About Compass 2.0
Compass is a structural geology toolbox for the interpretation and analysis of virtual outcrop models. It combines a flexible data structure for organising geological interpretation with a series of tools for intuitive and computer-assisted digitisation and measurement. Broadly, the tool comprises two modes: Map Mode, for delineating geological units, and Compass Mode for measuring orientations and thicknesses. Additionally, topological features such as crosscutting relationships and younging directions can also be defined and stored for later analysis. The combination of these two modes roughly correspond to the functionality of a good field geologist (armed with a trusty compass and notebook), but require less beer and sunscreen. They are described in detail below.
Compass Mode: Extracting structural measurements
The compass mode, activated by clicking the "compass" icon in the main dialog, contains three tools for measuring orientations: the Plane Tool, Trace Tool and Lineation Tool. A self-explanatory Picking Tool is also included for convenience.
Plane Tool: Measure surface orientations
The plane tool is used to measure the orientations of fully exposed planar structures, such as joint or bedding surfaces. When it is activated, a red circle is shown in the 3D window. On picking a point in the virtual outcrop model (left-click), a plane is fitted to all points sitting within the circle (using least squares), giving an orientation estimate (dip/dip direction). The circle radius can be changed with ctrl-scroll wheel. Note that for this to work correctly, Cloud Compare needs to be using orthographic projection (Display->Toggle Centred Perspective).
Trace Tool: Digitise and measure traces and contacts
The trace tool allows the estimation of a structure or contact's orientation based on its intersection with a non-flat surface. It uses a least-cost path algorithm to 'follow' these intersection traces along the surface between user defined start and end points, and then calculates the best fit plane to estimate the orientation. To use, select the tool and then left click the start and end of the structure you would like to digitise/measure. The tool will then try to find a fracture trace that links these points. Generally this works remarkably well, though, intermediate 'waypoints' can be added (also using left click) to modify the trace. To finish a segment click 'Accept' (green tick) or press the Space Key. To cancel a segment, click 'Close' or press the Escape key.
The results of this tool depend on the cost function used by the least-cost path algorithm. A variety of these have been implemented, and can be activated by clicking the algorithm dropdown (settings icon). The different cost functions that have been implemented are:
- Darkness [default] - traces follow dark points in the cloud. This is good for fracture traces defined by shadow.
- Lightness - traces follow light points in the cloud. This is good for light-coloured structures such as veins.
- RGB - traces avoid colour contrasts, following points with a similar colour to the start and end points.
- Curvature - traces follow points on ridges and valleys. This can be good for fracture traces with high relief. Note that for performance reasons, the tool will pre-compute a scalar field describing the curvature. This can be slow for large datasets.
- Gradient - traces follow colour boundaries such as lithological contacts. As above, the gradient will be precomputed as a scalar field,
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