Basic Interpretation
Interpretation in VRGS means drawing on the outcrop: tracing a bed, measuring its dip, outlining a channel body. Every tool works the same way — arm the tool from the ribbon, then click on the model — and every tool works on both meshes and point clouds.
This guide covers the tools on the Interpretations ribbon tab. If you are completely new to VRGS, walk the tutorial first; this page is the fuller reference behind it.
How every tool behaves
Learn these five things once and every tool below makes sense.
| Arming | Clicking a ribbon tool puts the mouse into that mode. The cursor changes to show which tool is armed. |
| Finishing | Single-point tools finish on the click. Multi-point tools (polylines, faults, geopolygons, scan lines) finish on a double-click. |
| Dragging still navigates | Left-drag orbits the view even with a tool armed, so you can re-frame mid-interpretation without disarming anything. |
| One object, then back to navigating | When a tool finishes it returns you to normal navigation — unless Sticky Interp is on. |
| Undo | Ctrl+Z, or Undo on the Edit panel, steps back through digitising. |
Structure → Sticky Interp keeps the tool armed after each object, so you can digitise twenty fractures without going back to the ribbon between each one. Turn it off (or press the tool again) when you are done.
Mid-polyline and need to fly round the corner of the outcrop? Edit → Pause Digitising suspends the tool so the mouse navigates normally, and pressing it again resumes the same line. It is not the same as finishing.
Before you start: the Active Descriptor
The Active Descriptor panel sits at the left of the Interpretations tab and carries two dropdowns:
| Facies | The facies assigned to each new object as you create it. |
| Horizon | The stratigraphic horizon pick assigned to each new object. |
Setting these before you digitise is far quicker than editing objects afterwards, and it is what makes the interpretation colour-coded and filterable later. Each has a button to create a new entry, and a button to apply the current value to everything currently selected — which is how you fix a batch you digitised under the wrong setting.
The lists themselves live in the Collections tree, under Facies and Picks.
Orientations — Dip Azimuth
The Dip Azimuth tool records a planar orientation. Five methods, and the choice matters:
| Method | How you drive it | Use it when |
|---|---|---|
| 3 Points | Click three points on the plane. The best-fit plane is built on the third click. | The default, and right most of the time — a bedding surface with three clear points on it. |
| N Points | Click as many points as you like; double-click to finish. Click-and-drag digitises points along a trace. | The surface is curved or noisy and three points would be unrepresentative. |
| 1 Point | Click the location, then type the dip and dip direction. | You measured it in the field with a compass-clinometer and want it in the model at the right place. |
| Area | Lasso a region on screen; every point inside is gathered and a plane fitted through all of them. | A large, well-exposed surface — the most robust fit available. Point clouds only. |
| Attributes | Click a point on a mesh that has been analysed for dip and azimuth. | The mesh already carries per-vertex orientation attributes and you just want to sample one. |
Whichever method you use, the result is an orientation object in the Interpretation tree, plotted on the stereonet and usable by every downstream tool.
The lasso gathers points from visible point clouds. On a mesh-only project use N Points, or analyse the mesh for dip and azimuth and use Attributes.
Polylines
A polyline is the workhorse: a bed trace, a contact, a channel margin, a cross-section line.
| Mode | How you drive it |
|---|---|
| Digitise Points | One vertex per click. Double-click to finish. |
| Draw | Freehand — hold the left button and drag, and vertices are dropped at the project's minimum digitiser spacing. |
| Geodesic Distance | Click sparse points; the line follows the shortest path across the surface between them, with a running along-surface length. |
Geodesic Distance is the one worth knowing about. A straight segment between two clicks cuts through the rock; a geodesic stays on it. That makes it the right tool for measuring a distance along an undulating surface, and for tracing a contact across topography with only a handful of clicks.
Editing a line as you build it
On the Digitise Polyline dropdown:
| Command | What it does |
|---|---|
| Split Line | Click a vertex to cut the line into two. |
| Join Lines | Select two or more lines (Ctrl-click), then join them into one. |
| Add Node | Click anywhere along a line to insert a vertex. |
| Draw Contour | Digitise a contour line. |
| Close Polyline | Close the line into a ring. |
| Snap To Mesh | Pull the line's vertices onto the mesh surface. |
| Create Geobody | Build a geopolygon from the selected polylines. |
The Edit panel
These apply to polylines and faults alike:
| Command | What it does |
|---|---|
| Move Vertices | Click a line to show its vertices, then drag one. Click the command again to finish. |
| 3D Translate | Move a vertex along an axis or in a plane, using on-screen handles. |
| Snap Vertices | While digitising, snap each new vertex onto an existing one — for making traces meet exactly. |
| Erase Vertices | Click vertices to delete them. |
| Reposition Object | Click an object to pick it up, then click where it should go. |
| Pause Digitising / Undo | As described above. |
Faults
Faults digitise exactly like polylines, and get their own tools because a fault is usually built from several sticks rather than one line:
| Command | What it does |
|---|---|
| New Fault | Start a fault. |
| Add stick | Add another stick to the existing fault. |
| Extend Fault | Continue a stick from its end. |
| Split / Join | Divide a fault, or merge two. |
VRGS calculates the fault's orientation from its geometry, so a digitised fault plots on the stereonet alongside your measured planes.
Auto Tracker
Auto Tracker follows a feature across the mesh for you rather than making you click along it — you give it a start and an end and it finds the path between, guided by the mesh's own attributes. Settings puts its parameters in the properties panel.
The Auto Tracker digitising tool is handled by the OpenGL view. In a Vulkan view the button is inert — use Geodesic Distance for surface-following traces, or switch renderer. (The mesh-following tracker used by measured sections works in both.)
Scan lines
A scan line samples fracture intensity along a line, the way a tape across the outcrop does in the field. Two ways to build one:
| Mode | What it does |
|---|---|
| From intersections | Digitise the line; VRGS intersects it with the orientations already displayed and records where each one crosses. |
| From Points | Digitise the line, then mark each fracture position along it by hand. |
Both finish on a double-click and need at least two points. The result feeds fracture-intensity analysis and DFN work.
Plunge
The Plunge tool measures a lineation: click the start and end of the line and VRGS returns its plunge and trend. Faults get a plunge calculated automatically.
Geopolygons
A geopolygon outlines a body on the outcrop and extrudes it into a solid, so you can measure it and attach observations to it.
| Mode | How you drive it |
|---|---|
| Digitise Points | One vertex per click around the outline; double-click to close and finish. |
| Draw | Freehand around the outline. |
| Key Pillar | Add a pillar that controls the extrusion. |
| Create from polylines | Build the outline from polylines you have already digitised. |
At least three points are needed. The ring is closed for you — you do not have to click back onto the first point.
Working on a geopolygon after you have drawn it
Right-click it in the Interpretation tree and choose Make Active. A Geobody ribbon tab appears with its editing tools: showing and moving nodes, interpolating new nodes between selected ones, and adding palaeocurrents.
Extract Volume… on the tree menu measures the volume the outline encloses — against a horizontal datum, or against the outline's own best-fit plane where the ground slopes. See the Geopolygons, Fill & Volume guide.
Palaeocurrents
A geopolygon can carry many palaeocurrent measurements, each with an azimuth, an optional bidirectional flag (for a trend rather than a direction) and a structure type. The types are edited in the Collections tree under Sed Structures.
This is not decoration. An outcrop only exposes a body at whatever angle the cliff happens to cut it, so apparent widths are biased. Palaeocurrents recorded on the body are what let that bias be corrected into a true width.
There is also a standalone Palaeocurrent tool on the Sedimentology panel for a measurement that does not belong to a body.
What to do with an object once you have made one
Everything below is on the object's right-click menu in the Interpretation tree.
- Give it meaning — set its facies, horizon or stratigraphy. Faster still, set the Active Descriptor before digitising.
- Group it — Group collects selected objects under one tree entry. Groups are the unit almost everything downstream works on: colour-by-group on the stereonet, orientation sets for a DFN, surface gridding inputs.
- Convert it — a polyline can become an orientation; orientations can become a gridded surface; polylines can become a geopolygon.
- Clean it up — Refine by Spacing thins vertices to a spacing you choose, Smooth removes digitising jitter, Remove Dip flattens a best-fit dip out of a line.
- Version it — put an interpretation under version control to keep several admissible interpretations of the same feature side by side.
- Export it — to file, or as GeoJSON.
Where interpretation leads
| You have | Next |
|---|---|
| Orientations | Stereonet — plot, contour, cluster into sets |
| Orientation sets | DFN — build a fracture network from them |
| Polylines / orientations | Surface Gridding — interpolate a surface through them |
| Traces on photographs | 2D image interpretation |
| A measured section | Sedimentary logging |
| A distance, height or angle | Measuring |
Tips and troubleshooting
- The tool disarmed itself after one object. That is the default. Turn on Sticky Interp to keep it armed.
- Clicking does nothing. The click has to land on a mesh or point cloud — a click on empty sky is discarded. Check the object you are aiming at is visible.
- My polyline is a mess of vertices. Draw mode drops a vertex every minimum-spacing step. Use Refine by Spacing afterwards, or digitise with Digitise Points.
- Two traces should meet but do not. Turn on Snap Vertices before digitising the second one.
- Area dip/azimuth does nothing. It gathers points from point clouds; there is nothing to gather on a mesh-only project.
- Everything came out with the wrong facies. Select them all, set the facies in the Active Descriptor, and use its apply to selected button.
See also
- Basic interpretation tutorial — the guided walkthrough.
- Auto Structural Mapping — extracting orientations automatically instead of by hand.
- Annotations tutorial — labels, waypoints, samples and multimedia.