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:
Import Tie Points
Import and export marks (Optional)
Add Manual Tie Points
Mark Tie Points
Automatic target detection, AutoGCP algorithm
Automatic marking, Auto-mark algorithm
Use Tie Points for processing
Check Tie Points errors
Tie points panel
Quality report panel
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...
- In the Import marks dialog, enter the File name and click Open.
For more information see How to import GCP marks from PIX4Dmapper into PIX4Dmatic article.
To export tie point marks:
- Click Tie Points > > Export marks...
- In the Export marks dialog, enter the File name and click Save.
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.
The Automatic marking (Auto-mark) algorithm is used to automatically propagate marks on images. It is available when:
- The Calibrate step is completed.
- At least two marks are added manually on images.
The Auto-mark algorithm reduces time needed for marking as it automatically places marks on additional images.
The algorithm searches for automatic color correlation of a feature on the rest of images. Thereby, the position of the marked feature will be optimized in additional images if the color correlation is good. The images that have a yellow cross are taken into account for processing during Calibrate step.