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Author Topic: To CTL or not to CTL that is the RTK question?  (Read 3244 times)

mks_gis

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To CTL or not to CTL that is the RTK question?
« on: June 29, 2023, 06:59:34 PM »
Hi,

When processing imagery with RTK/PPK air control points, what is the best practice re GCP? I've seen a Wingtra webinar where Alexey recommends to only use check points, but an Agisoft guide on this mentions using some CTL. Other sources state that some GCP is still best practice. I'd appreciate your views on this, with regard to Metashape processing. I'm not talking about check points, they are always useful to gauge result accuracy. If CTL are used, what are the priority settings in Metashape, is that controlled through the accuracy settings, as I understand it?

With RTK/PPK Alexey also recommends using Fit additional corrections for PPK data in Optimize, specifying all General parameters, but I've found better results with Adaptive on its own. Tbh, I always use adaptive, without fully understanding when to NOT use it. A simple guide on that would be helpful.

Thanks
M

Paulo

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Re: To CTL or not to CTL that is the RTK question?
« Reply #1 on: June 29, 2023, 07:14:23 PM »
Hello mks_gis,

When using just airborne RTK/PPK, you are just controlling position at level of photo capture. So in case of aerial drone, the XY controlled part is no problem but the Z (height) is as you are controlling Z from a position maybe 100 or more meters from the ground you wish to model. So as in photogrammetry, extrapolation is a nono, I would suggest to have a few GCPs on the ground to solve.. Another solution is to fly a control field prior (with varying viewing geometry) to get a solid camera calibration and then use these calibrated values in your further work. Note that in case of mainly vertical imagery focal length and Z are highly correlated and small error in f will impact Z quite a bit and having GCPs or good prior calibration will avoid this...

Hope this makes some sense,
Best Regards,
Paul Pelletier,
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mks_gis

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Re: To CTL or not to CTL that is the RTK question?
« Reply #2 on: June 30, 2023, 03:22:47 PM »
Thanks Paulo, that makes sense. I fully agree, my curiosity stems from the webinar where Alexey and Wingtra say not to use CTL, just to use any GCP as CHK.

Although my recent, and first, PPK processing without CTL or using 4 CTL didn't make a significant difference in x/y or z. I may need to set up a proper test of this.