Plotting geological attitudes in stereonets using QGIS

A new plugin for QGIS, geocouche, allows to plot geological data attitudes stored in stereonets, using the plotting utilities provided by the apsg module by Ondrej Lexa. Data can be stored in geological layers or entered as text, and plotted as great circles or axis. The current release, as of April, 4th,  is vers. 1.0 and  can be downloaded from: https://github.com/mauroalberti/geocouche/releases

How it works? The following description derives from the tool help. 

With the current geocouche  release, it is possible:
  1. to calculate the angles between planes stored in a layer and a reference plane
  2. to plot data from layers or texts in stereonets
These two tools are available from the QGIS plugin interface.

Fig. 1. geocouche interface.


Geological angles

This tool allows to calculate the angles (as degrees) between a reference plane and the (eventually selected) features in a point layer (Fig. 2).

Fig. 2. Geological angles calculation interface.
It can be applied to determine the degree of misalignement between a reference (for instance, regional) measure and local geological measures.

Definition of parameters

The user has to define the two fields storing the azimuth (dip direction or RHR strike) and the dip angle of each feature, the attitude of the reference plane, and the name of the output shapefile with a new field storing the calculated angle (Fig. 3).

Fig. 3. Definition of parameters for angle calculation.

Geological stereonets

It allows to produce stereonets depicting geological plane and axis attitudes (Fig. 4). There are three steps:
  1. Choice of input data
  2. Definition of plot style
  3. Data plotting

Fig. 4. Stereonet interface.
Choice of input data
It is possible to use input data from data stored in a point layer (Layer tab, Fig. 5) or to use text input (Text tab, Fig. 6).
Input from point layer
When using a point layer (already loaded in the TOC), plane and/or axis attitudes are defined via the fields storing their values (Fig. 5). When a selection is defined, only the selected features will be considered.
Fig. 5. Input from point layer interface.

Input from text
The input can be inserted into a text window (Fig. 6), defining if data consist of:
  • planes
  • axes
  • planes and axes
Fig. 6. Input from text interface.

Another option to take care of, is,  for plane data, whether orientations are expressed using dip direction or RHR strike.
Plot style
Styles can be defined for both great circles and poles: color, width/size, line/marker style, and transparency (Fig. 7). Settings are stored in memory.
Fig. 7. Plot style interface.

Stereonet plotting
Plots can use a new or a pre-existing stereonet (provided it has not been previously closed) (Fig. 8).
Fig. 8. Stereonet plot interface.

Planes can be plotted as great circles or as plane normals, axes as poles or as normal great circles. An example of stereonet is shown in Fig. 9.
Fig. 9. Stereonet example.


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