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 or lines lying on a topographic surface (example in Fig. 1).
This is a common task in geological mapping and analysis.
Fig. 1. Example of estimation of a fault attitude in the western border of Mt. Alpi zone (Southern Italy).
Input data
Elevation information is provided by a Digital Elevation Model (DEM). Points can be digitized by the user directly
on the map canvas or alternatively loaded from a point/line layers.
When the source DEM crs 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),
it is crucial to assign to the project a planar CRS with resulting
horizontal and vertical measures expressed in the same distance units (e.g., meters both for horizontal and vertical
distances), in order to avoid incorrect results.
Plugin usage
The user defines the DEM to be used as elevation source, by choosing from 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), or alternatively by choosing a loaded point/line layer (ref. 3 and 4).
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).
It is possible to save one or more set of best-fit plane solutions, coupled to their source points, in a shapefile.
The user can create a new shapefile with the Create new shapefile for result storage button (ref. 8).
Computed results are added with the Add current solution in shapefile button (ref. 10),
together with a short description (max. 50 characters).
The added results can be permanently saved to the shapefile with the
Save and stop edits in shapefile button (ref. 11) and then loaded in the QGIS project.
During following sessions, this shapefile can be loaded with the Use previous shapefile button
(ref. 9) for adding new results.
Fig. 2. Screenshot of the module window.
Methodology
The algorithm is based on the application of singular value decomposition technique, in order to derive the eigenvectors of a set of measures.
See the discussion in
Best fit plane algorithms why different results?.
