Add a gallery example for plotting Cartesian, circular and geographic vectors

[kmaterna]

Contributing a gallery plot with vectors on a map. Feel free to make any and all changes. I thought it would be good to have vector example since not everyone knows the new 'plot' options in GMT6.

[kmaterna]

/format

[weiji14]

Hi @kmaterna, welcome back! Appreciate your Pull Request, just wanted to note that we added a gallery example for vectors yesterday (#890) at https://www.pygmt.org/dev/gallery/line/vector-heads-tails.html#sphx-glr-gallery-line-vector-heads-tails-py. So sorry if you missed that!

If you want, you can take a look at https://github.com/GenericMappingTools/pygmt/blob/master/examples/gallery/line/vector-heads-tails.py and see if there's anything you would like to add to that example. Or, you can make a full tutorial for plotting vectors instead (see #892). Ping @michaelgrund who might be interested in working together on a full vector plotting tutorial.

[weiji14]

Actually, forget what I said above. Issue #892 also mentions we should add a tutorial for "How to plot Cartesian, circular and geographic vectors/arrows". Should be good to focus on that, I see your example covers plotting geographic arrows over continental US.

[kmaterna]

Oh wow, that's wonderful. Funny timing. I missed that. Feel free to close the PR, looks like the tutorial has it all covered.

The only thing that might be nice is the +z option with direction=[dx,dy] for us geodesists who didn't know about all of PYGMT's features. Definitely not urgent. I might play around and try another PR later. Thanks to Michael for the nice demo.

[kmaterna]

Yes, or keep. Up to maintainers. +1 for the idea of an example with cartesian vectors.

[weiji14]

@michaelgrund might be able to advice. I'd say keep this open for a while, it's easier to modify an already open Pull Request than to start a new one!

[michaelgrund]

Oh wow, that's wonderful. Funny timing. I missed that. Feel free to close the PR, looks like the tutorial has it all covered.

The only thing that might be nice is the +z option with direction=[dx,dy] for us geodesists who didn't know about all of PYGMT's features. Definitely not urgent. I might play around and try another PR later. Thanks to Michael for the nice demo.

Hey @kmaterna, great that you work on this. Besides the example provided in https://github.com/GenericMappingTools/pygmt/blob/master/examples/gallery/line/vector-heads-tails.py (which only shows different options for heads and tails) in #892 we decided to add another gallery example showing cartesian, circular and geographic vectors. So you covered 1/3 😉 Maybe you can add the two remaining ones in any way? Here's some content I pre-produced for the circular vectors (not comitted so far) you may add:

import pygmt
import numpy as np

fig = pygmt.Figure()
fig.basemap(region=[0, 6, -15, 15], projection="X15c/10c", frame=True)

# plot math angle arcs with different radii
x=3
y=0
startdir=90
stopdir=180
radius=4
pen="1.5p,black",
xtext=24.5

arcstyles = np.repeat("m0.5c+ea", 7)

for arcstyle in arcstyles:
    data = np.array([[x, y, radius, startdir, stopdir]])
    fig.plot(data=data, 
             style=arcstyle, 
             color="red3", 
             pen=pen)
    
    stopdir+=40
    radius-=0.5 

fig.show()

Additonally, a full tutorial on vectors is planned.

[kmaterna]

Nice, I like those circular vectors. By cartesian, do you mean regular vectors (i.e., an x-y plot with vectors given by magnitude/angle)? They're no different from geographic vectors except they're not on maps, right? I can work on something that shows all 3.

[seisman]

By cartesian, do you mean regular vectors (i.e., an x-y plot with vectors given by magnitude/angle)? They're no different from geographic vectors except they're not on maps, right?

I think what you're plotting (using style="v") is Cartesian vectors, although they're plotted on maps.

Geographic vectors (using style="=") should be vectors along great circle paths.

See https://docs.generic-mapping-tools.org/latest/cookbook/features.html#the-drawing-of-vectors for details.

[kmaterna]

/format

[kmaterna]

I made an example with circular/cartesian/geographic vectors - you guys can decide whether you think it's appropriate for the gallery. Output looks like this.

[seisman]

Nice example!

[michaelgrund]

Made some minor suggestions, all in all the example looks good to me.

[kmaterna]

/format

[seisman]

The example looks pretty good and is close to merging.

@GenericMappingTools/python @GenericMappingTools/python-maintainers Do you have any further comments?

[weiji14]

This is coming along really nicely. Just a few recommendations on documenting what the various modifiers in the style string represent (e.g. +e, etc).

[kmaterna]

/format

[weiji14]

Thanks @kmaterna for this amazing example (and to @michaelgrund and @seisman for the helpful review suggestions)!

Feel free to suggest any new ideas if you have any, and do consider adding your name to AUTHORS.md when you have time 😄