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GCP Error

I have GCP error and check points error and Mean reprojection error

How should I find Total error of complete 3D model.

 

Hi Chandrasekhar,

There is no 3D model error unless you have the ground truth 3D model and can compute the distance of the reconstruction to this ground truth.

Instead, you have different error measures that can be used to assess the quality of the reconstruction:

  • mean reprojection error is the average error in pixels of each 3D point projected onto the images on which they are visible. This should be less than 1 pixel.
  • GCP error and checkpoints error are the distance between the optimized 3D position and the user-input 3D position. This should be within the accuracy of the GCPs and checkpoints you input (usually in the centimeter range).

Validating accuracy in photogrammetry and even LiDAR requires an independent set of measurements, like hand measuring an object or something else at the scale you want for accuracy.  For instance on a cell tower where I know from drawings and tower climber measurements that a piece of steel is a certain size, then I can use software after Pix4D to approximate the difference from the point cloud to the catalog steel shape.  Once you are down to 1/16" to 1/32" relative accuracy then, as a Professional Engineer, I call that “engineering-grade”.  And there are plenty of online resources to do the same in “survey-grade” validation of accuracy.  You can reach me at Adam.Jordan@NHIAE.com if you want to get into more detail.

Thanking you sir.

But my question is I have mean reprojection error, GCPs RMSE xy and RMSE z values and also checkpoints RMSE xy and RMSE z values.

I need the absolute accuracy of my 3D model in terms of RMSE xy and RMSE z.

For me as a Professional Engineer, the reported error numbers mean nothing as absolute or relative accuracy are only determined by independent validation methods. The nature of the photogrammetry process requires extra validation than LiDAR. From my experience the error/accuracy values in the Quality Report are much “better” than true accuracy.

I am sure others have their own opinions as photogrammetry accuracy is a much debated topic on the Internet.

Yup!

Hello Chandrasekhar,

There is no such measure as a global accuracy of the model. The accuracy of the reconstructed model depends on many parameters and it varies across the model.

In this article, you can find more information about the accuracy of the Pix4D outputs. The quality report is one mean of assessing the accuracy of the reconstruction but there are more, like the use of verification points.

Regards,