@MichaelL The workflow you are doing either with DD or Pix4D is typical for photogrammetry using a “normal” p4p. Even using a DJI drone that is advertised as “RTK” is no different. DJI’s current use of the term is misleading and not accurate.
If you were to use a precision gnss system on your p4p, and you have your camera calibrated, you will achieve highly accurate results without the need for GCP in the photogrammetry process. I will say again how this is possible.
With a normal p4p, when it snaps a picture it records it’s (the p4p’s) location in the image meta. That location could be meters off from the 3d point it records to the image. So when you process, the resulting geotiff can be meters off. To bring that geotiff closer to accurate we add suryveyed GCPs to tell the processor where certain points on the ground are “really” located and the processor adjusts the geotiff to account for them.
With a p4p equipped with a tuned precision gnss, it records the location of where the image was taken in 3d space very accurately. Not meters off. When you are done flying the mission, you have the image set with the meta recorded by the p4p and you also have a log file from the precision gnss system. You use software that takes the images and the log from the gnss, and the log from a survey network station as close to your site as possible, that first applies the corrections from the local CORs network (this is called PPK) and then takes the images and replaces the location meta with the ppk’ed locations from the precision gnss.
So, now you have your image set and the location data has all be replaced. Now, when you process with DD or Pix4D, the camera locations are very precise and not meters off. The resulting geotiff will (should) be very precise without adding GCPs into the mix. To “prove” your results, you take your surveyed points that you used to call GCPs and use them as “Check Shots” What that means is that you don’t add them to the processing. After the processing, you select any of those points and it’s location on the geotiff should be very close to it’s surveyed values. That “proves” your geotiff is accurate.
This is still a simple explanation but hopefully it will be enough for you to get the big picture in order to be able to research if further. There is a big difference between using the gps on the p4p to write locations to images when compared with replacing those locations with precision gnss/ppk corrected locations.
DJI is ready to release an RTK P4P. We will have to see if it is a “real” RTK capable craft or not though. And I would bet if it is it will cost a bit more than the normal p4p.