qgSurf - DEM-plane intersection - help

Rationale

This module allows to calculate the intersections between a geological plane and a topographic surface, expressed by a DEM. Moreover, it allows to determine the DEM pixels that are within a given distance range, above and below the geological plane.
The results can be saved as point or line shapefiles, in the former case, or as a grid, in the latter.



Fig. 1. Example of plane-DEM intersections (white markers) as calculated with the module.

DEM data definition

Before opening the module, load the required DEM layer and whatsoever vector or image layers can be useful in the QGis project.
From the combo box to the right of "Choose current DEM layer" text, define the DEM to use.
Important: do not use geographic DEMs since the plugin will provide only approximate results.
If you need to use a raster that has been loaded after the opening of the module window, you can update the raster list with the "Update raster layer list" command.

Geographic parameters definition

In the "Geographic parameters" tab you will find the commands and parameters needed for specifying the point (named "source point" afterwards) that locates the geological plane in the space (Fig. 2).

You can define the source point directly in the map, using the "Set source point in map" command. In the X, Y and Z spinboxes, the coordinates of the source point are displayed. You can modify them from within the spinboxes. You can also choose to use Z values not fixed to the DEM surfaces, by setting the "lock z value to DEM surface" checkbox off.

The current value can be erased with the "Reset source point" command or by just defining a new one clicking again in the map.



Fig. 2. Definition of geographic parameters representing the source point used to position the geological plane.

Geological parameters definition

In the "Geological parameters" tab, you can define the orientation of the geological plane (Fig.3).

The required parameters are the plane dip direction and dip angle.



Fig. 3. Definition of the orientation of the geological plane.

Intersection calculation

Two types of result can be calculated (Fig. 4):
  1. the intersections between the geological plane and the topographic surface DEM ("Intersections of DEM with geological plane");
  2. the distances, orthogonal to the geological plane and within a specified distance range below and above the plane, between the plane and the DEM cells, ("Distances of DEM cell centers from geological plane").



Fig. 4. Calculation of results tab.


1. Intersections of DEM with the geological plane
The intersections are calculated by pressing the "Calculate intersection" command. You can change the last intersection color by choosing a color from the "Marker color" combo box, and delete all the intersections with the "Reset" command.

2. Distances of DEM cell centers from geological plane
This part allows to calculate the distances, orthogonal to the geological plane, from the plane to each DEM cell center.
The defined range of distances from the geological plane will limit the cells exported in a new grid: only cells with orthogonal distances within the range will be exported (Fig. 5).
Grid cells below the plane will be considered to have negative distances from the plane, and so a negative value will be defined as the maximum distance below the geological plane (value '-50' in Fig. 4), while grid cells above the geological plane will have positive distances (so value '50' in Fig. 4).



Fig. 5. Map of DEM cell distances from a geological plane: blue colors represent negative values, red colors positive ones.

Output

In the "Output" section, you can save the last intersections as a point or line shapefile, also loading it within the current project (Fig. 6). The output shapefile projection is the same of the current QGIS project.



Fig. 6. Example of saving the intersections between plane and DEM as a line shapefile.

Known limitations and bugs



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Doc version 2022-11-20, by Mauro Alberti - alberti.m65@gmail.com