Reprojection error

The reprojection errors indicate the difference between points' calculated 3D position and their marked position on images.

What is the reprojection error

When a point is computed automatically (ATP), or marked by the user (MTP, GCP, or CP) on at least two images, the 3D coordinates of this point are computed using the camera's internal and external parameters as well as the position of the point in the images.

Photogrammetry reprojection error

External and internal camera positions help defining the correlation between 2D keypoints on different images and calculate its' 3D position.

Once the 3D coordinates of the point are computed, the 3D point is reprojected on all the images that it appears. This error depends on the quality of the camera calibration (position and orientation), as well as on the quality of the marked point on the images (position and zoom level at which the point is marked).

Photogrammetry reprojection error

The distance between the marked and the reprojected point on one image is the reprojection error.

How to check the reprojection error in the project

In the rayCloud

In Pix4Dmapper and Pix4Dmatic, The reprojection error can be visualized in the rayCloud by selecting a 3D point.

 
Example: The reprojection of the point is the green cross that can be seen on the images in the Images section of the right sidebar. If the point is reprojected in the same position on multiple images, then this is a good indicator of the quality of the reconstruction of a model.

A 3D point reprojected on images in Pix4Dmapper.

Pix4Dmatic_reprojection_error.jpg

A 3D point reprojected on images in Pix4Dmatic. 

The reprojection error is one indication of the quality of the calibration process and should be less than or equal to one pixel.

In the quality report

 
Access: The reprojection error is visible in the quality report:

Troubleshooting

Multiple factors can cause excessive tie points reprojection error, including, but not limited to:

  1. Blurry or poorly exposed images.
  2. An insufficient number of tie points.
  3. An insufficient number of tie points marks on images.
  4. Inaccurately tie points marks on one or more images.

Tie points with a significantly higher reprojection error may indicate the presence of a problem in the project and should be verified. Checkpoints should be used to assess the accuracy of the project.

For more information see Tie points in photogrammetry project (ATP, GCP, MTP, and CP) article. 

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