Why Use Ground Control Points?
While our platform generates high-quality relative models, connecting your data to the real world requires Ground Control Points (GCPs). GCPs act as the "anchors" that lock your digital twin to a specific coordinate system (e.g., UTM, State Plane) through a process called a Rigid Body Affine Transformation.
Unlike "rubber sheeting" (which stretches a model to fit points), a rigid body transformation simply rotates, translates, and scales your model to fit your control points without distorting the data.
Selecting the Right Targets
As the GCPs are extracted from the point cloud after the cloud based processing is complete, it is important to ensure the targets used will be visible in the point cloud.
- Temporary control points
- Painted lath
- Aerial target (checkerboard)
- Semi-Permanent control points
- Magnetic nails
- Painted stencil target
- Natural targets (expansion joint)
- Can be placed on ground or walls
- Targets will need to be visible in the point cloud to extract
Placement of GCPs for capture
To perform a successful Rigid Body Affine Transformation, the geometry of your GCP placement is just as important as the quantity.
Bound Your Site
Imagine drawing a perimeter line connecting your outermost GCPs. This is your "Convex Hull."
Inside the Hull: Data is interpolated and highly accurate.
Outside the Hull: Data is extrapolated and error increases rapidly the further you go.
The Fix: Always place GCPs at the very corners or edges of your project area to "box in" your site.
Avoid the "Line of Death" (Collinearity)
Never place your targets in a straight line (e.g., only along a road centerline).
The Risk: If points are linear, the model can essentially "spin" around that axis like a rotisserie chicken.
The Fix: Create triangles. Stagger points on either side of the corridor or site to lock down the rotation.
Survey Quality: "Garbage In, Garbage Out"
Your model can never be more accurate than the survey coordinates you provide. If you require a model accuracy of 2 cm, your GCPs must be surveyed with an accuracy better than 2 cm.
Hierarchy of Survey Methods
- Total Station (Highest Accuracy): Essential for indoor work or sites where GNSS signals are obstructed. Millimeter-level precision.
- RTK/Network GNSS (High Accuracy): Ideal for open outdoor sites. Typically offers 1–3 cm accuracy.
- Handheld/Phone GPS (Low Accuracy): Not recommended for control. These often have errors of 2–5 meters, which will degrade the accuracy of the photogrammetry model.
Stability is Key
Ensure the rod or tripod is perfectly level and stable during the shot. Even a slight wobble during a 5-second observation can introduce enough variance to conflict with the photogrammetry processing, leading to high "Residual Errors" (RMSE) in your final report.