As an example I processed a map twice. Once with GCP’s and once without. I used the single point elevation calibration to force the non-GCP map to an elevation as established by the GCP. I then went and checked two other GCP’s and they were off 2-3ft. Unfortunately with the lack of accuracy the results won’t be reproducible.
As an alternative if you do not have a way to set GCP’s you can setup what we like to call Rough GCP’s (RGCP) where we basically fly the first map and then create points based on the values. We then use those points as a GCP file moving forward. Error gets better but you are still around 1-2ft vertically. Horizontal should be good unless you are doing a large site, larger than 40-50 acres. The most beneficial thing this does is force the maps to align near perfectly horizontally whereas they can shift up to 3 meters with any kind of control. Problem is that you have to be on at least a Business plan and you would have to pay per map. This is a shortcoming of DD but is where we are at right now.