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      How to mark ground control points (GCPs) in PIX4Dcatch

      Users can manually mark ground control points in the PIX4Dcatch application before uploading the data to PIX4Dcloud for processing. This article describes how to mark images.

      Ground control points (GCPs) are reference points with known coordinates used to accurately georeference and align photogrammetry projects. In PIX4Dcatch, images containing GCPs can be manually marked before upload. These marked images, along with the associated GCP data, can then be uploaded to PIX4Dcloud for processing.

      GCPs are managed within a Point Collection, which stores the coordinate information for each point. For GCPs to be used during processing, at least three points must be marked, and each must appear in a minimum of two images.

      Important: The Point Collection must be in a projected coordinate reference system.

      To create Manual Ground Control Points follow these steps:

      1.  Open the Project management file_blk.jpg dialog.
      2. Select the project.
        Select project in PIX4Dcatch
      3. Click the Tools tools icon icon.

      4. Open the Point Collection GCP_management.png and the GCP file.

        Select point collection in PIX4Dcatch

      5. Close the dialog. The GCP icon will turn blue, and the selected Point Collection will be displayed. The Mark points option is now available.Mark points in PIX4Dcatch
      6. Select Mark pointsMark points button in PIX4Dcatch.
      7. Select the GCP name from the list to mark.
        Select GCP in PIX4Dcatch
      8. Mark a GCP on an image with a single tap. To place it precisely, tap and hold to open the magnifier tool. To expand the image or remove a marking, tap the three-dot menu at the top-right corner of the image.
        GCP magnifier in PIX4Dcatch
      9. Mark the GCP in at least two images.
      10. GCPs will appear on the map, and images where a GCP has been marked will be highlighted in green.
      11. Tap Save.
      12. Select and mark additional GCPs for the project as before.

      Tip: To achieve the highest absolute accuracy, mark each GCP in as many images as possible and place the marker at the pixel level using the highest zoom level.

      To process a project with GCPs, please visit Processing projects - PIX4Dcatch.