Remote datasets: Add load_earth_deflection to load "IGPP Earth east-west and north-south deflection" datasets

[yvonnefroehlich]

Description of proposed changes

Add function load_earth_deflection to download the "IGPP Earth east-west and north-south deflection" datasets.

• Official documentation:
  • https://www.generic-mapping-tools.org/remote-datasets/earth-edefl.html
  • https://www.generic-mapping-tools.org/remote-datasets/earth-ndefl.html
• Recipe:
  • https://github.com/GenericMappingTools/gmtserver-admin/blob/master/recipes/earth_edefl.recipe
  • https://github.com/GenericMappingTools/gmtserver-admin/blob/master/recipes/earth_ndefl.recipe

Adresses #2431

Preview: https://pygmt-dev--3728.org.readthedocs.build/en/3728/api/generated/pygmt.datasets.load_earth_deflection.html

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash command is:

• /format: automatically format and lint the code

[yvonnefroehlich]

If people have a more suitable name for this new parameter, feel free to make suggestions. I just picked the "east-west deflection" dataset as default.

[seisman]

I think the parameter is OK, but for the arguments, "edefl"/"ndefl" is not readable. Maybe "e"/"n" or "east"/"north" or "east_west"/"north_south", but I have to admit that I have no idea about the physical meaning of this dataset

[yvonnefroehlich]

See comment #3728 (comment).

[seisman]

I think it should be "north_south" rather than "south_north", mainly because:

1. East and North are pointing to the positive values
2. The GMT dataset names are "edefl" and "ndefl".
3. The original source files are named as "east_xxx" and "north_xxx"

Suggested change

	direction: Literal["east_west", "south_north"] = "east_west",
	direction: Literal["east_west", "north_south"] = "east_west",

I think we should also fix "south-north" to "north-south" in the remote-dataset page.

[yvonnefroehlich]

Yes, I am also a bit wondering about the dataset names and coordinate system axes directions, and currently looking at in the literature about this naming.

[yvonnefroehlich]

Just saw that the remote dataset docs are actually not consistent, and both "south-north"
(title and image describtion https://www.generic-mapping-tools.org/remote-datasets/earth-ndefl.html#igpp-earth-south-north-deflections) and "north-south" (in section usage https://www.generic-mapping-tools.org/remote-datasets/earth-ndefl.html#usage) are used.

Wikipedia says is's "east-west" and "north-south" (https://en.wikipedia.org/wiki/Vertical_deflection) and here https://geodesy.noaa.gov/GEOID/GSVS/deflection-vertical.shtml the deflection of the vertical is described via "east" and "north" components. So I think, as suggested by @seisman, we should use "north-south" and update the remote dataset docs. For the PyGMT side, I am wondering, maybe component is better than direction for the parameter name.

[yvonnefroehlich]

Maybe we should at a note in the docs, that positive values mean a deflection to the east (north) and negative to the west (south) for the edefl (ndefl) dataset; similar as we did it for the "distance to shoreline" dataset.

east-west (edefl)	north-south (ndefl)

[seisman]

For the PyGMT side, I am wondering, maybe component is better than direction for the parameter name.

Yes, I think so.

Maybe we should at a note in the docs, that positive values mean a deflection to the east (north) and negative to the west (south) for the edefl (ndefl) dataset; similar as we did it for the "distance to shoreline" dataset.

Sounds good.

[seisman]

The CI failures on macOS are unrelated to changes in this PR and will be tracked in #3732.

Plan to merge this PR tomorrow if there are no further comments.