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Author Topic: RTK + single GCP  (Read 1232 times)

tom_rc33

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RTK + single GCP
« on: June 04, 2021, 06:47:56 PM »
Hi,

I've recently started working on a project with an RTK setup (DJI Phantom 4 Multispectral + DJI D-RTK 2 base station). My understanding is that this sort of RTK positioning will be centimetre-precise and internally consistent, but may be offset in real world coordinates by some decimetres.

I have read that this uniform offset can be corrected using a single GCP (e.g. https://doi.org/10.3390/rs9020172). This would be great because we are operating in an environment where, due to time and access limitations, we can only place a single GCP.

My plan was to use the RTK camera positions during image alignment to obtain a precise geometry, and then introduce the single GCP during the camera optimisation process. Effectively, I want perform a translation on my dataset to align it with the single GCP.

However, after going through the full Metashape processing chain it appears that the GCP marker still reports nearly a metre of error, which I can see when I export the orthophoto and view it in GIS software. After searching this forum, I'm thinking my problem may be that I do not uncheck the photos in the Reference window before performing an optimisation, leaving Agisoft to try and model both the camera positions and GCP position together. Is this the case? If so, my new processing chain would look as follows:

  • Import photos (and RTK camera positions)
  • Align photos
  • Import GCP and mark in photos.
  • UNCHECK all photo locations. < I currently do not this
  • Optimise cameras
  • Dense cloud, DEM, ortho, etc.

Is this the case? Or would unchecking photo locations before performing an optimisation mean that I 'lose' the benefits of drone RTK positioning?

Thanks in advance!

Alexey Pasumansky

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Re: RTK + single GCP
« Reply #1 on: June 04, 2021, 08:15:49 PM »
Hello tom_rc33,

The coordinate information for cameras and markers from the Reference will be considered for optimization and georeferencing process only for those items that are checked on in the pane.

And you need to have at least three reference points that do not lay on the same line (cameras and/or markers) to georeference the model - using only one reference points will not work.

You can play with the measurement accuracy values for the cameras and markers, if one is preferred over others, in order to increase the GCP  weight in georeferencing process.
Best regards,
Alexey Pasumansky,
Agisoft LLC

tom_rc33

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Re: RTK + single GCP
« Reply #2 on: June 07, 2021, 11:07:00 AM »
Hello tom_rc33,

The coordinate information for cameras and markers from the Reference will be considered for optimization and georeferencing process only for those items that are checked on in the pane.

And you need to have at least three reference points that do not lay on the same line (cameras and/or markers) to georeference the model - using only one reference points will not work.

You can play with the measurement accuracy values for the cameras and markers, if one is preferred over others, in order to increase the GCP  weight in georeferencing process.

Thanks Alexey, this is really useful. I will play around with the measurement accuracy values - although I guess by vastly increasing the measurement accuracy value for the cameras, I might be losing the benefits that the RTK positioning provides to the process?

I guess my best option might be to literally perform a translation in other software after processing.

MaciekK

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Re: RTK + single GCP
« Reply #3 on: June 07, 2021, 11:17:01 PM »
tom_rc33,

Can you try to set real values, e.g. 0.03m for cameras and 0.01 for GCP? It usually works for me. If these near-true error values produce very bad results, you have to look elsewhere for the problem.
MK

SAV

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Re: RTK + single GCP
« Reply #4 on: June 22, 2021, 02:53:36 AM »
Hi tom_rc33,

P4 RTK with base station (= internally consistent; cm accuracy) + 1 accurate GCP to shift the model works.
However, to do it manually is quite complicated and time-consuming in Agisoft Metashape Pro.

But with Python scripting it can be done :)

Best regards,
SAV


Hi,

I've recently started working on a project with an RTK setup (DJI Phantom 4 Multispectral + DJI D-RTK 2 base station). My understanding is that this sort of RTK positioning will be centimetre-precise and internally consistent, but may be offset in real world coordinates by some decimetres.

I have read that this uniform offset can be corrected using a single GCP (e.g. https://doi.org/10.3390/rs9020172). This would be great because we are operating in an environment where, due to time and access limitations, we can only place a single GCP.

My plan was to use the RTK camera positions during image alignment to obtain a precise geometry, and then introduce the single GCP during the camera optimisation process. Effectively, I want perform a translation on my dataset to align it with the single GCP.

However, after going through the full Metashape processing chain it appears that the GCP marker still reports nearly a metre of error, which I can see when I export the orthophoto and view it in GIS software. After searching this forum, I'm thinking my problem may be that I do not uncheck the photos in the Reference window before performing an optimisation, leaving Agisoft to try and model both the camera positions and GCP position together. Is this the case? If so, my new processing chain would look as follows:

  • Import photos (and RTK camera positions)
  • Align photos
  • Import GCP and mark in photos.
  • UNCHECK all photo locations. < I currently do not this
  • Optimise cameras
  • Dense cloud, DEM, ortho, etc.

Is this the case? Or would unchecking photo locations before performing an optimisation mean that I 'lose' the benefits of drone RTK positioning?

Thanks in advance!

HansR

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Re: RTK + single GCP
« Reply #5 on: July 01, 2021, 12:42:58 PM »
Hi, if you like to "shift / modify" the RTK coordinates of all jpgs from a flight to match one GCP, you can easily use REDtoolbox software https://www.redcatch.at/redtoolbox/

The related tutorial video is available on YouTube: https://www.youtube.com/watch?v=8Zyz9sQFbKI

cheers
« Last Edit: July 01, 2021, 12:44:32 PM by HansR »

c-r-o-n-o-s

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Re: RTK + single GCP
« Reply #6 on: July 03, 2021, 09:47:09 AM »
Hey, "HansR" sounds like the owner of RedCatch! ;)
RedToolBox is really a good software.

However, I have done many tests with UAV PPK.
One GCP is only enough if you have manageable projects.
As soon as they get a bit bigger, the whole system "tilts" around this one point.

The "golden" 5 points (dice 5) rule should still be the rule.
But of course that depends on the accuracy requirements.

HansR

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Re: RTK + single GCP
« Reply #7 on: July 04, 2021, 10:04:03 AM »
Yes, be careful with the tilt especially for large areas with multiple flights.
We identified three reasons for this:
- Wrong "f" ... did you deactivate "dewrap" on drone? and Set "f" fix?
- Changes in geoidundulation expecially in long distance flights (street mapping, powerlines) might influence this too (we're here in mountain areas, 1km is a few centimetres difference).
- Different Geoid information for GCP measurements.

"dice 5" is always a good rule  :) :)    And in addition to this (if you have time and possibility) the highest and lowest point should be included too :-)  (more important for non-RTK drones)

Cheers, Hans   ;)    (yes... but newbie here in forum  ;))