Cross Sections
A cross section in VRGS is a vertical curtain hung through the scene along a line you pick on the model. It is not a slice computed through data — it is a surface you place, and it earns its keep in two ways:
- as somewhere to hang a section drawing: a scanned field section, a correlation panel, a seismic line or a published figure, put where it belongs in 3D so the outcrop can be read against it;
- as a backdrop to interpret on: the curtain is a pickable surface in both 3D views, so polylines, orientations and measurements can be placed directly on it.
The section line is a map trace. Whatever the elevations of the points you click, the curtain that comes out is vertical: the picked Z values set only where the section starts and how tall it is by default.
Drawing a section
- Annotations tab → Cross Section.
- Click along the model, one click per bend in the section line.
- Double-click to finish.
The section appears under Sections in the Interpretation tree, named
Section. It behaves the same way in both 3D views.
A two-point section is a single flat panel, which is usually what you want for a scanned section drawing. Add intermediate clicks only where the section really does have to bend to follow a valley or a coastline — each bend divides the image between segments in proportion to their length.
What you get, and from where
| Quantity | How it is set |
|---|---|
| Plan shape | The X / Y of the points you clicked, in order. |
| Section Base | The elevation of the curtain's bottom edge. Starts at the lowest picked point. |
| Section Height | The height of the curtain. Starts at whichever is larger: the relief across the picked points, or one fifth of the section's length. |
| Section Scale | A multiplier on the height — vertical exaggeration for the curtain. Starts at 1. |
The top of the curtain therefore sits at Section Base + Section Height × Section Scale, and the base is flat: every vertex of the section line is pulled down to the base elevation when the curtain is built.
Draping an image on it
Right-click the section in the Interpretation tree and choose Import Image.
Accepted formats are .jpg, .bmp, .tif and .tiff.
The image is stretched once across the whole section: horizontally in proportion to distance along the line — so a bent section divides the image between its segments by length, and the picture stays continuous around the bend — and vertically over the full height of the curtain.
Three properties control the fit:
| Property | What it does |
|---|---|
| Auto Height | Sets Section Height from the image's own proportions and the section's length, so the picture is drawn undistorted. |
| Flip Image | Reverses the image along the section, for when the drawing runs the opposite way to the line you picked. |
| Auto Update UV | Recomputes the horizontal fit whenever section vertices move. Leave it on while you are still adjusting the line; turn it off to pin a fit you are happy with. |
A workflow that fits a section drawing quickly
- Pick the two ends of the section on the model, following the real line of section as closely as you can.
- Import Image.
- Turn Auto Height on. The curtain takes the image's proportions.
- Type Section Base so a datum you can recognise in the image — sea level, a marker bed — sits at the right elevation.
- If the drawing is exaggerated, turn Auto Height off and use Section Scale to match it.
Adjusting a section in 3D
Interpretations tab → Edit → 3D Translate arms the edit gadget and puts the section's handles on screen:
| Handle | What it does |
|---|---|
| Cubes at mid-height, one per bend | The section line's vertices, each grabbed at the centre of its pillar — the vertical edge of the curtain above it. Drag sideways (the X or Y arrow, or the flat plane) to move that vertex in plan and re-route the line. Drag up or down (the Z arrow) to move the whole section: the base elevation changes, not the one vertex, because the curtain is a single height and a lone pillar cannot be taller than its neighbours. |
| Sphere at the centre | Moves the whole section in any direction, base elevation included. Clicking anywhere on the curtain itself grabs this handle too. |
| Cube above the centre | Changes Section Height. |
Click the handle you want, then drag the gadget arms. The gadget stays on the handle as the curtain rebuilds, so a long adjustment can be made in several drags without re-picking. In the Vulkan view the centre and top handles are coloured differently — blue for move, orange for height — so they are not easily confused; in the OpenGL view both are drawn in the default handle colour.
Every drag is one step for Edit → Undo (Ctrl + Z), including the base elevation, so an accidental lift of the whole section comes straight back. The Section Base and Section Height rows in the property page follow the drag when you release the button.
The handles sit at half the curtain's height rather than on the section line itself, which is usually below the ground surface and out of reach once the section is draped over an outcrop.
Ctrl + wheel with the cursor over a section changes Section Scale by 5% per notch, in both views. It is the quickest way to try a vertical exaggeration. In the Vulkan view, close the 3D Translate gadget first — while the gadget is up Ctrl + wheel resizes the gadget instead.
In the OpenGL view, double-clicking a section makes it active, after which the plain wheel slides it up and down in steps of a tenth of its height. This is a preview only — the offset is thrown away when you double-click again to deactivate the section, and it is never saved. Use Section Base for a position you want to keep.
Interpreting on a section
The curtain counts as a model surface for picking, so once it is in place you can work on it exactly as you would on the outcrop:
- digitise polylines along horizons drawn on the section image;
- place orientations and measurements on it;
- pick points on it as input to surfaces and geobodies.
That is what makes a draped section useful beyond illustration — it turns a scanned figure into something you can trace and correlate against the mesh standing next to it.
Display
Sections are shown and hidden with their tick in the Interpretation tree.
| Property | What it does |
|---|---|
| Opacity | See-through level of the curtain, 1 to 100. Useful for reading the model through the section. |
| Section Base / Section Height / Section Scale | The geometry above, typed exactly. |
| File Name | The imported image, shown read-only. |
| Vertex rows | Each section-line vertex, typed as X / Y (the first also carries Z). |
The Opacity slider is a display option for cross sections as a class, not a property of the one section you have selected. Changing it on any section changes all of them.
Known limitations
- Sections are always vertical. There is no inclined or folded section surface; for an arbitrary plane through the model use an orthopanel instead.
- Sections are not included in interpretation export. Export the polylines you traced on the section rather than the section itself.
- Normal Colour has no effect on the drawn curtain in either view.
- Auto Height is not saved with the project; it returns to off on reload, although the height it produced is kept.
- The image import filter excludes PNG, unlike the billboard import which accepts it. Convert a PNG section drawing to JPEG or TIFF first.
- There is no Rename on a section's context menu; rename it from the Name row of the property page.
See also
- Measuring — where cross sections sit among the measurement tools.
- Basic Interpretation — the polyline tools you will use on the section.
- Cross-Sections context menu — the Interpretation tree commands.
- Using Billboards — for an image that should face the viewer rather than stand on a fixed line.