Geopolygons, Fill & Volume

Geopolygons are closed interpretation outlines digitised on an outcrop model — typically used to map facies bodies, lithological units, weathering zones or any areal feature on the exposure. Each geopolygon carries a facies assignment, a structural orientation, and area/thickness statistics, and can be filled with its display colour so mapped regions read clearly on the model.

This guide covers digitising geopolygons, the surface fill introduced in VRGS 3.3 (which drapes the fill directly onto the outcrop surface), and volume extraction for stockpile-style measurements.

Vulkan view

Surface fill is a feature of the new Vulkan 3D view. The OpenGL view continues to use the classic projected (extruded stencil) fill.

Digitising a geopolygon

Geopolygons are created from the Geobody tools — see the Geobody ribbon and Interpretations ribbon for the full button reference. The essentials:

1. Select the target facies (or assign one afterwards in the Properties panel).
2. Start the geopolygon tool and click vertices around the region on the model surface. Points are picked directly on the outcrop.
3. Double-click (or press Enter) to finish — the outline closes back to the first vertex automatically.

Vertices can be added, moved and deleted afterwards with the vertex editing tools; the fill updates automatically after every edit (and after undo/redo).

Surface fill

With Fill enabled in the geobody display options, the enclosed region is painted onto the outcrop surface itself:

• The fill follows the terrain exactly — across gullies, ridges and overhanging faces — instead of being projected through space, so it no longer bleeds onto ground in front of or behind the outcrop, and it cannot smear across the two sides of a folded or overhanging surface.
• The fill edge follows your digitised outline precisely: boundary triangles are trimmed per-pixel, so the edge stays crisp at any zoom level.
• Vertex spacing is up to you. Between vertices the fill boundary follows the straight line you drew, draped onto the surface — a handful of points around a large region works fine.
• The fill uses the geopolygon's colour and opacity, and respects the Use Lighting display option.

The region is computed in the background the first time a geopolygon is shown (and again after an edit). On very large tiled models this can take a second or two per polygon — the classic projected fill is displayed in the meantime and is replaced automatically when the surface fill is ready. When a project opens, fills wait for the model tiles to finish loading before computing.

Overhangs and complex faces

The fill region is worked out along the surface, so it is reliable on vertical and overhanging faces. For the truest boundary on severely overhanging ground, digitise the outline with the geodesic (surface-following) polyline mode so the boundary itself hugs the surface between clicks.

Fill Style

Each geopolygon has a Fill Style property (Properties panel → Display Options) controlling how its fill is rendered:

Fill Style	Behaviour
Auto (default)	Surface fill whenever the outline lies on a triangulated mesh and the region can be determined; otherwise falls back to the projected fill. Shows the projected fill while the surface fill is being computed.
Surface	Surface fill only. Nothing is shown until the surface region is ready; if it cannot be determined, no fill is drawn.
Projected	Always uses the classic projected fill (the outline extruded along the display dip vector). Use this for geobodies that intentionally project onto other surfaces.

The projected fill is also used automatically where a surface fill is not possible:

• in the OpenGL 3D view;
• on streamed layers (i3s/SLPK and Cesium 3D Tiles), which carry no editable triangle data;
• when the outline does not lie on a triangulated mesh (for example, digitised on a point cloud);
• when the enclosed region cannot be determined unambiguously (for example, an outline crossing a hole in the mesh).

Measuring volume (stockpile estimation)

A geopolygon can also be used to measure the volume of the surface it encloses — for example a stockpile, spoil heap, borrow pit or any bounded change in ground level. VRGS integrates the volume between the visible triangulated mesh(es) inside the outline and a base level (the datum), and reports it split into cut (material above the datum) and fill (material below it).

Workflow

1. Make sure the surface you want to measure is visible in the 3D view, and hide any other meshes that overlap the same footprint (see the caveats below).
2. Digitise a geopolygon around the base of the feature — trace the toe of a pile or the rim of a pit.
3. In the project tree, right-click the geopolygon and choose one of:
   • Extract Volume — horizontal datum;
   • Extract Volume (Best-Fit Plane) — tilted datum.
4. Read the result in the report dialog.

Choosing the datum

The two menu items differ only in how the base level is defined and which way the volume is measured:

Method	Datum	Measured	Use when
Extract Volume	A horizontal plane at the lowest surface elevation found around the outline.	Vertically (straight down).	The ground around the feature is roughly level.
Extract Volume (Best-Fit Plane)	The best-fit plane through the outline, which follows the local slope.	Perpendicular to that plane.	The feature sits on sloping or benched ground.

For a pile on flat ground the two methods give almost the same answer. On a slope they differ: the best-fit plane removes the underlying gradient, so the result reflects the material sitting on top of the slope rather than the slope itself.

The report

The dialog reports:

• Cut — volume of surface above the datum (the pile).
• Fill — volume below the datum (hollows, or a pit).
• Net — cut minus fill.
• Plan area — the footprint area actually covered by mesh.
• Datum — the base elevation used (for the best-fit plane, its mean elevation).
• Meshes measured — how many visible surfaces contributed.

Volumes are reported in cubic metres and areas in square metres, assuming the project uses metric units.

What gets measured

Volume is computed on every visible triangulated surface inside the outline, and overlapping surfaces are added together. To measure a single feature, show only the surface it sits on and hide the rest. The tool works on triangulated meshes — including kriged grid surfaces — but not on point clouds or streamed (i3s/SLPK, Cesium) layers; build a mesh from those first.

Getting a clean result

Draw the outline on the ground right around the base of the feature, and keep it clear of holes in the mesh. Parts of the outline that fall outside the mesh simply contribute nothing, and the Plan area value tells you how much of the footprint was actually covered — if it is much smaller than expected, the outline is reaching past the edge of the surface.

Related pages

• Geobody ribbon — geopolygon creation and editing tools.
• Geobodies tree — managing geopolygons in the project tree.
• Geological object properties — the full property reference, including Fill Style.