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<!DOCTYPE html>
<html>
<body>
<h1>About GeoTrace</h1>
<p>GeoTrace is a a QGIS plugin developed by <a href="mailto:[email protected]">Lachlan Grose</a> and <a href="mailto:[email protected]">Sam Thiele</a> containing some helpful tools for extracting and analysing the orientations of geological structures. It can be used to rapidly digitize structural traces in raster data, estimate their 3D orientations using an associated DEM, and then visualise the results on stereonets and rose diagrams. There is a complementary plugin, Compass, for CloudCompare that provides similar functionalities for 3D point clouds</p>
<p>The trace extraction method (Trace tab) uses a least-cost path algorithm to "follow" linear features in the raster. This relies on a single-channel cost raster in which the structures of interest are represented by low values, and the background by high values. A variety of functions for quickly calculating such a cost function have been included in the Cost Calculator tab.</p>
<h2>Useage Instructions</h2>
<h3>Trace tab</h3>
<p>The trace tab is used for computer-assisted digitization. Before starting, select:
<ul>
<li> An output layer (polyline .shp file) to write digitized traces to </li>
<li> A point layer to store the control points in (optional) </li>
<li> A cost layer. This must be a one-channel raster, in which traces will follow low values (though the Invert Cost check will make the trace follow high values).
Note that the <b>Cost Calculator</b> tab can be used to assist creation of the single-channel cost raster.</li>
<li> A DEM layer, used to estimate 3D orientations from the traces (optional)</li>
</ul>
</p>
<p>Once the relevent information has been set, start interpreting by clicking the Start Digitizing button.
Left-click adds control points to your trace and Right-Click completes a trace. Hit Backspace to undo.</p>
<p>If a DEM layer has been included (see above), best-fit-planes for each trace will be computed using the trace eigenvectors. It is important
to note that this will often produce poor results, especially in flat topography or where the traces have variable orientations. To help identify
traces with poor orientation estimates, trace eigenvalues are recorded in the output, along with a "planarity" metric that approaches 1 as as plane
orientation becomes well constrained (cf. Thiele et al., 2015). For convenience, orientation estimates are also classified as "Good" (planarity
<0.75), "Average" (0.5<planarity<0.75), and "Poor" (planarity<0.5). Also be aware that well-constrained ("Good") orientation estimates sub-parallel to
the DEM surface can be produced if the structures being digitised have variable orientations.</p>
<h3>Advanced Trace</h3>
<p>The Advanced trace tab is used to generate traces from predefined control points. This uses a Cost layer and writes to an Output layer, as above, but rather than requiring manually inserted control points it takes a point feature layer (Control Points) and ID field defining which trace each point belongs to (Unique ID Field), and will then automatically generate traces on clicking Run.</p>
<h3>Cost Calculator</h3>
<p>This tab wraps a variety of python functions from the scikit-image package for easy generation of cost rasters. Please refer to the <a href="http://scikit-image.org/">scikit-image</a> website for detailed descriptions of each of these functions.</p>
<h3>Stereonet</h3>
<p>The stereonet tab can be used to plot stereonets of planar orientation estimates (strike/dip or dip/dip direction) created using this plugin, or otherwise. Simply select the layer and associated fields containing the orientation estimates and then use the plotting tools to draw the stereonet. </p>
<p>The stereonet utility is a wrapper around the python library <a href="nehttps://github.com/joferkington/mplstereonet"><i>mplstereonet</i></a></p>
<h3>Rose Diagram</h3>
<p>This tab works as above, but creates a rose diagram rather than a stereonet.</p>
<h2> Licence </h2>
<p>GeoTrace was developed by Lachlan Grose and Sam Thiele and is free software licenced under the GNU licence v2</p>
<h2>Further Reading and Citation</h2>
<p>If you found this tool useful, please cite <i>Thiele et al., 2017</i>. The publication also contains a more detailed
description of the methods employed by this plugin.</p>
<p>
<i><a href="https://doi.org/10.5194/se-8-1241-2017">Thiele, S. T., Grose, L., Samsu, A., Micklethwaite, S., Vollgger, S. A., and Cruden, A. R.: Rapid, semi-automatic fracture and contact mapping for point clouds, images and geophysical data, Solid Earth, 8, 1241-1253,10.5194/se-8-1241-2017, 2017</a></i>
</p>
<p>For further information on the plane-fitting approach and planarity metric please refer to:<p>
<p>
<i><a href="https://doi.org/10.1016/j.jsg.2015.05.006">Thiele, S. T., Micklethwaite, S., Bourke, P., Verrall, M., and Kovesi, P.: Insights into the mechanics of en-echelon sigmoidal vein formation using ultra-high resolution photogrammetry and computed tomography, Journal of Structural Geology, 77, 27-44, 10.1016/j.jsg.2015.05.006, 2015.</a></i>
</p>
<p>Where the derived orientation estimates are of critical importance, the following will also be a useful reference:</p>
<p>
<i><a href="https://doi.org/10.1016/j.jsg.2015.11.004">Seers, T. D. and Hodgetts, D.: Probabilistic constraints on structural lineament best fit plane precision obtained through numerical analysis, Journal of Structural Geology, 82, 37-47, 10.1016/j.jsg.2015.11.004, 2016.</a></i>
</p>
<p></p><p></p>
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