By coupling mapping or remote sensing information with topographic data, this plugin allows to estimate the attitude
of the best-fit-plane to a set of points, lines or polygon geometries lying on a topographic surface (example in Fig. 1).
This is a common task in geological mapping and analysis.
As an example, for the points option, place these as a cluster on a structural surface such as a dip-slope, while
for the line or polygon options, this typically could be the trace of a resistant geological unit forming an escarpment.
Fig. 1. Example of estimation of a bedding attitude in the Mt. Alpi area (Basilicata, Southern Italy).
Input data
Elevation information is extracted from a Digital Elevation Model (DEM).
Points can be digitized by the user directly in the map canvas
or, alternatively, loaded from an existing point/line/polygon layer.
IMPORTANT: note that when the CRS of the source DEM is in polar coordinates (i.e., latitude and longitude) or its vertical distance unit is
different from the horizontal one (e.g., meters for the height and decimal degrees for the horizontal location),
in order to avoid the production of incorrect results, the user must assign to the project a planar CRS with resulting
horizontal and vertical measures expressed in the same distance unit (e.g., meters both for horizontal and vertical
distances).
Plugin usage
Calculating a solution
The user chooses the DEM to be used as elevation source, by selecting one of the rasters loaded in the project (ref. 1 in Fig. 2).
Afterwards the locations to be used for defining the geological plane, are entered via digitization of three or more
points in the map canvas (ref. 2 in Fig. 2), or alternatively by choosing a loaded point/line/polygon layer (refs. 3 and 4 in Fig. 2).
In the latter case, selected elements will be used for the best-fit-plane calculation. When there is no active selection, all records will be used.
The coordinates of the points defined in the map are added in the list of source points (ref. 6).
It is possible to reset the list of points by clicking the Reset source points button (ref. 5).
When finished with the point definition, the best-fit-plane can be calculated by pressing the
Calculate best-fit plane button (ref. 7). A pop-up window will display the resulting plane dip direction
and dip angle (example in Fig. 1, left-side window).
Fig. 2. Screenshot of the module window.
Saving a solution in the database
It is possible to save the solution in a permanent SQLite database, by using the "Save solution"
button (bottom of the stereoplot window in the left-side of Fig. 3).
Notes can be added to the saved solution (Fig. 3, central window).
Fig. 3. Saving a solution in the database.
Prior to saving, the SQLite database has to be defined in the "Configurations" tab (Fig. 4).
It is possible to use both a new database or a previously created one, where new data will be appended.
Fig. 4. Definition of the internal SQLite database.
Processing database records
Saved solutions can be inspected, plotted in stereonets, removed or exported in a shapefile
from the "Results" tab (Fig. 5).
Fig. 5. Definition of the internal SQLite database.
Selected records can be exported in a new or a previous shapefile, both as points or as lines
connecting the source points, for subsequent GIS processings (Fig. 6).
The output shapefile CRS will always be EPSG:4236.
Fig. 6. Export of selected records in a shapefile.
The fields added in the exported point/line shapefiles provide information about the point or line
features.
In particular, fields named 'mpt_x', 'mpt_y' and 'mpt_z' provide the x, y and z Cartesian coordinates of the
mean point of the inverted points, while 'distance' is the orthogonal distances of the source points from
the inferred best-fit-plane passing through the mean point (BFP) (Fig. 7). Points above the plane will have positive distances, while points below the mean BFP will
have negative distances (see example in Fig. 8). The fields 'dist_x', 'dist_y' and 'dist_z' are the Cartesian
components of the distance between each input point and the corresponding orthogonal projection onto
the BFP.
Fig. 7. Example of results for export as points.
Note that when exporting as a point shapefile, the orthogonal distances of the source points from the inferred
best-fit-plane (passing through the mean point) will be inserted in the output, as well as the Cartesian
components (Fig. 8). Points above the plane will have positive distances, while points below the BFP will
have negative distances.
Fig. 8. Example of points along original polygonal source, symbolized by distance from
the best-fit-plane.
