qProf plugin help

The purpose of this plugin is to help create geological profiles. Georeferenced data describing topography, geological outcrops and attitudes can be used.

This plugin was realized by M. Alberti (www.malg.eu) and M. Zanieri.
The original concept is by M. Zanieri, while the implementation is by M. Alberti.
Some people contributed with their suggestions and testing, among them we want to thank S. Peduzzi, for his vigorous testing.

A few posts describing the plugin are:

In Italian:

• qProf: un nuovo plugin QuantumGIS per la creazione di profili topografici (2013-03-16)
• Qprof (2015-06-04)

In English:

• Plotting geological planes in profiles with Quantum GIS (2013-10-26)
• qProf supports on-the-fly reprojection and multiples profile lines (2014-03-02)
• Construction of geological cross sections in QGIS (2016-03-01)

In French:

• Plugin qProf - Qgis (2013-04-02)

To construct a geological profile, the first step consists in the construction of a topographic profile, with input data represented by one or more DEMs.

While for constructing topographic profiles lines made up by more than just two points may be used, for further geological projections or intersections it is required that the profile line is made up by just two points, i.e., its start and end point. This constraint will probably be relaexed in future versions at least for geological intersection processings.

Subsequently, for profiles created by lines with jst two points, we can process geological data, expressed by point, line or polygon layers with useful geological attributes stored in the their attribute tables.
The subsequent processings can be:

• the projection of geological attitudes on the profile section;
• the projection of geological lines;
• the intersection of geological lines (e.g., faults) on the profile section;
• the intersection of geological polygonal elements (e.g., outcrops) on the profile section.

Last step is the export of the produced datasets as graphics and/or GIS data, for further elaborations in graphic or geological software.

It is possible to create a topographic profile in two ways:

1. from one or more DEMs and a line element, digitized or stored in a layer;
2. directly from a GPX file storing track points.

In the former case, data sources can be projected in different CRS, while the created profile will be in the assigned project CRS.
In the latter case, i.e. when creating a profile from a GPX file, the original latitude-longitude values are automatically converted to cartesian metric values, and from those the topographic profile is constructed.

One or more DEM(s) sources have to be defined, as well a source profile line. The line can derive from a source line layer, a on-the-fly-digitized line or a list of points.

When using a line layer, all the elements will be used, irrespective of eventual selections in the layer. Multiple lines will be merged into a single line, based on the chosen order field when available, or otherwise based on the internal line order. Some errors in derived profiles can be due to erroneous line ordering, not adjusted via the choice of a correct sequential order stored in an integer field (order values start from 1).

When directly digitizing a line in the map, you add points wtih left clicks, and stops a line with a right click.

It is also possible to define the profile as a list point coordinates. Each point is defined by a x-y coordinates pair, comma-separated, on a single row, for instance:
549242.7, 242942.2
578370.3, 322634.5

Fig. 1. The plugin interface for topographic profile creation from DEM(s).

The elevation statistics along the profile for each chosen DEM may be calculate with the "Calculate profile statistics" button. These statistics can be used as a help in the definition of the profile plot minimum and maximum elevations ("plot z min value" and "plot z max value" options). Otherwise, automatic values will be applied.

You may define the "Line densify distance" option, i.e. the distance between consecutive sampling point automatically added when the original vertices of the used path have distances larger than this value. It is advisable to use a value comparable to the resolution of the used DEM, for instance 30 m for Aster DEMs. A suggested value is added automatically based on the resolution of the DEM with the highest spatial resolution.

By using "Invert profile line orientation", the profile line will have its orientation inverted, i.e., the start will become the end and the former end will become the profile start.
With the option "Flip x-axis direction", the start of the x-axis in the plot will be at the right, not at the left, so that distances will increase from left to right.

The profile plot is created by using the "Create profile" button. Sometimes the plot creation may require more than just a few seconds, so please be patient... ;)

Fig. 2. Topographic profile extracted in the eastern border of Mt. Alpi zone (southern Italy).

Topographic profiles can be created also using a GPX file as a source. Beware that in the current version, only profile lines with just two points (start and end points) may be used for further geological data projections and intersections. So if you just want to create a topographic profile, GPX profile sources are fine. In future releases, the only-two-points constraint for geological intersections should be removed.

Fig. 3. The plugin interface for topographic profile creation from GPX files.

The source for geological attitudes is a point layer. Only selected points will be projected, unless, in case of no point selection, all layer points will be projected.

Required fields are the geological point id and its surface orientation, expressed by dip direction and dip angle values.

Fig. 4. The plugin interface for geological attitude projections on the profile.

The geological attitudes can be projected on the section plane according to three methods:

1. nearest point;
2. projection along a common axis;
3. projection along individual axes, for each geological record.

When choosing the axes from option, trend and plunge fields, storing the fold axis values along which to project each observation, are required in the source point layer.

Fig. 5. Example of geological attitudes projection along a profile in the eastern sector of Mt. Alpi zone (Basilicata, Southern Italy).

Geological traces can be projected on the section plane, based on a fold axis for which trend and plunge values have to be defined.

The "Line densify distance" option describes the distance used to densify the lines to plot, when original points are separated by distances larger than the densify distance.

Fig. 6. The plugin interface for geological lines projections on the profile.

The "Id field is used to label the project lines in the resulting plot, as evident in Fig. 7.

Fig. 7. Example of geological line projections on a profile in the Mt. Alpi zone (Basilicata, Southern Italy).

It is possible to determine the intersections of the profile (composed by just two points) with lines ("Intersect line layer") or with polygons ("Intersect polygon layer"), representing geological features.

An Id field and a Classification field can be provided, as an aid in plot visualization.

Previously created intersection can be removed by using the "Reset intersections" button.

Fig. 8. Example of geological line intersection on a profile in the Mt. Alpi zone (Basilicata, Southern Italy).

Fig. 9. Example of geological polygon intersection on a profile in the Mt. Alpi zone (Basilicata, Southern Italy).

The last created figure can be saved as PDF, svg or tif. The export graphic parameters can be saved in a text file and loaded for applying them to further plots.

Result data can be exported ad point/line shapefiles and/or csv files, depending on the particular type of exported data. The exported data will have the same CRS as the current QGis project.

Note: the slope is saved as relative (positive when upward, negative when downward), even if plotted as absolute. To change to absolute value, apply the absolute function on the relative field in a GIS or spreadsheet software.

Fig. 10. The plugin interface for result export.

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Doc version 2016-04-17, by Mauro Alberti - alberti.m65@gmail.com