Abstract
Geological planar facets (stratification, fault, joint…) are key features to unravel the tectonic history of rock outcrop or appreciate the stability of a hazardous rock cliff. Measuring their spatial attitude (dip and strike) is generally performed by hand with a compass/clinometer, which is time consuming, requires some degree of censoring (i.e. refusing to measure some features judged unimportant at the time), is not always possible for fractures higher up on the outcrop and is somewhat hazardous. 3D virtual geological outcrop hold the potential to alleviate these issues. Efficiently segmenting massive 3D point clouds into individual planar facets, inside a convenient software environment was lacking. FACETS is a dedicated plugin within CloudCompare v2.6.2 (<a href="http://cloudcompare.org/"target="_blank">http://cloudcompare.org/</a> ) implemented to perform planar facet extraction, calculate their dip and dip direction (i.e. azimuth of steepest decent) and report the extracted data in interactive stereograms. Two algorithms perform the segmentation: Kd-Tree and Fast Marching. Both divide the point cloud into sub-cells, then compute elementary planar objects and aggregate them progressively according to a planeity threshold into polygons. The boundaries of the polygons are adjusted around segmented points with a tension parameter, and the facet polygons can be exported as 3D polygon shapefiles towards third party GIS software or simply as ASCII comma separated files. One of the great features of FACETS is the capability to explore planar objects but also 3D points with normals with the stereogram tool. Poles can be readily displayed, queried and manually segmented interactively. The plugin blends seamlessly into CloudCompare to leverage all its other 3D point cloud manipulation features. A demonstration of the tool is presented to illustrate these different features. While designed for geological applications, FACETS could be more widely applied to any planar objects.
Highlights
Acquiring dense 3D point cloud has been a challenge up until a decade ago
A discussion will call on some aspects of caution to relate plugin performance with respect to 3D point cloud surveys
Facets can be grouped by orientation into single planes and plane families (Plugins > Facet/fracture detection > Classify facets by orientation)
Summary
Acquiring dense 3D point cloud has been a challenge up until a decade ago. With the advent of fast lidar scanners and Structure-from-Motion techniques, non-technical communities have started producing their own point clouds shifting the emphasis towards making actual use of this 3D data. In 3D environments, and point clouds thereof, one specific geometrical property retains many users attention: planes. They are both the simplest 2D geometric figure and one of the most meaningful elementary objects for many applications. FACETS, a structural geology plugin, implanted in CloudCompare (from version 2.6.2), was designed to extract planes from unstructured 3D point clouds. FACETS contains three aspects: (i) a data processing aspect with two different algorithms, each with a minimum number of parameters; (ii) a stereogram rendering tool to produce structural geology diagnostics with a community established standard, assorted with an interactive query and subset interface; and (iii) export facilities towards third party software (GIS specific and all-purpose ASCII export) for further specialized interpretation. A discussion will call on some aspects of caution to relate plugin performance with respect to 3D point cloud surveys
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