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Author Topic: Agisoft PPK process  (Read 4292 times)

V Aero

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Agisoft PPK process
« on: November 07, 2018, 05:14:00 AM »
Hi All,
I am running the latest Agisoft version, trying to use a PPK GPS system, and checking the accuracy using GCP's, however I seem to be getting 1m error.

Could someone check that my workflow is correct?
I am doing the following:
- Import Images
- read in image co-ordinates (saved in a file using WGS84)
- Set image accuracy (0.05m) and select rolling shutter compensation.
- align images on medium (40,000 and 4,000)
- Using gradual selection, filter reconstruction Uncertainty (limit 10), Reprojection error (limit 0.5) and Projection Accuracy (Limit 10). optimising after each filter.
- Import GCP file (GDA 94 MGA Zone 56 with AHD Height for the Vertical CS)
- measuring the difference between GCP and projected location

there seems to be roughly 1.1m error across the GCP's.
Any thoughts as to my error source? I'm fairly confident in the accuracy of my camera locations (have measured to 5cm by placing over GCP).
is there an error in my process, or a co-ordinate conversion error?
Link to data is:

https://www.dropbox.com/sh/0a2bjkkw6zz9ixs/AADZ2mXaBZ2-Y4oRTgqlc9zqa?dl=0

Any help would be greatly appreciated.
Cheers,
Andrew

Paulo

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Re: Agisoft PPK process
« Reply #1 on: November 07, 2018, 08:18:32 AM »
Andrew,

I think it is a datum transformation problem…..

The GDA94 no longer corresponds to WGS94 datum as from its creation (2000), positions have changed due to tectonic motion...That is why there is a new datum for Australia GDA2020  which is about 1.5 meters offset from GDA94 in your data set area...
I would suggest to transform your GCPS from GDA94 to GDA2020 and from there your datum will correspond to WGS94....

Here is file of transformed geo coordinates in GDA2020 using on line transformation service...

Hope this helps,
Best Regards,
Paul Pelletier,
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johnhalifax

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Re: Agisoft PPK process
« Reply #2 on: November 07, 2018, 10:24:29 AM »
Hi
See my post I am experiencing exactly the same problem. There is a major issue here.
I have tried a number of ways to correct this and they don't work.
I am convinced that if the correct transformation parameters can be set in agisoft this will work but there is no way we can access that part of the software. I have to resolve this if you or anyone else would like to contact me directly and we can see what's to be done.

karad

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Re: Agisoft PPK process
« Reply #3 on: November 07, 2018, 11:18:21 AM »
hi,
1. alignment constructs the model in an arrbitrary coordinate system
2. after importing GCP (georeferencing) you perform a 7-parameters transformation that rotates and scales the model only (linear transformation).
3. Optimization step ( classical bundle adjustment) wraps the model to conform to the GCP (non linear transformation).
I think you must perform the optimization step (consult the latest user manual p.59) to improve the results.