Tie Points (GCPs, MTPs, and CPs) in PIX4Dmatic

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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.

 
Tip: 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. 

Pix4Dmatic_GCPs_import.gif

To import Tie Points coordinates:

  • Click gcp.pngTie 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. 

 
Note: It is possible to define the GCP coordinate system before importing the .txt or .csv file. Note that the coordinate system of GCPs also defines the output coordinate system. For more information about the use of coordinate systems and geoid models in PIX4Dmatic, please see:

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.

To import tie point marks:

  • Click gcp.pngTie Points > Pix4D_more_icon.png > Import marks... or
  • Click Menu bar > FilePix4D_import_icon.png 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.

Export marks

To export tie point marks:

  • Click gcp.pngTie Points > Pix4D_more_icon.png > Pix4D_export_icon.pngExport marks...
  • In the Export marks dialog, enter the File name and click Save.

Add Manual Tie Points

To add MTPs in the project:

  • [1] In the Image viewer, click Pix4Dmatic_add_tie_point.png 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 Pix4D_more_icon.png and Pix4Dmatic_add_tie_point.png Add tie point.

Pix4Dmatic_add_tie_point.jpg

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.

Pix4Dmatic_mark_tie_points.gif

To do so: 

  1. In the gcp.png 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 - Pix4Dmatic_marked_tie_point.png 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 see Navigation and panels article.
  3. (Optional) Repeat steps 1 and 2 for the rest of the Tie Points.
 
Tip: To speed up the marking procedure we recommend to:
  • [1] Pix4Dmatic_center_on_marker.png Center to center and resort images based on marks. 
  • [2] Change the number of displayed columns.
  • [3] Use the  Zoom Slider to change the zoom level of all images, or use the Alt + Scroll. Once in the full-screen marking mode, CTRL + 1 (minimum zoom) and CTRL +2 (maximum zoom) are enabled.
  • [4] Sort images and display by Distance, Reprojection error or Name.
  • [5] Pix4Dmatic_view_all.png Maximize image viewer to display the image viewer full screen, or use the Ctrl + F shortcut
    PIX4Dmatic_marking_tie__points_panel.png
See the Pix4Dmatic keyboard shortcuts article and the Navigate in the images article for more information.

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.

 
Access: On the menu bar click Process > Run AutoGCP.

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.

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.

PIX4Dmatic_Auto-Mark.gif

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.

 
Important: At the moment, PIX4Dmatic's Auto-mark is only supported for nadir projects. We strongly recommend to check the results of the Auto-mark algorithm and manually correct the positions if needed.

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 processing_options.png or,
  • On the Menu bar, click Process > Calibrate.
 
Tip: When adding, changing, or removing Tie Points marks after the Calibrate step is finished, we recommend using the Reoptimize cameras option as it reduces the processing time after applying changes to Tie Points. For more information, see Processing options - Reoptimize cameras.

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 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:

  • [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 Tie Point mark that does not fit with the calculated position is detected.

Pix4Dmatic_tie_point_quality_check.jpg

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

 
Note: The values are automatically updated every time a mark is added or removed in the images. However, the project needs to be reprocessed or reoptimized to see the real impact of changes on the reconstruction.

Quality report panel

The following is available after running the Calibrate or Reoptimize cameras step:

  • Detailed quality report (.txt):
    • In the Report panel, Click exportButton.png
    • On the Menu bar > File, click Export quality report...
  • General quality report :
    • On the right bar, click Report Pix4Dmatic_report_panel.png
      Pix4Dmatic_quality_report_panel.jpg

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

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4 comments

  • Jack Wilcox

    How can you assess the accuracy of the model using checkpoints? I do not see an option for checkpoints therefore I have no way to verify the accuracy of the model which makes this SW pretty much obsolete for any surveying applications.

  • Blaž (Pix4D)

    Hi Jack, 

    Nice to see you are using PIX4Dmatic (:

    At the moment, only GCPs are available inPIX4Dmatic. We are working on supporting checkpoints in order to give users a better way to verify the accuracy of the reconstruction. If things go as planned, checkpoints could be available by the end of the year. 

    Best,

    Edited by Blaž (Pix4D)
  • Thomas Cerchiara

    Check points would be an awesome addition. Right now we're using Global Mapper.

    Tom - MJ Engineering and Land Surveying

  • Blaž (Pix4D)

    Hi Thomas and Jack,

    Checkpoints are available in PIX4Dmatic and you will be able to use them for quality assessment. 

    After you import the Tie Points, make sure to change the type from GCP to Checkpoint. 

    Once the calibrate step is finished, you can assess the errors either directly in the Tie points table or in the Quality report. 
    Best,

    Edited by Blaž (Pix4D)
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