To load a design surface into a machine guidance system: export the TIN (Triangulated Irregular Network) surface from the design software (Civil 3D, AGTEK) as a .gc3, .svd, or system-specific format; import it into the base station or machine computer; verify a sample of points from the surface match the design elevations from the plans. A wrong surface file — wrong version, wrong datum, wrong coordinate system — will grade the entire site to the wrong elevation.
Design Surface File Formats by System
| Machine Guidance System | Primary Surface Format | Notes |
|---|---|---|
| Topcon 3D-MC | .tp3, .gc3 | Import via Topcon Link software |
| Trimble GCS900 / Earthworks | .svd (SVL) | Created in Trimble Business Center |
| Leica iCON | .is3, .3d | Via Leica Exchange |
| Caterpillar AccuGrade / GRADE | .svd or proprietary via Cat Connect | Check machine software version compatibility |
| Komatsu iMC | Proprietary; import via Komconnect | — |
Creating the Surface File
The design surface is typically created by the engineer or the contractor’s machine control specialist from the civil plans:
- Open the design drawing in Civil 3D or AGTEK and identify the target surface (finished grade, subgrade, or base course)
- Export the target surface as a TIN in the machine guidance system’s required format
- Verify the coordinate system of the export matches the machine guidance system’s configured coordinate system (State Plane, local grid, or WGS84/UTM)
- Verify the vertical datum (NAVD88 for most US projects)
Loading the Surface on the Machine
- Transfer the surface file to a USB drive or to the machine computer via office software or direct download
- On the machine display, navigate to the job/surface menu and import the surface file
- Verify the surface loaded correctly by navigating the display to a known design point and confirming the elevation shown matches the plans
- Check at least three representative points across the site before beginning grading — including a high point, a low point, and a structure location
What Can Go Wrong
- Wrong surface version: The engineer issues a design revision; the machine is still running the old surface. Always check the surface file date against the current design revision.
- Coordinate system mismatch: The surface file is in State Plane but the machine is configured for a local project grid. Every point will be offset. Catch this in the verification step.
- Vertical datum mismatch: Surface in ellipsoid height, machine expects orthometric. All elevations wrong by the geoid separation (typically 50–100 ft in the US). Obvious error but catastrophic if not caught.