# GRDSAMPLE

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
GRID VALUES PRECISION
GRID FILE FORMATS
HINTS
EXAMPLES
SEE ALSO

## NAME

grdsample − Resample a grid file onto a new grid

## SYNOPSIS

grdsample in_grdfile −G out_grdfile [ −F ] [ −I xinc[unit][=|+][/yinc[unit][=|+]] ] [ −L flag ] [ −Q[b|c|l|n][[/]threshold] ] [ −R west/east/south/north[r] ] [ −T ] [ −V ] [ −f colinfo ]

## DESCRIPTION

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.
in_grdfile

The name of the input 2-D binary grid file. (See GRID FILE FORMAT 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 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 −R grdfile 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 recognised 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 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).

| −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 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 (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 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.

## 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 important. 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 −I1m −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 −Gpixel.grd

## SEE ALSO

GMT(1), grdedit(1), grdfft(1), grdfilter(1)