"""
Function to download the Earth magnetic anomaly datasets from the GMT data server, and
load as :class:`xarray.DataArray`.
The grids are available in various resolutions.
"""
from collections.abc import Sequence
from typing import Literal
import xarray as xr
from pygmt.datasets.load_remote_dataset import _load_remote_dataset
from pygmt.exceptions import GMTValueError
__doctest_skip__ = ["load_earth_magnetic_anomaly"]
[docs]
def load_earth_magnetic_anomaly(
resolution: Literal[
"01d", "30m", "20m", "15m", "10m", "06m", "05m", "04m", "03m", "02m"
] = "01d",
region: Sequence[float] | str | None = None,
registration: Literal["gridline", "pixel", None] = None,
data_source: Literal["emag2", "emag2_4km", "wdmam"] = "emag2",
) -> xr.DataArray:
r"""
Load the Earth magnetic anomaly datasets in various resolutions.
.. list-table::
:widths: 50 50
:header-rows: 1
* - Earth Magnetic Anomaly Model (EMAG2)
- World Digital Magnetic Anomaly Map (WDMAM)
* - .. figure:: https://www.generic-mapping-tools.org/remote-datasets/_images/GMT_earth_mag.jpg
- .. figure:: https://www.generic-mapping-tools.org/remote-datasets/_images/GMT_earth_wdmam.jpg
This function downloads the dataset from the GMT data server, caches it in a user
data directory (usually ``~/.gmt/server/earth/earth_mag/``,
``~/.gmt/server/earth/earth_mag4km/``, ``~/.gmt/server/earth/earth_wdmam/``), and
load the dataset as an :class:`xarray.DataArray`. An internet connection is required
the first time around, but subsequent calls will load the dataset from the local
data directory.
The dataset can also be accessed by specifying a file name in any grid processing
function or plotting method, using the following file name format:
**@**\ *earth_mag_type*\_\ *res*\_\ *reg*. *earth_mag_type* is the GMT name for the
dataset. The available options are **earth_mag**, **earth_mag4km**, and
**earth_wdmam**; *res* is the grid resolution; *reg* is the grid
registration type (**p** for pixel registration, **g** for gridline registration).
If *reg* is omitted (e.g., ``@earth_mag_01d``), the gridline-registered grid will be
loaded for grid processing functions and the pixel-registered grid will be loaded
for plotting functions. If *res* is also omitted (i.e., ``@earth_mag``), GMT
automatically selects a suitable resolution based on the current region and
projection settings.
The default color palette tables (CPTs) for this dataset are ``@earth_mag.cpt`` for
``data_source="emag2"`` and ``data_source="emag2_4km"``, and ``@earth_wdmam.cpt``
for ``data_source="wdmam"``. To use the dataset-specific CPT when plotting the
dataset, explicitly set ``cmap="@earth_mag.cpt"`` or ``cmap="@earth_wdmam.cpt"``,
otherwise GMT's default CPT (*turbo*) will be used. If the dataset is referenced by
the file name in a grid plotting method, the dataset-specific CPT file is used
automatically unless another CPT is specified.
Refer to :gmt-datasets:`earth-mag.html` and :gmt-datasets:`earth-wdmam.html` for
more details about available datasets, including version information and references.
Parameters
----------
resolution
The grid resolution. The suffix ``d`` and ``m`` stand for arc-degrees and
arc-minutes. The resolution ``"02m"`` is not available for
``data_source="wdmam"``.
region
The subregion of the grid to load, in the form of a sequence [*xmin*, *xmax*,
*ymin*, *ymax*] or an ISO country code. Required for grids with resolutions
higher than 5 arc-minutes (i.e., ``"05m"``).
registration
Grid registration type. Either ``"pixel"`` for pixel registration or
``"gridline"`` for gridline registration. Default is ``None``, which means
``"gridline"`` for all resolutions except ``"02m"`` for
``data_source="emag2"`` or ``data_source="emag2_4km"``, which are ``"pixel"``
only.
data_source
Select the source of the magnetic anomaly data. Available options are:
- ``"emag2"``: EMAG2 Earth Magnetic Anomaly Model. It only includes
data observed at sea level over oceanic regions.
See :gmt-datasets:`earth-mag.html`.
- ``"emag2_4km"``: Use a version of EMAG2 where all observations
are relative to an altitude of 4 km above the geoid and include
data over land.
- ``"wdmam"``: World Digital Magnetic Anomaly Map (WDMAM).
See :gmt-datasets:`earth-wdmam.html`.
Returns
-------
grid
The Earth magnetic anomaly grid. Coordinates are latitude and
longitude in degrees. Units are in nano Tesla (nT).
Note
----
The registration and coordinate system type of the returned
:class:`xarray.DataArray` grid can be accessed via the *gmt* accessor. Refer to
:class:`pygmt.GMTDataArrayAccessor` for detailed explanations and limitations.
Examples
--------
>>> from pygmt.datasets import load_earth_magnetic_anomaly
>>> # Load the default grid (gridline-registered 1 arc-degree grid)
>>> grid = load_earth_magnetic_anomaly()
>>> # Load the 30 arc-minutes grid with "gridline" registration
>>> grid = load_earth_magnetic_anomaly(resolution="30m", registration="gridline")
>>> # Load high-resolution (5 arc-minutes) grid for a specific region
>>> grid = load_earth_magnetic_anomaly(
... resolution="05m",
... region=[120, 160, 30, 60],
... registration="gridline",
... )
>>> # Load the 20 arc-minutes grid of the emag2_4km dataset
>>> grid = load_earth_magnetic_anomaly(
... resolution="20m", registration="gridline", data_source="emag2_4km"
... )
>>> # Load the 20 arc-minutes grid of the WDMAM dataset
>>> grid = load_earth_magnetic_anomaly(
... resolution="20m", registration="gridline", data_source="wdmam"
... )
"""
# Map data source to prefix
prefix = {
"emag2": "earth_mag",
"emag2_4km": "earth_mag4km",
"wdmam": "earth_wdmam",
}.get(data_source)
if prefix is None:
raise GMTValueError(
data_source,
description="earth magnetic anomaly data source",
choices=["emag2", "emag2_4km", "wdmam"],
)
grid = _load_remote_dataset(
name="earth_wdmam" if data_source == "wdmam" else "earth_mag",
prefix=prefix,
resolution=resolution,
region=region,
registration=registration,
)
return grid