Okay, results are in......
The same quarry site was processed using GCP data and I am pleasantly relieved that the closer to the truth has been recorded than the horrendous lie it gave last time.
The double pitched roof has more or less parallel contours that reflect the same horizontal line of the ridge:-
....and I don't have anywhere near as much of the bowl effect I had before:-
I've entered the surveyed GCP data and although the measurements given from the processed DD image are different, they are proportionate to the levels I gave. You will also see that 7no. GCPs were used and I measured 4no. points on 2no. roofs also. These areas were not so accurate unfortunately. The shorter flat roof was out by 1.265m and the double pitched roof was out by 1.7m.
The drone's path started in the SW corner of the map and flew east before returning west and making its way north so I wouldn't have expected the drone to have suffered that much sagging at this point so I can only put it down to an inaccuracy in how the data has been processed. I dare say had I have climbed up onto the roofs and pinned a marker there those points would also have tied in as the GCPs, but that's obviously impractical.
Due to the nature of the site, only those GCPs I've shown were able to be located and it's interesting to see that beyond them, the bowling affect starts to take place again, suggesting the surrounding land falls dramatically away to the east, which it doesn't. The road out to the east is almost flat but according to DD it has an 8m fall on it.
It's obvious that DD requires GCPs to be used for its elevation data to be considered remotely accurately, but it is disappointing to find that despite entering GCP data around the buildings, it's still managed to introduce discrepancies of up to 1.7m. The discrepancies appear to increase the greater the height from the nearby GCP.
With regards to the difference in measured levels, GCP1 was measured at 24.745 and despite entering that when tagging the GCP and despite entering the correct EPSG of 27700 and exporting as such, the same point has been determined at 71.65m by DD. I'm assuming it's me not using the correct export settings?