Matrix grid doesn't always work with different central meridians

[seisman]

Description of the problem

The issue was first reported in GenericMappingTools/pygmt#390, but was thought to be fixed by GMT PRs #3813 and #3829. The GMT master branch works for some projection parameters but doesn't always work.

Full script that generated the error

import pygmt
fig = pygmt.Figure()

fig.grdimage("@earth_relief_01d_g", projection="Q123/0/15c", cmap="geo")

fig.shift_origin(yshift="8c")

dem = pygmt.datasets.load_earth_relief(resolution="01d", registration="gridline")
fig.grdimage(dem, projection="Q123/0/15c", cmap="geo")
fig.savefig("map.pdf")

Output

Image difference

In the PyGMT, we test several different projection parameters. Here are the ones that are known to fail:

• Q123/0/15c
• Q123/30/15c
• Q180/0/15c
• Q180/30/15c

Here are the ones that are known to work:

• Q0/0/15c
• Q0/30/15c

The same parameters work well for General Stereographic projection (i.e., changing Q to S).

Note: Those tests were originally written by @weiji14.

[PaulWessel]

Is this a gridline registration issue only or is it the same for pixel as well?

[seisman]

Same issue for pixel-registered grids (just tried Q123/0/15c).

[PaulWessel]

And just to be absolutely clear, the plot on the bottom is what you expect? That rougher plot is what I get with long=0 which you said works, so just checking.

[seisman]

And just to be absolutely clear, the plot on the bottom is what you expect?

Yes, the bottom one uses the earth relief grid file, which gives the same plot as GMT CLI.

[PaulWessel]

So what happens is this: We have a read-only grid and it has a certain central meridian. This kicks in:

if (got_z_grid && (gmt_whole_earth (GMT, Grid_orig->header->wesn, wesn) == 1))
	need_to_project = true;	/* This can only happen if reading a remote global geographic grid and the central meridian differs from that of the projection */

So now we go the way of resampling the images - hence the smoother apparance. The other image is not resampled, just plotted, but handles the change in longitude. I will see if I can make it work the same. You would think that should be doable, but the big difference is that the file is read in and the matrix is rotated to fit the -R while in the xarray case the pixels are still -180/180. So not the same case.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 90 days. It will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 90 days. It will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[joa-quim]

What is the status of this? Mane related changes have been committed meanwhile.

[seisman]

I'll find some time to check it.