Introduction

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. 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 GUI showing the available modes, customizable cost functions and option for data export.

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.

Schematic showing the least-cost path approach to trace detection in a point cloud implemented in the Compass plugin.

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 but can be slow and sensitive to noise.
Gradient - traces follow colour boundaries. Excellent for tracing contacts between lithologies with different colour.
Distance - traces take the shortest path. I'm not sure what this is good for.
Scalar Field - traces follow low values in the active (displayed) scalar field. Use this to implement custom cost functions.
Inverse Scalar Field - traces follow high values in the active (displayed) scalar field. Use this to implement custom cost functions.

When using this tool it is important to note that its performance (i.e. speed) scales with trace length. Hence, it can be used with large point clouds if trace lengths are kept small (though long traces can be quickly digitised as multiple segments). Asking the tool to find long traces may result in system crashes.

A plane is fitted to each trace (using least squares) when it is finalized (green tick or space key), providing an estimate of the structure orientation. Unfortunately, in some situations the best-fit plane does not provide a robust estimate of structure orientation, particularly in low relief. To help avoid these issues, planes deriving from traces which are co-linear or within 10° of the average surface orientation along the trace (if point normals have been calculated). Further details on the automatic vetting process can be found at [1].

Automatic plane fitting can be enabled/disabled (it is disabled by default) in the algorithm menu (settings icon) or holding the shift key when accepting the trace. Plane orientations are expressed using the dip/dip direction convention.

Lineation tool: Measure lineations

This tool measures the trend and plunge of a (straight) line between two points. Left-click selects points (as above).

Map mode

Map Mode provides functionality for storing and organising interpretations in larger projects, where many different geological features need to be recorded. On entering Map Mode (map icon), a second dialog contains functionality for creating and managing GeoObjects appears. GeoObjects are a data structure for organising and describing geological features in a flexible way, and are comprised of an Interior, Upper Boundary and Lower Boundary. Hence, measurements from a dyke for example, can be assigned such that they are representative of either contact (somewhat arbitrarily called upper and lower) or the interior. The GeoObject dialog contains functionality for creating GeoObjects and defining their active part (upper, lower or interior), to which any new measurements get assigned.

When Map Mode is active, the Compass functionality described above remains as described above with a few subtle difference. As already mentioned, new measurements are stored in the active GeoObject rather than in a Measurements folder. Furthermore, traces defined using the Trace tool are also kept visible in this mode (rather than converted to planes), so the Trace tool can be used to digitise contacts.

Other tools

The Other Tools dropdown (Plus icon) contains additional functionality, and includes tools for measuring true thicknesses, recording notes and exporting interpretation for 2D visualisation. These Other Tools are summarised in the order they appear below.

Add pinch node

Pinch nodes are used to record locations were the upper and lower surfaces of a GeoObject meet, such as at dyke tips or were sedimentary layers pinch-out. Pinch nodes are simply represented by a single point, but in being assigned to a GeoObject can record important information.

Measure one-point/two-point thickness

The Measure One-Point Thickness and Measure Two-Point Thickness tools can be used to measure the true-thickness of geological units by selecting a plane representing the orientation of a unit and one or two points on its boundary. Measure One-Point Thickness will measure the plane-perpendicular distance between the selected plane and each successively chosen points (use this if the fit-plane represents one of the contacts). Measure Two-Point Thickness measures the plane-perpendicular distance between pairs of successively chosen points (used when the fit-plane doesn't fall on one of the contacts).

Add note

Quick and dirty way of taking notes. Will add a new object to the picked location that displays the object name (this will be the contents of the note).

Fit plane to GeoObject

On some occasions, a GeoObject comprised of multiple different trace objects will define a well-constrained plane. As the trace-tool only allows plane-fitting to an individual trace, the Fit Plane to GeoObject tool will calculate a fit plane for all traces defining the upper and lower surfaces of a GeoObject. Use with care, as this can often produce spurious results.

Merge selected GeoObjects

Merges all of the GeoObjects selected in the DB Tree into the currently active (most recently selected) GeoObject.

Estimate structure normals

Uses a bayesian plane-fitting method and moving window search algorithm to identify optimum best-fit planes for each point along the selected structure trace and hence estimate structure normals. If one (or more) GeoObjects are selected SNEs will be created for all traces within each object. If objects have upper and lower surfaces defined then true-thicknesses corresponding to each orientation can also be estimated. Results are stored on a "SNE" point cloud, including thickness, dip and dip direction scalar fields. Details of the different options that can be used for this tool are shown in the figure below. Note that fit-planes are not allowed to include pinch-nodes, meaning these can be used to separate different sections of a structure (e.g. a segmented dyke).

Screenshot of the SNE estimation dialog and associated explanation of each option.

When using this algorithm it is critical to carefully filter the results, as geometric quirks will often lead to wildly inaccurate results. The "segment" tool (scissors) can be used to delete unwanted SNEs as with any other point cloud. SNEs are also compatible with the stereonet tool in the qFacets plugin, which can aid in filtering erroneous results.

Export SVG

Exports trace objects to a .svg file for "beautification" in vector-graphics packages such as Inkscape or Adobe Illustrator. Traces will projected into 2D using the current view-direction and camera settings, so should appear as viewed on the screen. Other objects (e.g. point clouds) can be included by rendering an image with standard CloudCompare functionality (Display->Render to File)

Saving and exporting measurements

Interpretation generated using this tool is represented internally by Cloud Compare as planes and polygons, and so can be natively stored in CC's .bin files. For ease of use, a save tool has also been included. This tool exports plane and trace data to .csv files, from which data can be easily imported into other software packages for further analysis. Polylines created by the Trace and Lineation tools can also be exported as shapefiles through the normal Save dialog in Cloud Compare. To avoid confusion regarding different strike/dip conventions (British vs American RHR), we suggest using the exported dip and dip-direction values instead, or at least treating the strike values with care.

About the author

Sam Thiele (see his website: https://www.samthiele.science)

Compass (plugin)

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