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.

Tie points can be imported and marked at different stages:

• Before the Calibrate step: This requires more manual work as the position of Tie Points on images is not precise as it is determined only based on the initial image geolocation and orientation. However, marking the Tie Points before processing can eliminate the need for reprocessing or reoptimizing the Calibrate step.
• After the Calibrate step: The position of Tie Points on images is more precise as external and internal camera parameters are calculated. However, you will need to reprocess or reoptimize the Calibrate step.

Import Tie Points

It is possible to import GCPs and Checkpoints in the .txt or .csv file format. For more information see GCPs import format article. 

Importing a GCP .txt file into PIX4Dmatic.

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, the 2D and 3D view.

Import and export marks (Optional)

Import marks

It is possible to import marks from PIX4Dmatic and PIX4Dmapper projects. This way it is not needed to mark them in the images again.

For more information: How to import and export Tie Points marks - PIX4Dmatic.

Export marks

Add Manual Tie Points

To add MTPs in the project:

• [1] In the Image viewer, click MTP, or
• [2] In the Image viewer, right-click the image and click Add & and mark new MTP, or
• [3] In the Tie points panel, click and Add tie point.

It is possible to add MTPs from the image viewer [1], [2], or from the Tie points panel [3].

Mark Tie Points

Once Tie Points are imported or added, they can be marked either before running the processing or after.

To do so: 

1. In the Tie Points panel, click the Tie Point, or select the point in the 3D view.
2. On images, use left-click to mark the Tie Point. A yellow cross - appears each time the Tie Point is marked.
   
    

   Important: To change the display of an individual image:

   • Hold Ctrl and left-click to pan on the image.
   • Hold Ctrl and rotate the scroll wheel to zoom in and out of the image.

   For more information:e Navigation and panel.
3. (Optional) Repeat steps 1 and 2 for the rest of the Tie Points.

Automatic target detection - AutoGCP algorithm

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. 
• GCP and Checkpoint type of tie points are available.

More information about the algorithm and the best practices is available here: Automatic target detection, AutoGCP algorithm.

Automatic marking, Auto-mark algorithm

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.

Use Tie Points for processing

We recommend adding the Tie Points before processing the project. In case that GCPs and Checkpoints with a different coordinate system are imported after processing, the images will need to be reprocessed and previous calculations will be lost.

To process the project:

• Click Process  or,
• On the Menu bar, click Process > Calibrate.

Checking Tie Points errors

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: What is the relative and absolute accuracy of drone mapping. 

Tie points panel

In the Tie Points panel the following can be used to assess the quality of the reconstruction:

• [1] Reprojection error [px], the reprojection error of the Tie Point on images.
• [2] 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.
• [3] 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.
• [4] 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.
• [5] Outliers, a detected Tie Point mark that does not fit with the calculated position.

The Tie Points panel includes information about reprojection and positional errors of each marked point.

Outliers

When a Tie point does not fit within the calculated position it is then considered an outlier. In PIX4Dmatic, this is indicated by a red circle in the Tie points table found in the Status center.

For more information on how to handle tie point outliers please click here:

How to handle tie point outliers - PIX4Dmatic

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 
    
    

For a detailed description of values included in the quality report: Quality report - PIX4Dmatic.