Cache the 03s earth relief tile @N35E135.earth_relief_03s_g.nc

[core-man]

Description of proposed changes

Originally posted in #976 (comment).

33.5MB https://github.com/GenericMappingTools/pygmt/suites/2150973318/artifacts/44049313

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 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 commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

😕 I even didn't know that we can use gmt which -Ga @N35E135.earth_relief_03s_g.nc to download a single tile.

[seisman]

@PaulWessel It seems we can use the following command to download a tile of 03s earth relief grid:

gmt which -Ga @N35E135.earth_relief_03s_g.nc

I never saw it in the GMT documentation. Is it an undocumented feature? Is it the recommended way to download one tile?

I don't understand why it works, since the tiles are stored in jp2 format on the GMT server.

[PaulWessel]

Certainly undocumented since we don't really want to tell people to get specific tiles, except indirectly via a -R setting. But it works as it does because the filename matches a known tile and off it goes. It knows what to do with these (jp2->nc).

I think we should keep quiet about this because we may decide to change our tiling structure etc. It should not be something users should be aware of.