OK so based on @andrewm's post about tweaking settings, and on this seemingly "oh yeah you might want to try this" statement in the FAQ on the Pix4D forum thread for processing P4P RTK data:
What could cause a big offset when comparing the results with checkpoints? - The horizontal offset could be caused by the horizontal grid corrections and transformations.
In some cases, when flying fast, close to the ground or in high winds, enabling the linear shutter optimization could reduce the offset. - The vertical offset could be present due to wrongly selected vertical coordinates system or custom geoid models.
In case the optimized camera parameters are far from the initial values, e.g. focal length, we recommend using the All Prior processing option.
(Highlight mine)
And the fact that the author Blaž also mentioned "All Prior" in my correspondence with him, I decided to reprocess the 4 flights from 20190513 with only that parameter changed. The setting is in Processing Options>Initial Processing>Advanced>Calibration>Camera Optimization>Internal Parameters Optimization (whew!).
Lo and behold, my 80m AGL flight Z accuracy improved (went from 0.434m and 0.621m to 0.165m and 0.111m, for D-RTK 2 and NTRIP, respectively). However, 40m Z accuracy suffered, although not to the same degree as the improvement @ 80m (0.039m and 0.043m to 0.062m and 0.053m, for D-RTK 2 and NTRIP, respectively).
The accuracy at 80m AGL is still nowhere close to 3xGSD (that would be 0.072m, so it's closer to 6-7XGSD) but it is an improvement. The accuracy @ 40m AGL using the "All" setting rather than "All Prior" camera optimization setting is at least close to 3XGSD (0.037m).
What does it all mean? I dunno. Except that better results are possible, and hopefully we'll end up with some processing guidelines that produce more consistent results across the range of altitudes we need to fly.
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