geo-script: Plot of geological attitudes and traces in vertical sections along fold axes with Quantum GIS and qProf

Sunday, 25 May 2014

Plot of geological attitudes and traces in vertical sections along fold axes with Quantum GIS and qProf

The new released version of qProf (0.2.4), a Python plugin for Quantum GIS, allows to project geological attitudes and traces onto a vertical section along fold axes. On-the-fly reprojection of layers is supported.

The first step in using this plugin is creating a topographic profile by defining one (or more) source DEMs and a line layer that define the section, plus a line densify distance (generally of the order of the used DEM cell resolution) and an optional line order field (required only when the input geometry of the line layer is not sufficiently simple, see previous posts on the qProf plugin).
While for creating only topographic profiles the input line can be made up of many points and more than one DEM can be used, for geological projections the input line must be made up by just two points and only one DEM may be selected.
After having defined and created the profile using the tools in the "profile" tab, it is then possible to project geological attitudes or traces by using the commands in the "Project" tab.

Projection of geological attitudes

Geological attitudes will be stored in a point layer, with the measured dip direction and dip angle of the features in two distinct fields of the attribute table.
The commands are available from the "Geological attitudes" subwindow.
The projection of geological attitudes may be applied via different methodologies: (a) the nearest intersection point between the geological plane of a feature and the section plane; (b) using a common projection ("fold") axis for all features; (c) using individual axes for each attitude, with two numeric fields storing trend and plunge values.
If some data are selected, only those selected will be plotted, otherwise all layer features will be used. Labels can also be added to projected measures.
It is possible to cumulate in a single analysis session more sub-analyses, distinguishing them via different colors. Previous sub-analyses can be removed with the 'Reset geological attitudes' button.
Data can be saved as csv files or shapefiles.

A plot example, with attitudes labelled by original trend and plunge values, is in the figure below.

Projection of line traces

Line traces can represent the intersection of geological surfaces with the topography. When these surfaces are cylindrically folded, it is possible to projected the traces onto the section via a user-defined fold axis. The elevation information will be derived from the chosen DEM.
The commands are available from the "Geological traces" subwindow.
A line layer has to be chosen, with the name of the id field and a value for the line densification.
Trend and plunge of the projection axis will be defined, and a color chosen for the line plotting.
Different subanalyses, with different projection axes, can be differentiated by using different colors.
Previous analyses can be eliminated with the "Reset traces" command.
Results can be saved as text file, storing id-s-z values (where s is the horizontal distance from the section start) for each resulting point.

An example of the output is below. In this case, only geological traces are plotted. In general, both geological traces and attitudes can be plotted in the same section.