How to determine image geolocation accuracy

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

I did a PPK test last week, which didn’t yield the results I was hoping for. I used an Emlid Reach M+ on my Matrice 600 to log shutter events from my Sony A6000 mapping camera and post-processed this with raw base and rover data. After that, I geotagged the images in Pix4D prior to Step 1. I also placed 4 GCPs as checkpoints and this was the quality report:

Needless to say this is not acceptable. I posted this on a FB group and one of the users said I needed to change the accuracy of the photos in the Images Properties Editor. By default, it’s set to 5m/10m… and he said I should change it to 0.05m (since every picture does indeed have a very high accuracy). Painful process, since I had to do that manually for every picture, but okay. And this is the quality report after re-running Step 1:

 

I can’t really explain my feelings about this, but it feels like cheating or something. Besides that, the suggested 0.05 value for accuracy isn’t based on anything. The question here is twofold:

  • How does Pix4D process this image accuracy?

  • What is a realistic value, so that the quality report is trustworthy and can be shown to customers?

 

And I just did another test, whereby I changed the checkpoints to 3D GCPs… take a look at the absolute vertical error. I just don’t understand how this works…

2

Nobody? :confused:

3

A few years ago I experimented with Piksi and got inconsistent results using rtk/ppk techniques.

I’m not sure where your problem is, sorry.

4

The problem is that I want to know if the 5 cm image geolocation accury is a good number. Because the QR looks fine with that setting, even with checkpoints only. Nobody from Pix4D Support?

5

Hi Jan,

As other users already mentioned, it is crucial that the image geolocation accuracy geolocation indicated in the Pix4D project reflects the real values. 

The image geolocation accuracy of PPK systems is typically written directly in the EXIF of the images and represents the accuracy at the time the image is triggered. In some cases, the image geolocation accuracy is also written in the file that can be imported into Pix4Dmapper. 

As a side note, it is possible to change all the accuracy values at once, by right-clicking on the column you want to edit and using the Edit All option:

When the accuracy values are not computed by the software that is used for PPK computation one can estimate the image geolocation values by marking the GCPs in the project and analyzing the Quality Report after Step 1. is completed. 

In your case, the value (0.05m) is already a good estimation for the accuracy as most of the image coordinates lay inside the expected range and there are only a few outliers, approximately 3% (in red) of all images in the X and Y direction. 

One thing that might be worth checking is if the coordinates that are computed correspond to the image projection center or the center of the GPS antenna (lever arm correction). In case that the values represent the center of the GPS antenna instead of the camera center I would recommend decreasing the estimated accuracy value even more. 

Best, 

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