Provided by: gmt_4.5.11-1build1_amd64

**NAME**

grdfilter - Filter a 2-D grid file in the space (or time) domain

**SYNOPSIS**

grdfilterinput_file-Ddistance_flag-F<filtertype><width>[mode]-Goutput_file[-Ixinc[unit][=|+][/yinc[unit][=|+]] ] [-Ni|p|r] [-Rwest/east/south/north[r] ] [-T] [-V] [-f[i|o]colinfo]

**DESCRIPTION**

grdfilterwill filter a.grdfile in the time domain using one of the selected convolution or non-convolution isotropic filters and compute distances using Cartesian or Spherical geometries. The output.grdfile can optionally be generated as a subOPT(R)egion of the input and/or with a new-Increment. In this way, one may have "extra space" in 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.input_fileThe grid file of points to be filtered. (See GRID FILE FORMATS below).-DDistanceflagtells how grid (x,y) relates to filterwidthas follows: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.-FSets the filter type. Choose among convolution and non-convolution filters. Append the filter code followed by the full diameterwidth. Available convolution filters 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. Non-convolution filters are: (m) Median: Returns median value. (p) Maximum likelihood probability (a mode estimator): Return modal value. If more than one mode is found we return their average value. Append - or + to the filter width if you rather want to return the smallest or largest 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 initial sign test; in that case the filter will return 0.0.-Goutput_fileis the output grid file of the filter. (See GRID FILE FORMATS below).

**OPTIONS**

-Ix_inc[and optionallyy_inc] is the output Increment. Appendmto indicate minutes, orcto indicate seconds. If the newx_inc,y_incare NOT integer multiples of the old ones (in the input data), filtering will be considerably slower. [Default: Same as input.]-NDetermine how NaN-values in the input grid affects the filtered outout: Appendito ignore all NaNs in the calculation of filtered 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 coregistered), andpwhich will force the filtered 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 registration as the input grid].-VSelects verbose mode, which will send progress reports to stderr [Default runs "silently"].-fSpecial formatting of input and/or output columns (time or geographical data). Specifyioroto make this apply only to input or output [Default applies to both]. Give one or more columns (or column ranges) separated by commas. AppendT(absolute calendar time),t(relative time in chosenTIME_UNITsinceTIME_EPOCH),x(longitude),y(latitude), orf(floating point) to each column or column range item. Shorthand-f[i|o]gmeans-f[i|o]0x,1y(geographic coordinates).

**GRID** **FILE** **FORMATS**

By defaultGMTwrites out grid as single precision floats in a COARDS-complaint netCDF file format. However,GMTis 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]], whereidis a two-letter identifier of the grid type and precision, andscaleandoffsetare optional scale factor and offset to be applied to all grid values, andnanis 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. Seegrdreformat(1) and Section 4.17 of the GMT Technical Reference and Cookbook for more information. When reading a netCDF file that contains multiple grids,GMTwill read, by default, the first 2-dimensional grid that can find in that file. To coaxGMTinto reading another multi-dimensional variable in the grid file, append?varnameto the file name, wherevarnameis 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?varnamesuffix can also be used for output grids to specify a variable name different from the default: "z". Seegrdreformat(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.

**GEOGRAPHICAL** **AND** **TIME** **COORDINATES**

When the output grid type is netCDF, the coordinates will be labeled "longitude", "latitude", or "time" based 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 byTIME_UNITandTIME_EPOCHin the .gmtdefaults 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_dbdb5.grd 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 calculations, you need:grdfilternorth_pacific_dbdb5.grd-Gfiltered_pacific.grd-Fm600-D4-R150/250/10/40-I0.5-V

**LIMITATIONS**

When working with geographic (lat, lon) grids, all three convolution filters (boxcar, cosine arch, and gaussian) will properly normalize the filter weights for the variation in gridbox size with latitude, and correctly determine which nodes are needed for the convolution when the filter "circle" crosses a periodic (0-360) boundary or contains a geographic pole. However, the spatial filters, such as median and mode filters, do not use weights and thus should only be used on Cartesian grids (or at very low latitudes) only. If you want to apply such spatial filters you should project your data to an equal- area projection and run grdfilter on the resulting Cartesian grid. 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).

**SEE** **ALSO**

GMT(1),grdfft(1)img2mercgrd(1)