### 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. You may also load previously saved analysis parameters by using the "Load geographical and geological parameters from file" button (Fig. 1).
Fig. 1. Example of plane-DEM intersections (pink markers) as calculated with the module. The red outline represents a thrust klippe.

#### 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.
Define the DEM to use by choosing a layer listed in the combo box positioned to the right of "Choose current DEM layer" text.
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.
Important: do not use geographic DEMs since results calculated by the plugin will be erroneous.

#### Geographic parameters definition

In the "Geographic parameters" tab you define the parameters needed for specifying the point (afterwards named "source point" ) that defines the spatial position of the geological plane (Fig. 2).
The "Longitude"", "Latitude" and "Elevation" text boxs display the current coordinates of the source point. You can set the source point directly in the map by using the "Set source point in map" command or editing the values displayed in the boxes.
The elevation values may be fixed or not fixed to the DEM surface, depending on the "Get elevation from DEM surface" checkbox.
The current values can be reset by using the "Reset source point" command or by just defining a new point location directly in the map.
It is also possible to limit the extent of the calculated intersections by flagging the "Limit calculation to buffer zone around source point" and defining a calculation radius in the "Buffer radius" input. When not flagged, the intersections will be calculated for the whole source grid.
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. The azimuth of the plane dip direction may be relative to the geographic North or to the map top direction (that obviously changes dependant on the used CRS).
Fig. 3. Definition of the orientation of the geological plane.

#### Intersection calculation

The intersections between the geological plane and the topographic surface DEM are calculated by using the "Calculate" button in the "Calculate" tab. 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.
Fig. 4. Calculation of results tab.

#### 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. 5). The output shapefile projection will be EPSG:4326.
Fig. 5. Example of saving the intersections between plane and DEM as a line shapefile.

#### Known limitations and bugs

- DEMs in geographic coordinates (i.e., lat-lon) are not supported. Using the plugin with them will only provide approximate results, particularly with regards to the dip angle.
- Very large DEM could produce memory errors. Please resize your DEM to a smaller extent or resample it to a larger cell size.
- If you try to define source points outside the DEM extent (for instance, because you have on-the-fly reprojection to a project CRS different from that of the DEM), a message warning can be repeated more that once.

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Doc version 2025-12-29, by Mauro Alberti - alberti.m65@gmail.com