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General / Re: RTK + single GCP
« on: February 08, 2022, 09:25:40 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!
Hey tom_rc33,
I've meddled with using or dropping image geotags in addition to incorporating GCP markers for additional spatial refinement. In my experience, there are differences, but they can be pretty modest. Your post got me wondering: how did you survey the location of your GCP, and did you survey the location of the base station? As mentioned, DJI's base station will have a global accuracy of about 1-2 m, but RTK should still offer a local accuracy of approx. 1-2 cm.