Control Points

A control point is a known piece of information about a calibration result, known prior to calibration. It can be used to improve or measure the calibration results.

Checkpoint vs Control Point

A control point may be used either to improve or to measure the quality of calibration results. In the former case, it is referred to simply as a control point; in the latter case, it is referred to as a checkpoint.

Short name	Engine API	Goal
Control point	ControlPoint.is_checkpoint == false	Improve calibration quality
Checkpoint	ControlPoint.is_checkpoint == true	Measure calibration quality

Definitions

Control point

A control point is either an MTP or a GCP.

Summary

Acronym	Formal name	Engine API
MTP	Tie point	MTP
GCP	Position constrained tie point	GCP

MTP

MTP is an acronym for Manual Tie Point. Because historically all tie points were created manually, this acronym is still used as an equivalent of tie point.

A tie point represents a set of 2D positions on images, where each position is the projection of the same 3D point in space.

There is only one 2D position per image, and only one 3D point in space.

Unless specifically stated, this 3D point is unknown prior to calibration processing. If this 3D point is known prior to calibration processing, use GCP as described below.

Examples of tie points

A tie point can be any of, but not limited to:

• manual tie point

• semantic tie point

• intersection tie point

GCP

GCP is an acronym for Ground Control Point.

A GCP is an MTP with a known 3D position (together with the uncertainty on that position). It represents known projections onto different images of the same known physical 3D position.

There is no constraint for the GCP coordinate to be on the ground.

A GCP is a type of position constrained tie point, where the known 3D position uses a known coordinate system.

Because historically all position constrained tie points were GCPs, the terms are still used as synonyms.