GCPs and MTPs are used to improve the absolute and relative accuracy of photogrammetric projects while Checkpoints are used for quality assessment:
- Ground Control Points (GCPs) are points with known coordinates. Typically, GCP coordinates are very precise and are measured using RTK/PPK GNSS receivers or total stations. GCPs are used to accurately georeference a project to assess the accuracy of the reconstruction.
- Manual Tie Points (MTPs) are 3D points corresponding to features that are marked (clicked) by the user in the images. They are used for assessing and improving reconstruction accuracy.
- Checkpoints are used to assess the absolute accuracy of the model and are not used for georeferencing the project.
In this article, we will refer to GCPs, MTPs, and Checkpoints with the general term Tie Points.
In this article:
It is possible to import GCPs and Checkpoints in the .txt or .csv file format. For more information see GCPs import format article.
To import Tie Points coordinates:
- Click Tie Points to open the Tie Points table.
- Drag and drop, or use the Select from disk option to specify the file with GCPs.
- Define the Horizontal and vertical coordinate reference system.
- (Optional) Define the Geoid or Geoid height.
- Click Apply.
After the Tie Points are imported, they are displayed in the Tie points table and the 3D view.
It is possible to import marks from PIX4Dmatic and PIX4Dmapper projects. This way it is not needed to mark them in the images again.
To import tie point marks:
- Click Tie Points > > Import marks... or
- Click Menu bar > File > Import marks...
For more information see How to import GCP marks from PIX4Dmapper into PIX4Dmatic article.
To add MTPs in the project:
-  In the Image viewer, click MTP, or
-  In the Image viewer, right-click the image and click Add & and mark new MTP, or
-  In the Tie points panel, click and Add tie point.
It is possible to add MTPs from the image viewer , , or from the Tie points panel .
Once Tie Points are imported or added, they can be marked either before running the processing or after.
To do so:
- In the Tie Points panel, click the Tie Point, or select the point in the 3D view.
- On images, use left-click to mark the Tie Point. A yellow cross - appears each time the Tie Point is marked.
- (Optional) Repeat steps 1 and 2 for the rest of the Tie Points.
AutoGCPs algorithm automatically locates targets in images and detects their centers with pixel-level accuracy. It reduces the time needed for tie points marking.
The Run AutoGCP is available when:
- The Calibrate step is completed.
- Tie points are imported.
- Tie points with GCP type are available.
More information about the algorithm and the best practices is available in the Automatic target detection - AutoGCP algorithm article.
We recommend adding the Tie Points before processing the project. In case that GCPs and Checpoints with a different coordinate system are imported after processing, the images will need to be reprocessed and previous calculations will be lost.
To project the project:
- Click Process or,
- On the Menu bar, click Process > Calibrate.
Introducing Tie Points in the project gives insights into the accuracy of the reconstruction:
- GCPs and Checkpoints are used to assess absolute accuracy.
- MTPs are used to assess the relative accuracy.
For more information see the What is the relative and absolute accuracy of drone mapping article.
Tie points panel
In the Tie Points panel the following can be used to assess the quality of the reconstruction:
-  Reprojection error [px], the reprojection error of the Tie Point on images.
-  Position error X [units], The difference between the computed and the initial position of the ground control point in the X-axis, i.e., initial position - computed position.
-  Position error Y [units], The difference between the computed and the initial position of the ground control point in the Y-axis, i.e., initial position - computed position.
-  Position error Z [units], The difference between the computed and the initial position of the ground control point in the Z-axis, i.e., initial position - computed position.
-  Outliers, a Tie Point mark that does not fit with the calculated position is detected.
The Tie Points panel includes the information about reprojection and positional errors of each marked point.
Quality report panel
The following is available after running the Calibrate or Reoptimize cameras step:
- Detailed quality report (.txt):
- In the Report panel, Click
- On the Menu bar > File, click Export quality report...
- General quality report :
- On the right bar, click Report
- On the right bar, click Report
For a detailed description of values included in the quality report, see Quality report - PIX4Dmatic.