How to use GCPs in Pix4Dmatic

A Ground Control Point (GCP) is a point 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.

Import GCPs

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

Pix4Dmatic_import_GCPs.jpg

To import GCPs:

  • Click gcp.pngGCP to open the GCP table.
  • Drag and drop, or use 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 Import.
 
Access: For more information about the use of coordinate systems and geoid models in Pix4Dmatic see:

After the GCPs are imported, they are displayed in the GCP table and the 3D view. 

Mark GCPs

Once GCPs are imported, GCPs can be marked either before running the processing or after. To do so,  click Mark GCPs in the gcp.pngGCP table to mark the position of GCPs on imported images:

  1. In the GCP table, click on the GCP. 
  2. On images, use left-click to mark the GCP. A green cross appears each time the GCP is marked on the image.
     
    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 GCPs.
  4. When all GCPs are marked on the images, click Done

Use GCPs for processing

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

Checking GCP errors

Introducing GCPs in the project gives insights into the accuracy of the reconstruction. 

The following is available:

  • General report is generated after running the Calibrate or Reoptimize cameras step:
    • On the right bar, click Report Pix4Dmatic_report_panel.png
      Pix4Dmatic_quality_report.jpg
  • Detailed quality report (.txt).

For a detailed description of values included in the quality report, see Quality report - Pix4Dmatic.

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