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Author Topic: How are GCP's "weighted" when also using ppk camera positions?  (Read 956 times)

jmaeding

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Hello,
We have a DJI Mavic 3E, and use a ppk workflow. Surveyors run a base station but we don't hook to it (no RTK).
We process our photo locations through TBC and get a text file with adjusted camera locations for metashape.
We use that text file as reference and works well.
TBC has an issue where the elevations are lifted up for some reason by 3 or 4 ft.
We typically set 4 GCP's which we have correct coords for (no error like ppk data...), and when we add those in, the elevations are corrected.
Why is that though? We have 300 cameras and only 4 gcps.

The question is how the GCP's modify the results, compared to just camera locations used.

This is quite important to know, but also likely has a few answers.
Is there anything "clear" than can be said about adding gcp's, like "they dominate the vertical placement of the results"?
This topic is super important to understand when discussing why gcp's are still recommended in ppk workflows.
In our case, our ppk elevation values have issues which we will fix eventually.

thanks,

Paulo

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Re: How are GCP's "weighted" when also using ppk camera positions?
« Reply #1 on: June 06, 2023, 08:26:42 PM »
Hello jmaeding,

the GCPs and camera PPK positions are weighted according to their accuracy. The weight is defined according to the inverse of their variance or squared accuracy.

So if the camera accuracy in z is set to 0.01 m and GCP z to 0.02 m  then the camera Z position will have a weight 4 times that of GCP. So to reflect correct weighting. I would set camera z accuracy to 2 cm and GCP z accuracy to 0.01 m or better. That way the adjustment will be tighter to GCP z. If you suspect less z camera accuracy then you can change...
Best Regards,
Paul Pelletier,
Surveyor

jmaeding

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Re: How are GCP's "weighted" when also using ppk camera positions?
« Reply #2 on: June 08, 2023, 02:04:30 AM »
@Paulo,
Ah, very useful, thanks.
I also posed the question to support so getting good info from both sides.
There is another aspect to this that actually may be more important:
Does metashape mix together the photo coords and GCP's and come up with one set of control it uses?
Or does it form a "photo network" from photo info, then apply gcp correction to that?

What is going on with me is I have ppk data that seems perfect horizontally, but the vertical seems off by about a foot.
If I then add in 4 gcp's and run, with both photos and markers checked, the results seem on vertically.
But, how are areas away from the markers affected? Do they try to stick to location based on photo coords?

Metashape behavior is very clear to me if just using photo coords, or just gcp's.
Its when they are mixed that I have to be careful about fooling myself.
thanks

Paulo

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Re: How are GCP's "weighted" when also using ppk camera positions?
« Reply #3 on: June 08, 2023, 06:16:46 AM »
Yes if they are both checked they will be used as control with weights inverse proportional to accuracy squared. If you have doubts about Z accuracy of camera Z put its accuracy to 10 cm and GCP to 1 cm Z acc. The adjustment will closely fit the GCP Z. In case of only vertical shoots with just PPK or RTK, Z is correlated to f so important to have at least some GCPS to decorrelate or do a calibration flight prior to your survey....
Best Regards,
Paul Pelletier,
Surveyor

jmaeding

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Re: How are GCP's "weighted" when also using ppk camera positions?
« Reply #4 on: June 12, 2023, 08:46:05 PM »
Oh, very good tip on setting the camera z accuracy low to let GCP dominate Z.
Thanks again for the reply!