"""
An xarray backend for reading raster grid/image files using the 'gmt' engine.
"""
from collections.abc import Sequence
from typing import Literal
import xarray as xr
from packaging.version import Version
from pygmt._typing import PathLike
from pygmt.clib import Session
from pygmt.exceptions import GMTValueError
from pygmt.helpers import build_arg_list, kwargs_to_strings
from pygmt.src.which import which
from xarray.backends import BackendEntrypoint
# TODO(xarray>=2025.10.1): Remove the __doctest_skip__ on GMTBackendEntrypoint
__doctest_skip__ = (
["GMTBackendEntrypoint"] if Version(xr.__version__) < Version("2025.10.1") else []
)
[docs]
class GMTBackendEntrypoint(BackendEntrypoint):
"""
Xarray backend to read raster grid/image files using 'gmt' engine.
Internally, GMT uses the netCDF C library to read netCDF files, and GDAL for GeoTIFF
and other raster formats. See :gmt-docs:`reference/features.html#grid-file-format`
for more details about supported formats. This GMT engine can also read
:gmt-docs:`GMT remote datasets <datasets/remote-data.html>` (file names starting
with an `@`) directly, and pre-loads :class:`pygmt.GMTDataArrayAccessor` properties
(in the '.gmt' accessor) for easy access to GMT-specific metadata and features.
When using :py:func:`xarray.open_dataarray` or :py:func:`xarray.load_dataarray` with
``engine="gmt"``, the ``raster_kind`` parameter is required and can be either:
- ``"grid"``: for reading single-band raster grids
- ``"image"``: for reading multi-band raster images
Optionally, you can pass in a ``region`` in the form of a sequence [*xmin*, *xmax*,
*ymin*, *ymax*] or an ISO country code.
Examples
--------
Read a single-band netCDF file using ``raster_kind="grid"``
>>> import pygmt
>>> import xarray as xr
>>>
>>> da_grid = xr.open_dataarray(
... "@static_earth_relief.nc", engine="gmt", raster_kind="grid"
... )
>>> da_grid # doctest: +NORMALIZE_WHITESPACE
<xarray.DataArray 'z' (lat: 14, lon: 8)> Size: 448B
[112 values with dtype=float32]
Coordinates:
* lat (lat) float64 112B -23.5 -22.5 -21.5 -20.5 ... -12.5 -11.5 -10.5
* lon (lon) float64 64B -54.5 -53.5 -52.5 -51.5 -50.5 -49.5 -48.5 -47.5
Attributes:...
Conventions: CF-1.7
title: Produced by grdcut
history: grdcut @earth_relief_01d_p -R-55/-47/-24/-10 -Gstatic_eart...
description: Reduced by Gaussian Cartesian filtering (111.2 km fullwidt...
actual_range: [190. 981.]
long_name: elevation (m)
Read a multi-band GeoTIFF file using ``raster_kind="image"``
>>> da_image = xr.open_dataarray(
... "@earth_night_01d", engine="gmt", raster_kind="image"
... )
>>> da_image # doctest: +NORMALIZE_WHITESPACE
<xarray.DataArray 'z' (band: 3, y: 180, x: 360)> Size: 194kB
[194400 values with dtype=uint8]
Coordinates:
* band (band) uint8... 1 2 3
* y (y) float64... 89.5 88.5 87.5 86.5 ... -86.5 -87.5 -88.5 -89.5
* x (x) float64... -179.5 -178.5 -177.5 -176.5 ... 177.5 178.5 179.5
Attributes:...
long_name: z
Load a single-band netCDF file using ``raster_kind="grid"`` over a bounding box
region.
>>> da_grid = xr.load_dataarray(
... "@tut_bathy.nc", engine="gmt", raster_kind="grid", region=[-64, -62, 32, 33]
... )
>>> da_grid
<xarray.DataArray 'z' (lat: 13, lon: 25)> Size: 1kB
array([[-4369., -4587., -4469., -4409., -4587., -4505., -4403., -4405.,
-4466., -4595., -4609., -4608., -4606., -4607., -4607., -4597.,
...
-4667., -4642., -4677., -4795., -4797., -4800., -4803., -4818.,
-4820.]], dtype=float32)
Coordinates:
* lat (lat) float64 104B 32.0 32.08 32.17 32.25 ... 32.83 32.92 33.0
* lon (lon) float64 200B -64.0 -63.92 -63.83 ... -62.17 -62.08 -62.0
Attributes:...
Conventions: CF-1.7
title: ETOPO5 global topography
history: grdreformat -fg bermuda.grd bermuda.nc=ns
description: /home/elepaio5/data/grids/etopo5.i2
actual_range: [-4968. -4315.]
long_name: Topography
units: m
"""
description = "Open raster (.grd, .nc or .tif) files in Xarray via GMT."
open_dataset_parameters = ("filename_or_obj", "raster_kind", "region")
url = "https://pygmt.org/dev/api/generated/pygmt.GMTBackendEntrypoint.html"
[docs]
@kwargs_to_strings(region="sequence")
def open_dataset( # type: ignore[override]
self,
filename_or_obj: PathLike,
*,
drop_variables=None, # noqa: ARG002
raster_kind: Literal["grid", "image"],
region: Sequence[float] | str | None = None,
# other backend specific keyword arguments
# `chunks` and `cache` DO NOT go here, they are handled by xarray
) -> xr.Dataset:
"""
Backend open_dataset method used by Xarray in :py:func:`~xarray.open_dataset`.
Parameters
----------
filename_or_obj
File path to a netCDF (.nc), GeoTIFF (.tif) or other grid/image file format
that can be read by GMT via the netCDF or GDAL C libraries. See also
:gmt-docs:`reference/features.html#grid-file-format`.
raster_kind
Whether to read the file as a "grid" (single-band) or "image" (multi-band).
region
The subregion of the grid or image to load, in the form of a sequence
[*xmin*, *xmax*, *ymin*, *ymax*] or an ISO country code.
"""
if raster_kind not in {"grid", "image"}:
raise GMTValueError(
raster_kind, description="raster kind", choices=["grid", "image"]
)
with Session() as lib:
with lib.virtualfile_out(kind=raster_kind) as voutfile:
kwdict = {"R": region, "T": {"grid": "g", "image": "i"}[raster_kind]}
lib.call_module(
module="read",
args=[filename_or_obj, voutfile, *build_arg_list(kwdict)],
)
raster: xr.DataArray = lib.virtualfile_to_raster(
vfname=voutfile, kind=raster_kind
)
# Add "source" encoding
source: str | list = which(fname=filename_or_obj, verbose="quiet")
raster.encoding["source"] = (
sorted(source)[0] if isinstance(source, list) else source
)
return raster.to_dataset()