@Sylvain_POULAIN
The P4 GPS is below and slightly forward of the groundplane. Yes there is some shielding, but it does a good enough job of getting a fix. Normally 10-14 satellites.
@Matthew_Cunningham
Thanks. I have processed the data through Photoscan, Photomodeller UAS, Zephyr 3D, MapsMadeEasy, DroneDeploy, and Pix4D and ODM.
Currently I favour MapsMadeEasy for delivery to a non technical client, Photoscan ('cos I have a license) because it uses the GPU well and am quite excited by the progress with ODM/WebODM for delivering a robust open source solution.
Once the GPS data in the headers is replaced with the RTK position any processing workflow will produce a reasonably tight orthomosaic.
I have had problems with absolute orthomosaic positioning (ReachView v2.9.3 - RTK performance boost - #62 by Simon_Allen) due to differences in understanding of what the base position averaging does and where it is used. This is now resolved for me and mitigated procedurally. BUT I think this issue is still likely to cause a lot of people to produce different (and less absolutely accurate) results in post processing than they collected in realtime in the field.