Topographic traces of planes can be derived by means of gSurf , a Python application that, given a DEM and a plane, determines the intersections (as points) between these two surfaces. This tool is now available also as a plug-in for Quantum GIS. Why Quantum GIS? Because it is a user-friendly, open source and free GIS software, that allows a simple and rapid integration between its core functionalities and Python- and Qt-based modules.
This plug-in can be installed by copying the plug-in folder into the standard Python plug-ins folder for Quantum GIS. Since it presents the same interface, tools and theoretical bases as the stand-alone Python application, the reader is referred to previous posts for additional information [1, 2, 3]. It was tested in Quantum GIS 1.7.3 in both Windows Vista and Ubuntu Lucid Lynx (10.4 LTS) and can be freely downloaded here (license: GPL v. 3).
Next I describe an example of application and result validation using data from the Valnerina zone between S. Anatolia di Narco and Cerreto di Spoleto (Umbria, Central Italy), where Plio-Pleistocene normal faults dissect the previous Miocene thrust-and-fold structure. Geological data derive from my PhD thesis. The example regards a normal fault, for which a structural measure and the topographic trace are available (Fig. 2). The structural measure value is (dip direction, dip angle) = (227°, 58°). However, it cannot be considered representative of the general orientation of this fault, since the computed trace (white line in Fig. 2) does not fit well the mapped fault trace.
|Fig. 2. Fault traces superposed onto DEM. White dots (mimicking a line) represent the computed intersections for a plane with measured values of dip direction and angle (227°, 58°). The red circle represents the measure location.|
A value of (219°, 61°) seems more appropriate to fit the mapped trace, even with some deviation due to local erroneous fault trace mapping or fault surface deviations from the general orientation (Fig. 3).
|Fig. 3. A plane-solution with a better fitting to the mapped fault trace.|
Plane orientation is (dip direction, dip angle) = (210°, 61°).
In order to demonstrate the correctness of the computed theoretical result, a method routinely used in geology has been applied to the intersections. The highest and lowest intersections between the results and the contour lines derived from the DEM has been extracted, and their vertical and horizontal separations computed. The vertical separation is equal to (1100 – 650) m = 450 m, while the horizontal separation is equal to about 245 m (Fig. 4). This values indicate a dip angle of 61.4°, to be confronted with the theoretical value of 61°. The orientations of the two intersection lines, that are parallel as expected, suggest a dip direction of about 220°, to be compared with the theoretical value of 219°.
We can be therefore confident of the correctness of the application results.
|Fig. 4. Screenshot showing part of the two lines representing the intersection directions with height of 1100 m asl (blue line, top) and 650 m asl (blue line, bottom-left). Their separation is approximately of 245.5 m.|
 gSurf: una applicazione Python per calcolare interattivamente l'intersezione fra piani e DEM.
 Calculating the intersections between planes and DEM: a Python implementation
 Intersezioni tra DEM e superfici planari, un tema di interesse in geologia