How to align more Crane Camera projects from a construction site

What is the site alignment?

To make sure that all the datasets capturing the same construction site are not shifted and to enable accurate measurements and comparisons, an alignment operation is required.

When a new project is processed, Pix4D uses an algorithm (ortho-alignment) which will find common points between an existing reference orthomosaic from a previous dataset and the new one from the new project.

As a result, the new 2D outputs (orthomosaic and DSM) will be shifted to match the reference one. The point cloud and 3D textured mesh will not be impacted.

How does the ortho-alignment work?

Ortho-alignment

  • This algorithm aligns only the 2D outputs (orthomosaic and DSM) of each site. The point cloud and 3D textured mesh will not be impacted.
  • When a new project is processed, the 2D outputs are aligned with respect to the latest reference project by finding common points.
  • A project is marked as a reference if:
    • It has the minimum number of images defined for each site. This number is defined internally by Pix4D considering the area of the site and the number of Crane Cameras installed.
    • It has at least 80% of images with respect to the last reference project.
Important: The above two criteria also affect the accuracy of the alignment itself. If they are not met for a specific project, the alignment of this project is not guaranteed to be accurate due to the lack of images.

Workflow example for a site with a minimum number of images set to 100. 

  1. The first project P1 of the site is processed on January 1st and has 120 images.
  2. P1 is marked internally as a reference by the Pix4D algorithm.
  3. A new project P2 is uploaded on January 2nd and has 60 images.
  4. The P2 will be aligned with respect to the latest reference project P1 but it will be marked as Low alignment confidence as the number of images is not 80% with respect to the latest reference project and it does not have the minimum number of required images. Therefore, it will not be a reference for future projects.
  5. A new project P3 is uploaded on January 3rd and has 105 images.
  6. P3 will be aligned with respect to the latest reference project P1. It will be marked as a reference for future projects as it has the minimum number of images defined for this site and it has at least 80% of images with respect to the latest reference project P1.
  7. A new project P4 is uploaded and will be aligned with respect to P3. It will be marked as a reference or as Low alignment confidence depending on the number of images.

 

Alignment errors on the Crane camera dataset

The dataset will be flagged with the notification Low alignment confidence when:

  • The dataset has fewer images than the minimum required number defined by the Pix4D algorithm.
  • It has fewer than 80% of the images with respect to the latest reference project.
  • The ortho-alignment algorithm failed.
 
Important: Low alignment confidence does not necessarily mean that the alignment is not accurate. it means that the alignment of this project cannot be guaranteed to be accurate due to the lack of images.

The Low alignment confidence is displayed:

  • With a warning icon on the error.pngOverlays and warning.png Annotations layers.
  • With a warning message on the lower-right corner when the Overlays and Annotations layers are selected.
  • With a warning message on the main page on the Compare mode.

 

 

 

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