qgSurf - Best Fit Plane - help
Module purpose
The purpose of this module is to allow the derivation of the planar attitude that best fits a set of points or lines on a topographic surface. This task is common in geological mapping and geological analysis. By coupling mapping or remote sensing information with topographic data, it is possible to estimate the orientation of the geological plane fitting a set of observations, all located on the topographic surface (example in Fig. 1).
Fig. 1. Example of estimation of a fault attitude in the western border of Mt. Alpi zone (southern Italy).
Points are drawn on the slope map derived by a DEM, with elevations extracted from DEM ("dem_w4u3" in this case).
Input data
The module requires a DEM as the source of the elevations for points digitized by the user on the map or loaded point/line layers. If the CRS of the DEM is in polar coordinates (i.e., latitude and longitude), it is crucial to assign a planar CRS (e.g., UTM, Lambert Conformal Conic) to the current QGis project so that both horizontal and vertical measure units are the same (e.g., meters). Otherwise, the module outputs are wrong. For the same reason, if the source DEM is in a planar CRS, the project CRS must be a planar CRS with horizontal locations expressed in the same unit as the vertical distance unit of the source DEM.
Usage
The user chooses a DEM from the list of rasters currently loaded in the QGis project [1 in Fig. 2]. This DEM provides the source elevation for points.
Then the user defines the x-y locations of the points, by drawing them directly in the map canvas or by choosing a loaded point/line layer. In the latter case, only selected elements are used for the best-fit-plane calculations, unless there is no active selection, in which case all records are used. In the former case, it is possible digitized the points in the map canvas, after clicking on the Define source points in map button [2]. The coordinates of the points defined in the map are added in the list of source points [6]. It is possible to reset the list of points by clicking the Reset all source points button [3]. Data can also originate by a loaded layer, chosen in the [5] list and then clicking the Get source points from layer. Input points are appended to the list [6].
When finished with the point definition, the best-fit-plane can be calculated by pressing the Calculate best-fit plane button [7]. A pop-up window should normally display the resulting plane dip direction and dip angle (see example in Fig. 1).
It is possible to save one or more set of best-fit plane solutions, together with their source points, in a shapefile. To this purpose, the user can create a new, ad-hoc shapefile with the Create shapefile for storing results button [8]. Computed results can be added to this shapefile with the Add current solution to shapefile button [10], that opens a window when the user can insert a description of the computed solution (max. of 50 characters allowed).
When completed an analysis session, the results can be saved to the shapefile with the Save and stop edits in shapefile button [11] and loaded in the QGis project.
In a successive session, the previously created shapefile can be loaded with the Use previous shapefile button [9] and then new results added with the described commands.
Fig. 2. Screenshot of the module window.
Methodology
The basis of the algorithm is the application of singular value decomposition to derive the eigenvectors of a set of measures.
See, for instance, the discussion in
Best fit plane algorithms why different results?.