical Reference and Cookbook for more
information.
When reading a netCDF file that contains multiple grids, GMT will read,
by default, the first 2-dimensional grid that can find in that file. To
coax GMT into reading another multi-dimensional variable in the grid
file, append ?varname to the file name, where varname is the name of
the variable. Note that you may need to escape the special meaning of ?
in your shell program by putting a backslash in front of it, or by
placing the filename and suffix between quotes or double quotes. The
?varname suffix can also be used for output grids to specify a variable
name different from the default: "z". See grdreformat(1) and Section
4.18 of the GMT Technical Reference and Cookbook for more information,
particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
HINTS
If an interpolation point is not on a node of the input grid, then a
NaN at any node in the neighborhood surrounding the point will yield an
interpolated NaN. Bicubic interpolation [default] yields continuous
first derivatives but requires a neighborhood of 4 nodes by 4 nodes.
Bilinear interpolation [-Q] uses only a 2 by 2 neighborhood, but yields
only zeroth-order continuity. Use bicubic when smoothness is impor‐
tant. Use bilinear to minimize the propagation of NaNs.
EXAMPLES
To resample the 5 x 5 minute grid in hawaii_5by5_topo.grd onto a 1
minute grid:
grdsample hawaii_5by5_topo.grd -I 1m -Ghawaii_1by1_topo.grd
To translate the gridline-registered file surface.grd to pixel regis‐
tration while keeping the same region and grid interval:
grdsample surface.grd -T -G pixel.grd
SEE ALSOGMT(1), grdedit(1), grdfft(1), grdfilter(1)GMT 4.5.14 1 Nov 2015 GRDSAMPLE(1)
