Support Website Contact Support Blog

Using checkpoints

Hi,

I want to use checkpoints to understand the accuracy of a reconstruction but I can’t find any information on the process I have to follow to incorporate them and then use them. Any help would be appreciated.

Thanks,

Nick

Hello Nick,

You can have information on how to import and mark GCPs here.

The checkpoints must be imported the same way and after the import, you can change the type of point from 3DGCP to Checkpoint:

You can then run Step 1 (or reoptimize in case you already run it) and those points will be used as check.

Regards.

Thanks, Daniel.

What is the process for then checking the accuracy of the reconstruction using the checkpoint? Where do I look?

Thanks.

Hi again,

The error is listed in the Quality Report right after the Ground Control Points.

More information can be found in these articles: QR help, QR specifications.

Regards.

Hello Daniel,

I have a similar question and found this information helpful. I have a job that we processed through the cloud yesterday using GCP’s. I did not import the check shots before processing however, and after reading this post I imported the checkpoints as you described, then Reoptimized. I went to the quality report and the results were not listed after the GCP’s. Do I need to also mark the checkpoints as I did the GCP’s, or is there something else I am missing in order to see the results of the checkpoints?

 

Thanks!

1 Like

Hello Andrew,

Yes, you have to mark the Checkpoints in the images as you did with the GCPs. Otherwise, the software cannot compute the coordinates and they will not be shown in the Quality Report.

Regards.

1 Like

Thank you, Daniel!

Hi Daniel,

Thanks for the above info. I have read the links you provided above, but I still have some questions. Given that we mark the Check Points as we do the GCPs, I’m not sure I understand the difference between them. Please could you explain how the check points differ from the GCPs and how this is understood within the QR. For example, I have attached a screenshot from a recent QR from one of our projects, which includes check points - what is the difference between the error data shown for the GCPs and that shown for the check points? If there is no difference then I guess my question is what is the point of the check points?

In terms of interpreting the information on check points can we assume that for checkpoint 8 the Y Error is within 0.0397m of the position taken using a survey grade GPS?

Many thanks.

-Hi Nick,

When you run Step1 with GCPs, the software will tie the block to them so that means that the computation is taken them into account.

The difference with the Check-Points is that they (Check-points) are not taken into account in the computation so the software is not trying to tie the block to them.

The check-points are very useful to check the accuracy as they are independent of the computation. 

Let´s say that you get a Pix4D project from someone else and you want to check the accuracy. One way to do it would be to go to the field, measure some points and compare them with the coordinates that you get in the Pix4D project. Your measurements in the filed are totally independent of the Pix4D project that you got. They would be check-points. 

There is an article about ithere.

By looking at the QR that you attach, we can estimate that the accuracy of the block is 3-5 cm (check points RMS).

I hope my answer helps.

Regards.