grdfilter - Filter a.grdfile in the Time domain

grdfilterinput_file.grd-Dflag-F<type><width>[mode]-Goutput_file.grd[-Ix_inc[m|c][/y_inc[m|c]] ] [-Rwest/east/south/north[r] ] [-T] [-V]

grdfilterwill filter a.grdfile in the time domain using one of the selected convolution or non-convolution filters and compute distances using Cartesian or Spherical geometries. The output.grdfile can optionally be generated as a sub-Region 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_file.grdThe file of points to be filtered.-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 two options are slower because they recompute weights for each East-West scan line.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.-FSets the filter type. Choose among convolution and non-convolu- tion filters. Append the filter code followed by the full diame- terwidth. 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. 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 ini- tial sign test; in that case the filter will return 0.0.-Goutput_file.grdis the output of the filter.

-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.]-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].-VSelects verbose mode, which will send progress reports to stderr [Default runs "silently"].

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 calcula- tions, you need:grdfilternorth_pacific_dbdb5.grd-Gfiltered_pacific.grd-Fm600-D4-R150/250/10/40-I0.5-V

grdfilteris not yet aware of boundary conditions and periodicities; hence filtering global grids will not necessarily give you the result you seek. You can remedy this partly by padding the east and west sides (e.g., use a grid that goes from -10 to 370 in longitude) but no such solution is possibly across the poles untill we rewrite the program for the next major release.

gmt(l),grdfft(l) GMT4.0 1 Oct 2004 GRDFILTER(l)

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