grdfilter(1) GMT grdfilter(1)

## NAME

grdfilter - Filter a grid in the space (or time) domain

## SYNOPSIS

grdfilteringrid-Ddistance_flag-Fxwidth[/width2][modifiers]-Goutgrid[-Iincrement] [-Ni|p|r] [-Rregion] [-T] [-V[level] ] [-fflags ]Note:No space is allowed between the option flag and the associated arguments.

## DESCRIPTION

grdfilterwill filter a grid file in the time domain using one of the selected convolution or non-convolution isotropic or rectangular fil- ters and compute distances using Cartesian or Spherical geometries. The output grid file can optionally be generated as a sub-region of the input (via-R) and/or with new increment (via-I) or registration (via-T). In this way, one may haveaextra spaceain the input data so that the edges will not be used and the output can be within one half-width of the input edges. If the filter is low-pass, then the output may be less frequently sampled than the input.

## REQUIRED ARGUMENTS

ingridThe grid file of points to be filtered. (See GRID FILE FORMATS below).-Ddistance_flagDistanceflagtells how grid (x,y) relates to filterwidthas follows:flag= p: grid (px,py) withwidthan odd number of pixels; Cartesian distances.flag= 0: grid (x,y) same units aswidth, Cartesian distances.flag= 1: grid (x,y) in degrees,widthin kilometers, Cartesian distances.flag= 2: grid (x,y) in degrees,widthin km, dx scaled by cos(middle y), Cartesian distances. The above options are fastest because they allow weight matrix to be computed only once. The next three options are slower because they recompute weights for each latitude.flag= 3: grid (x,y) in degrees,widthin km, dx scaled by cosine(y), Cartesian distance calculation.flag= 4: grid (x,y) in degrees,widthin km, Spherical distance calculation.flag= 5: grid (x,y) in Mercator-Jm1 img units,widthin km, Spherical distance calculation.-Fxwidth[/width2][modifiers] Sets the filter type. Choose among convolution and non-convolu- tion filters. Use any filter codex(listed below) followed by the full diameterwidth. This gives an isotropic filter; append /width2for a rectangular filter (requires-Dpor-D0). By default we perform low-pass filtering; append+hto select high-pass filtering. Some filters allow for optional arguments and modifiers. Convolution filters (and their codes) are: (b) Boxcar: All weights are equal. (c) Cosine Arch: Weights follow a cosine arch curve. (g) Gaussian: Weights are given by the Gaussian function, wherewidthis 6 times the conventional Gaussian sigma. (f) Custom: Weights are given by the precomputed values in the filter weight grid fileweight, which must have odd dimensions; also requires-D0and output spacing must match input spacing or be integer multiples. (o) Operator: Weights are given by the precomputed values in the filter weight grid fileweight, which must have odd dimensions; also requires-D0and output spacing must match input spacing or be integer multiples. Weights are assumed to sum to zero so no accumulation of weight sums and normalization will be done. Non-convolution filters (and their codes) are: (m) Median: Returns median value. To select another quantile append+qquantilein the 0-1 range [Default is 0.5, i.e., median]. (p) Maximum likelihood probability (a mode estimator): Return modal value. If more than one mode is found we return their average value. Append+lor+uif you rather want to return the lowermost or uppermost of the modal values. (h) Histogram mode (another mode estimator): Return the modal value as the center of the dominant peak in a histogram. Append /binwidthto specify the binning interval. Use modifier+cto center the bins on multiples ofbinwidth[Default has bin edges that are multiples ofbinwidth]. If more than one mode is found we return their average value. Append+lor+uif you rather want to return the lowermost or uppermost of the modal values. (l) Lower: Return the minimum of all values. (L) Lower: Return minimum of all positive values only. (u) Upper: Return maximum of all values. (U) Upper: Return maximum or all negative values only. In the case ofL|Uit is possible that no data passes the ini- tial sign test; in that case the filter will return NaN.-Goutgridoutgridis the output grid file of the filter. (See GRID FILE FORMATS below).

## OPTIONAL ARGUMENTS

-Ixinc[unit][+e|n][/yinc[unit][+e|n]]x_inc[and optionallyy_inc] is the grid spacing. Optionally, append a suffix modifier.Geographical(degrees)coordinates: Appendmto indicate arc minutes orsto indicate arc seconds. If one of the unitse,f,k,M,noruis appended instead, the increment is assumed to be given in meter, foot, km, Mile, nau- tical mile or US survey foot, respectively, and will be con- verted to the equivalent degrees longitude at the middle lati- tude of the region (the conversion depends on PROJ_ELLIPSOID). Ify_incis given but set to 0 it will be reset equal tox_inc; otherwise it will be converted to degrees latitude.Allcoordi-nates: If+eis appended then the corresponding maxx(east) ory(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 thenumberofnodesdesired by appending+nto the supplied integer argument; the increment is then recalcu- lated from the number of nodes and the domain. The resulting increment value depends on whether you have selected a grid- line-registered or pixel-registered grid; see App-file-formats for details. Note: if-Rgrdfileis used then the grid spacing has already been initialized; use-Ito override the values.-Ni|p|rDetermine how NaN-values in the input grid affects the filtered output: Appendito ignore all NaNs in the calculation of fil- tered value [Default],ris same asiexcept if the input node was NaN then the output node will be set to NaN (only applies if both grids are co-registered), andpwhich will force the fil- tered value to be NaN if any grid-nodes with NaN-values are found inside the filter circle.-Rwest,east,south, andnorthdefines the Region of the output points. [Default: Same as input.]-TToggle the node registration for the output grid so as to become the opposite of the input grid [Default gives the same registra- tion as the input grid].-V[level] (morea|) Select verbosity level [c].-f[i|o]colinfo(morea|) Specify data types of input and/or output columns.-^or just-Print a short message about the syntax of the command, then exits (NOTE: on Windows just use-).-+or just+Print an extensive usage (help) message, including the explana- tion of any module-specific option (but not the GMT common options), then exits.-?or no arguments Print a complete usage (help) message, including the explanation of all options, then exits.

## 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 calledapackingaof grids, writing out floating point data as 1- or 2-byte integers. (morea|)

## GEOGRAPHICAL AND TIME COORDINATES

When the output grid type is netCDF, the coordinates will be labeledalongitudea,alatitudea, oratimeabased on the attributes of the input data or grid (if any) or on the-for-Roptions. For example, both-f0x-f1tand-R90w/90e/0t/3t will result in a longitude/time grid. When the x, y, or z coordinate is time, it will be stored in the grid as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH in the gmt.conf file or on the command line. In addition, theunitattribute of the time variable will indicate both this unit and epoch.

## EXAMPLES

Suppose that north_pacific_etopo5.nc is a file of 5 minute bathymetry from 140E to 260E and 0N to 50N, and you want to find the medians of values within a 300km radius (600km full width) of the output points, which you choose to be from 150E to 250E and 10N to 40N, and you want the output values every 0.5 degree. Using spherical distance calcula- tions, you need: gmt grdfilter north_pacific_etopo5.nc -Gfiltered_pacific.nc -Fm600 \ -D4 -R150/250/10/40 -I0.5 -V If we instead wanted a high-pass result then one can perform the corre- sponding low-pass filter using a coarse grid interval asgrdfilterwill resample the result to the same resolution as the input grid so we can compute the residuals, e.g., gmt grdfilter north_pacific_etopo5.nc -Gresidual_pacific.nc -Fm600+h \ -D4 -R150/250/10/40 -I0.5 -V Here, the residual_pacific.nc grid will have the same 5 minute resolu- tion as the original. To filter the dataset in ripples.nc using a custom anisotropic Gaussian filter exp (-0.5*r^2) whose distances r from the center is given by (2x^2 + y^2 -2xy)/6, with major axis at an angle of 63 degrees with the horizontal, try gmt grdmath -R-10/10/-10/10 -I1 X 2 POW 2 MUL Y 2 POW ADD X Y MUL 2 MUL \ SUB 6 DIV NEG 2 DIV EXP DUP SUM DIV = gfilter.nc gmt grdfilter ripples.nc -Ffgfilter.nc -D0 -Gsmooth.nc -V

## LIMITATIONS

1. To use the-D5 option the input Mercator grid must be created by img2mercgrd using the-Coption so the origin of the y-values is the Equator (i.e., x = y = 0 correspond to lon = lat = 0). 2. If the newx_inc,y_incset with-Iare NOT integer multiples of the increments in the input data, filtering will be considerably slower. [Default increments: Same as input.]

## SEE ALSO

gmt(1),grdfft(1),img2grd(1)

## COPYRIGHT

2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe 5.4.2 Jun 24, 2017 grdfilter(1)

gmt5 5.4.2 - Generated Wed Jun 28 18:26:31 CDT 2017