.. index:: ! grdfilter ********* grdfilter ********* .. only:: not man grdfilter - Filter a grid in the space (or time) domain Synopsis -------- .. include:: common_SYN_OPTs.rst_ **grdfilter** *ingrid* |-D|\ *distance_flag* |-F|\ **x**\ *width*\ [/*width2*][*modifiers*] |-G|\ *outgrid* [ |SYN_OPT-I| ] [ |-N|\ **i**\ \|\ **p**\ \|\ **r** ] [ |SYN_OPT-R| ] [ |-T| ] [ |SYN_OPT-V| ] [ |SYN_OPT-f| ] |No-spaces| Description ----------- **grdfilter** will filter a grid file in the time domain using one of the selected convolution or non-convolution isotropic or rectangular filters 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 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. Required Arguments ------------------ *ingrid* The grid file of points to be filtered. (See GRID FILE FORMATS below). .. _-D: **-D**\ *distance_flag* Distance *flag* tells how grid (x,y) relates to filter *width* as follows: *flag* = p: grid (px,py) with *width* an odd number of pixels; Cartesian distances. *flag* = 0: grid (x,y) same units as *width*, Cartesian distances. *flag* = 1: grid (x,y) in degrees, *width* in kilometers, Cartesian distances. *flag* = 2: grid (x,y) in degrees, *width* in 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, *width* in km, dx scaled by cosine(y), Cartesian distance calculation. *flag* = 4: grid (x,y) in degrees, *width* in km, Spherical distance calculation. *flag* = 5: grid (x,y) in Mercator **-Jm**\ 1 img units, *width* in km, Spherical distance calculation. .. _-F: **-Fx**\ *width*\ [/*width2*][*modifiers*\ ] Sets the filter type. Choose among convolution and non-convolution filters. Use any filter code **x** (listed below) followed by the full diameter *width*. This gives an isotropic filter; append /*width2* for a rectangular filter (requires **-Dp** or **-D0**). By default we perform low-pass filtering; append **+h** to 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, where *width* is 6 times the conventional Gaussian sigma. (**f**) Custom: Weights are given by the precomputed values in the filter weight grid file *weight*, which must have odd dimensions; also requires **-D0** and output spacing must match input spacing or be integer multiples. (**o**) Operator: Weights are given by the precomputed values in the filter weight grid file *weight*, which must have odd dimensions; also requires **-D0** and 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 **+q**\ *quantile* in 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 **+l** or **+u** if 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 /*binwidth* to specify the binning interval. Use modifier **+c** to center the bins on multiples of *binwidth* [Default has bin edges that are multiples of *binwidth*]. If more than one mode is found we return their average value. Append **+l** or **+u** if 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 of **L**\ \|\ **U** it is possible that no data passes the initial sign test; in that case the filter will return NaN. .. _-G: **-G**\ *outgrid* *outgrid* is the output grid file of the filter. (See GRID FILE FORMATS below). Optional Arguments ------------------ .. _-I: .. include:: explain_-I.rst_ .. _-N: **-N**\ **i**\ \|\ **p**\ \|\ **r** Determine how NaN-values in the input grid affects the filtered output: Append **i** to ignore all NaNs in the calculation of filtered value [Default], **r** is same as **i** except if the input node was NaN then the output node will be set to NaN (only applies if both grids are co-registered), and **p** which will force the filtered value to be NaN if any grid-nodes with NaN-values are found inside the filter circle. .. _-R: **-R** *west*, *east*, *south*, and *north* defines the Region of the output points. [Default: Same as input.] .. _-T: **-T** Toggle 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]. .. _-V: .. |Add_-V| unicode:: 0x20 .. just an invisible code .. include:: explain_-V.rst_ .. |Add_-f| unicode:: 0x20 .. just an invisible code .. include:: explain_-f.rst_ .. include:: explain_help.rst_ .. include:: explain_grd_inout_short.rst_ .. include:: explain_grd_coord.rst_ 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 calculations, 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 corresponding low-pass filter using a coarse grid interval as **grdfilter** will 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 resolution 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 ----------- #. To use the **-D**\ 5 option the input Mercator grid must be created by img2mercgrd using the **-C** option so the origin of the y-values is the Equator (i.e., x = y = 0 correspond to lon = lat = 0). #. If the new *x\_inc*, *y\_inc* set with **-I** are NOT integer multiples of the increments in the input data, filtering will be considerably slower. [Default increments: Same as input.] See Also -------- :doc:`gmt`, :doc:`grdfft` :doc:`img2grd `