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Version: 3.4 (unreleased)

Ground Control Points

Surveyed control points held in the project. Each one is a physical target whose coordinates a surveyor measured, and it lives in the project as an ordinary object: visible and pickable in the 3D view, listed in the Data Tree, and saved with the project.

Control is normally brought in by the SfM workflow — see Ground control points in the SfM guide for importing a CSV, marking targets in photographs, and reading the accuracy — but the points themselves are not SfM-only. Anything in the project can use them.

Not the same as Tie Points

Tie Points belong to one mesh and record a correction for that mesh: this point on the model should be over there. A ground control point is the survey record — one identity per physical monument, with the accuracy the surveyor quoted — and it belongs to the project, not to any one object.

The two connect: an SfM workflow's Seed Mesh Tie Points from Ground Control turns measured control into a mesh's tie points, so the mesh ribbon's Corrections → GCP can pull that mesh onto the survey.

Properties​

Select a ground control point to see its properties.

Position​

  • Name — the surveyor's label for the point (e.g. GCP_07).
  • X / East, Y / North, Z — the surveyed position, in the project's coordinate system.

Survey​

  • Horizontal accuracy (m) — the one-sigma horizontal accuracy quoted by the surveyor. Weights this point against the photographs in the bundle adjustment.

  • Vertical accuracy (m) — the one-sigma vertical accuracy.

  • Use as control —

    • On (control point): the reconstruction is fitted to this point.
    • Off (check point): the point is held back from the fit and only measured.

    Check points matter. A control point's residual only shows how well the fit absorbed it; a check point's residual is the only independent estimate of how accurate the model really is.

Last reconstruction​

  • Residual 3D (m) — how far the reconstruction placed this point from where it was surveyed.
  • Residual dX, dY, dZ (m) — the same error per axis, in the project's coordinate system (fitted minus surveyed).
  • From workflow — the SfM workflow this point was published from.

A point that no reconstruction has measured reads not measured rather than showing a zero error — marking it in fewer than two photographs means there is no measurement, which is not the same as a perfect one.

Notes​

  • Editing a point's coordinate here does not feed back into a solve. The survey is the authority, so control flows from the survey into the model, never the other way. Re-import the CSV if the survey changes.
  • Points are matched by name when a workflow publishes them, so re-importing a CSV or re-running a reconstruction updates the existing points rather than stacking duplicates.