pygmt.dimfilter

pygmt.dimfilter(grid, outgrid=None, **kwargs)[source]

Filter a grid by dividing the filter circle.

Filter a grid in the space (or time) domain by dividing the given filter circle into the given number of sectors, applying one of the selected primary convolution or non-convolution filters to each sector, and choosing the final outcome according to the selected secondary filter. It computes distances using Cartesian or Spherical geometries. The output grid can optionally be generated as a subregion of the input and/or with a new increment using spacing, which may add an “extra space” in the input data to prevent edge effects for the output grid. If the filter is low-pass, then the output may be less frequently sampled than the input. pygmt.dimfilter will not produce a smooth output as other spatial filters do because it returns a minimum median out of N medians of N sectors. The output can be rough unless the input data are noise-free. Thus, an additional filtering (e.g., Gaussian via pygmt.grdfilter) of the DiM-filtered data is generally recommended.

Full option list at https://docs.generic-mapping-tools.org/6.5/dimfilter.html

Aliases:

  • D = distance

  • F = filter

  • I = spacing

  • N = sectors

  • R = region

  • V = verbose

Parameters:

  • grid (str or xarray.DataArray) –

    Name of the input grid file or the grid loaded as a xarray.DataArray object.

    For reading a specific grid file format or applying basic data operations, see https://docs.generic-mapping-tools.org/6.5/gmt.html#grd-inout-full for the available modifiers.

  • outgrid (str | None, default: None) – Name of the output netCDF grid file. If not specified, will return an xarray.DataArray object. For writing a specific grid file format or applying basic data operations to the output grid, see https://docs.generic-mapping-tools.org/6.5/gmt.html#grd-inout-full for the available modifiers.

  • distance (int or str) –

    Distance flag tells how grid (x,y) relates to filter width, as follows:

    • 0: grid (x,y) in same units as width, Cartesian distances.

    • 1: grid (x,y) in degrees, width in kilometers, Cartesian distances.

    • 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 two options are slower because they recompute weights for each latitude.

    • 3: grid (x,y) in degrees, width in km, dx scaled by cosine(y), Cartesian distance calculation.

    • 4: grid (x,y) in degrees, width in km, Spherical distance calculation.

  • filter (str) –

    xwidth[+l|u]. Set the primary filter type. Choose among convolution and non-convolution filters. Use the filter code x followed by the full diameter width. 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 +l or +h to the filter width if you want to return the smallest or largest of each sector’s modal values.

  • sectors (str) –

    xsectors[+l|u] Set the secondary filter type x and the number of bow-tie sectors. sectors must be integer and larger than 0. When sectors is set to 1, the secondary filter is not effective. Available secondary filters x are:

    • (l) Lower: Return the minimum of all filtered values.

    • (u) Upper: Return the maximum of all filtered values.

    • (a) Average: Return the mean of all filtered values.

    • (m) Median: Return the median of all filtered values.

    • (p) Mode: Return the mode of all filtered values: If more than one mode is found we return their average value. Append +l or +h to the sectors if you rather want to return the smallest or largest of the modal values.

  • spacing (str or list) – x_inc [and optionally y_inc] is the output increment. Append m to indicate minutes, or c to indicate seconds. If the new x_inc, y_inc are NOT integer multiples of the old ones (in the input data), filtering will be considerably slower. [Default is same as input.]

  • region (str or list) – [xmin, xmax, ymin, ymax]. Define the region of the output points [Default is same as input].

  • verbose (bool or str) –

    Select verbosity level [Default is w], which modulates the messages written to stderr. Choose among 7 levels of verbosity:

    • q - Quiet, not even fatal error messages are produced

    • e - Error messages only

    • w - Warnings [Default]

    • t - Timings (report runtimes for time-intensive algorithms)

    • i - Informational messages (same as verbose=True)

    • c - Compatibility warnings

    • d - Debugging messages

Returns:

ret (xarray.DataArray or None) – Return type depends on whether the outgrid parameter is set:

  • xarray.DataArray if outgrid is not set

  • None if outgrid is set (grid output will be stored in file set by outgrid)

Example

>>> import pygmt
>>> # Load a grid of Earth relief data
>>> grid = pygmt.datasets.load_earth_relief()
>>> # Create a filtered grid from an input grid.
>>> filtered_grid = pygmt.dimfilter(
...     grid=grid,
...     # Set filter type to "median" and the diameter width to 600 km
...     filter="m600",
...     # Set grid in degrees, width in km
...     distance=4,
...     # Create 6 sectors and return the lowest values in the sector
...     sectors="l6",
...     # Set the region longitude range from 55W to 51W, and the
...     # latitude range from 24S to 19S
...     region=[-55, -51, -24, -19],
... )