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Surfaces & Volumes

Build TIN and DEM surfaces, derive contours, compute cut/fill volumes with a heatmap, and export LandXML/DXF/GeoTIFF/PDF.

Surfaces turn your survey points into a terrain model you can measure. From a surface Groundtie derives the two classic deliverables — contours (the topo map) and cut/fill volumes (the earthwork number that prices a job) — and exports them to CAD/GIS. Everything lives in the Surfaces tab of a project. It's a Crew feature.

A surface is named and versioned: its inputs are snapshotted, so a volume you computed last month never silently changes when you rebuild. All geometry is computed server-side and rendered in the same 3D scene as your points.

Building a TIN from points

A TIN (triangulated irregular network) is a mesh of triangles through your survey shots — the digital terrain model everything else is derived from.

  1. Open the Surfaces tab and the Build a surface card.
  2. Give it a name (e.g. "Existing grade").
  3. Choose the points: all design points, one category, or one group.
  4. Optionally set a max edge length (meters) to drop long sliver triangles that reach across gaps in your data.
  5. Click Build surface. It appears in the Surfaces list, reporting its triangle and vertex counts. Click a surface to show it in the 3D scene.

Shade the surface from the scene's Display menu: an elevation ramp (hypsometric tint), slope analysis, or a wireframe for QC.

Constraints: breaklines, boundary, holes

A TIN is only as good as its constraints. Add them in the Constraints card, then rebuild the surface (a new version) to apply them.

  • Breaklines force triangle edges to follow a real linear feature — a curb, a swale, a ridge — so the surface doesn't smooth across it.
  • A boundary clips the surface to a region (nothing is interpolated outside it). Use Auto boundary to derive a concave hull from your points, then edit.
  • Holes cut voids out of the surface (a building pad, a pond).

You can capture constraints two ways:

  • Digitize — pick the kind, click Digitize, then click survey points in the scene to snap the vertices; Save.
  • Import DXF — map each DXF polyline layer to a kind. DXF has no elevation, so vertex heights are filled from the nearest survey point at build time.

Uploading a DEM

Instead of points, you can build a surface from a drone/LiDAR DEM. In the Build a surface card, click Upload DEM and choose a GeoTIFF. Groundtie reads it in your browser, downsamples it to a workable grid, reprojects it to your site using the GeoTIFF's own CRS, and builds a DEM surface. From there it behaves exactly like a TIN — contours, volumes, and export all work.

The background OpenTopography terrain (the shaded relief under your site) is context only — it is never used as a measurement surface. Only point-built TINs and uploaded DEMs are real surfaces.

Contours

With a surface selected, the Contours section (in the Surfaces card) draws iso-lines live in the scene:

  • Interval — spacing between minor contours, in your display unit.
  • Major every — heavier, labeled contours at this larger spacing.
  • Smoothing — rounds the lines (Chaikin) without changing which triangles they cross.
  • Elevation labels — printed on the major contours.

Contours are computed on demand and drape on the surface, so they update as you change the parameters.

Cut / fill volumes

The Volumes card computes earthwork between a base surface and a target:

  1. Pick the comparison: to another surface (e.g. existing vs proposed) or to a reference elevation (a flat pad).
  2. Choose the base surface and either the compare surface or the reference elevation.
  3. Set the cell size (meters) — the grid resolution; smaller is more precise but slower.
  4. Click Compute volume.

Results show cut, fill, net, and area, toggleable between cubic yards and cubic meters. Select a volume to shade the surface with its cut/fill heatmap (red = cut, blue = fill) with a legend in the scene. Because the result snapshots the surface versions, it stays reproducible after a rebuild.

Exporting

From the download menu on each surface or volume row:

  • Surface → LandXML (TIN faces, opens in Civil 3D), DXF (3D faces plus contour layers when a contour interval is set), or a GeoTIFF DEM raster.
  • Volume → a PDF report or a CSV — both carry the method, cell size, and the surface versions used, so the numbers are auditable.

Exports are written in your project's projected coordinate system, ready to drop into CAD or GIS.