Surfaces & volumes

Triangulated terrain models (TIN) for existing ground and design surfaces — and the earthwork between them.

What a surface is

A surface is a TIN — a Triangulated Irregular Network built from 3D points by Delaunay triangulation. Any (x, y) inside it interpolates an elevation from the triangle it falls in. Surfaces drive the ground line in Profile, the ground in Sections, daylighting, and cut/fill volumes.

Creating a surface

Breaklines

A breakline forces triangle edges to follow a line (a ridge, a ditch, an edge of pavement) instead of cutting across it. Tools › Add Surface Breakline prompts you to draw a polyline over the selected surface; press Esc to enforce it into the TIN. Corridor surfaces enforce their feature lines as breaklines automatically. Three types, as in Civil 3D (context menu of the surface): standard — the drawn vertices become surface points and the segments are enforced; proximity — each vertex snaps to the nearest existing surface point (position and elevation) and the line joins those points, adding nothing; wall — the drawn line is the top of a near-vertical face, a second line a centimetre to the chosen side carries the other elevation (you type the elevation change and pick the side when you press Esc). Smooth Surface… (Edit) runs Laplacian smoothing: each pass moves every point's elevation towards the mean of its TIN neighbours by the chosen weight. All of these are operations in the Definition.

Surface Style

Tools › Surface Style sets the display mode and contours:

Surface Properties (right-click the surface, or Tools › Surface Properties) opens the Civil 3D-style dialog: the Information tab (points, triangles, extents, elevation range, breaklines, boundaries, source) and the Definition tab — the surface's operations in order: Snapshot (a "create from …" step), Add Points, Add Point Group, Add Breakline, Add Boundary, and the edit operations. The TIN is what you get by replaying the ticked operations: untick one and the surface is rebuilt without it; tick it again and it is back. Rebuild replays the list as it stands, Remove drops an operation, Move Up / Down reorder, Revert to Selected drops everything after a step. Every change is one undo step, so you can try and Ctrl+Z. Breaklines drawn in Plan and boundaries are recorded as operations (and are undoable); a breakline's vertices become surface points before its segments are enforced, as in Civil 3D. Corridor trims (the ground hidden under a corridor) are derived, not operations. A surface that came from a corridor, a grading band or an import (LandXML, DXF) has no operations of its own: its geometry is the starting point, and anything you add — a boundary, a breakline, an edit — applies on top of it.

Boundaries come in three kinds (context menu of the surface): Outer keeps only what lies inside the ring, Hide voids the area inside the ring, Show re-shows an area inside a hide ring (a building pad inside a hidden lot, say). All three are Add Boundary operations and can be unticked or removed in the Definition.

Editing a surface

Civil 3D's Surface › Edits, from the surface's context menu or Tools › Surfaces › Edit Surface, each recorded as an operation in the Definition (undoable, untickable):

• Delete Point / Modify Point Elevation / Move Point — click near a surface point in Plan; for an elevation type the new level in Command; for a move click the new position.
• Swap Edge / Delete Line — click near a TIN edge: swap flips it to the other diagonal of its two triangles, delete removes those triangles (a void). Edge operations replay after the data operations. A point cannot be added inside a void — delete the void's operation, or add the point before it in the Definition list.
• Raise/Lower Surface… — shift every point by a level. The breaklines move with the points, so a stringline stays on the surface.
• Paste Surface… — another surface's points and breaklines override the area they cover. The link is live in both directions: edit the pasted surface and every surface that pastes it rebuilds at once.

Corridor surfaces

From Corridor Properties › Surfaces (or Tools › Corridor Surface + Volume) you choose which feature-line codes contribute (none = every section point), and on Boundaries whether to clip the TIN to the corridor extents. A corridor surface stays linked to its corridor and rebuilds when the corridor changes.

The ground surface can in turn be trimmed by a corridor: Trim the daylight surface under this corridor in Create Corridor (or Modules › Road › Trim Surface by Corridor) hides the ground inside the corridor's daylight footprint, and the hole follows the corridor as it changes. See Corridors › Boundaries.

When a surface goes out of date

A surface is baked: its TIN is what you get by replaying its definition, and it can therefore stop describing what it was made from. Edit the point group an Add Point Group operation reads, move the baseline under a corridor surface, and the triangulation on screen is still there, still looking authoritative, and no longer true.

Each surface now records what it was built from and notices when that changes. One that has gone out of date says so in the Properties dock, and Tools › Rebuild Out-of-Date remakes it — the menu entry carries how many objects are waiting and is greyed out when none are. Rebuild Selected beside it remakes only the object you have selected, which is what you want when one answer needs settling and forty corridor surfaces do not. Renaming a source changes nothing: only the values that would change the triangulation are watched.

A surface that reads nothing outside itself — a TIN built straight from imported points, with no referenced group and no source corridor — can never be out of date, and is never flagged. There is no source that could have moved.

Each derived object’s own card carries three fields: Out of date, a Rebuild now button, and Rebuild automatically.

A surface does not rebuild itself by default, and the reason is a measured number: re-baking a large one takes about 217 ms, which is a visible stall on every edit and multiplies in a document with several. The rail derived objects — clearance envelopes, fouling points, level plans, joint schedules — cost between 0.1 and 22 ms and therefore do keep themselves current by default. Any single object can be switched either way, and the switch is saved with it.

An automatic rebuild runs after the change that caused it has settled, never inside a redraw.

Corridors are not in this, and that is not an omission. A corridor re-bakes every time it is sampled, so it has no stored result that could go stale and an out-of-date badge on one would mean nothing. The case that matters still works: a corridor surface is a surface, and it follows the corridor it came from — move the baseline or change the assembly and the derived surface is marked.

Cut & fill volumes

Tools › Corridor Surface + Volume samples every corridor at 5 m, builds its TIN, and compares it to a ground surface on a regular grid. The report lists, per corridor:

• {name} surface ({pts} pts, {tris} tris, {ok}/{total} breakline edges) — the breakline count flags a degenerate section if it's partial. Each stringline segment is counted once, even where a column carries several codes.
• vs ground: cut {…} m³ fill {…} m³ net {…} m³.

The grid method and accuracy are in the Formula reference.

Import / export

Surfaces round-trip through LandXML (points + triangles) and export to DXF as 3D faces. See the Command reference.