Manual Reference Pages - GRDSAMPLE (1)
grdsample - Resample a grid file onto a new grid
Grid Values Precision
Grid File Formats
grdsample in_grdfile -Gout_grdfile [ -F ] [ -Ixinc[unit][=|+][/yinc[unit][=|+]] ]
[ -Lflag ] [ -Q[b|c|l|n][[/]threshold] ]
[ -Rwest/east/south/north[r] ] [ -T ] [ -V ] [ -fcolinfo ]
grdsample 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.
The name of the input 2-D binary grid file.
(See GRID FILE FORMAT below.)
The name of the output grid file.
(See GRID FILE FORMAT below.)
Force pixel node registration on output grid. [Default is same registration as input grid].
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
nautical miles, respectively, and will be converted to the equivalent degrees longitude at
the middle latitude of the region (the conversion 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.
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 recognised as periodic.]
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
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.
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 latitude). 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 delimiters and their type and positions must be exactly as indicated
(however, input, output and plot formats are customizable; see gmtdefaults).
Translate between grid and pixel registration; if the input is
grid-registered, the output will be pixel-registered and vice-versa.
Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
Special formatting of input and/or output columns (time or geographical 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 (absolute 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
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 facilitates 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.
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
important. Use bilinear to minimize the propagation of NaNs.
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 registration while
keeping the same region and grid interval:
grdsample surface.grd -T -G pixel.grd
|GMT 4.5.14 ||GRDSAMPLE (1) ||1 Nov 2015 |
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