New module: stereonet for plotting Schmidt/Wulff stereonets (structural geology)

[Esteban82]

Description of the desired feature

Structural geologists routinely plot fault/fracture planes and lineations on a stereonet — the projection of one hemisphere used to display the orientation of planes (as strike/dip or dip direction/dip) and lines (as trend/plunge). GMT can already produce the net itself with the existing azimuthal projections (-JA for the equal-area Schmidt net, -JS for the equal-angle Wulff net, see the Cookbook figure GMT_stereonets), but there is no built-in way to convert structural measurements into the coordinates those projections expect, or to draw the cyclographic trace of a plane.

This was discussed at length on the forum: https://forum.generic-mapping-tools.org/t/plot-a-schmidt-stereonet/6067

Status

I'm working on this — a new geology supplement with a stereonet module
(psstereonet in classic mode). Current scope:

• -JA/-JS (Schmidt/Wulff net), centered on 0/0, reusing GMT's existing
  azimuthal projections rather than inventing a new one
• -T selects what the input angles mean: strike/dip (right-hand rule),
  dip direction/dip, or trend/plunge of a line; +u for the upper hemisphere
• Draws the cyclographic trace of each plane (-W) and/or the pole or line
  (-S), colorable via -C
• -A for the azimuth ring around the perimeter
• -M to dump the converted lon/lat instead of plotting, for building custom
  figures

Opening this issue mainly so the community is aware work is underway, and to
get early feedback on naming/scope/options before a PR — happy to hear
opinions, especially from other structural geologists.

Are you willing to help implement and maintain this feature? Yes

[rhum1-geo]

Thanks! This module is going to be really appreciated. Exciting to see a GMT stereonet module coming!
A few feature suggestions that could be useful:

• D[k|e|s] for density plots, allowing the user to choose between Kamb (Kamb, 1959), Exponential Kamb (Vollmer, 1995), and Schmidt (no ref for that one) contouring. Perhaps modifiers such as +s for the sigma value (default = 2? 3?), +wstep for the contour interval and pen attributes, and maybe +c to fill the contours using a CPT file.
• -F for Fisher statistics, to report the mean plane and/or mean pole from an input dataset.
• This is not really an option, or maybe it could be handled through -T but it would be great if the module could recognize different orientation notations and convert them to RHR automatically. For example, 030/36W would be converted to 210/36 in RHR notation.

For pole plots, it would be nice if -S allowed the user to choose the plotting symbol, similar to gmt plot. For line plots, having access to vector attributes as well, again following the gmt plot syntax, could also be useful.

There is already a lot of work done in Python for stereonets, with packages such as mplstereonet or APSG, which could provide some inspiration for the options and features of the GMT stereonet module.

These are just a few ideas that come to mind as I work on different projects. I'll probably come back with more suggestions as I go!

[joa-quim]

We should very careful with inclusion of large chunks of code produced only by AI. And only top models (basically only Opus and above) should be used in its design. A module like this should be the most self contained as possible.