Sorry for the silence. No news here, the clouds are back, and the road to up above the clouds is closed due to landslides

Your explanation of the deviation seems very sound! I agree that the test was really a challenge, especially as my compass is in the observatory, and not here . But such extreme settings often help to pinpoint problems.
I absolutely agree that it's not really needed to change the plotting. If one gets such large corrections an iteration is needed anyhow, and that one will have better matched movement paths. So all fine for now. I'll pester you again once I find more silly ideas

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