Hi geomaticist,
Here is something that might be of interest for you:
Survey precision under direct georeferencing could be 2–3 times better than from GCP-control.
James et al 2017 (EARTH SURFACE PROCESSES AND LANDFORMS)
James et al have done some extensive research on marker precision. Their work suggests that photogrammetric models based on 'air control points' with cm accuracy are more accurate than models that have been processed using traditional GCPs at cm accuracy.
Hi SAV
I apologise for the partial deconstruction of some of your posts but I read them all in one go and I felt that I had to make some comments, so I am just leaving this here for future reference (even though the post diverged slightly off topic and was hijacked by Marija).
I think that replacing the term "precision" with "accuracy" is misleading.
James et al. reiterate the notion that a free bundle adjustment minimises the trace of the a posteriori covariance matrix (precision).
(Also, don't forget that their results are primarily based on computer simulations.)
The accuracy of the result will be unavoidably affected by the accuracy of the GCP coords, even if minimal constraints are used;
however, X cm on the ground are approximately X cm on the ground, whereas propagating X cm in the air to the ground will not only depend on the flying height and the imaging configuration but also on the quality of the interior orientation parameters.
As we are talking about non-metric cameras within a self-calibrating bundle adjustment, projective compensation should be expected and can potentially affect the "internal consisteny" of the result even if one is not interested in absolute georeferencing.
Hi geomaticist,
Using cm accuracy air control points allows you to achieve the same accuracy compared to cm accuracy ground control points. Theoretically it might be even more accurate because you generally have many more air control points (= number of images) then you would have ground control points.
I've done some tests where I first estimated the accurate location of geotagged UAV imagery based on cm accuracy ground control points. Then I used the estimated geolocation of the pictures (cm accuracy) and removed all ground control points from the project and tried to reconstruct the scene without them. The resulting model had basically the same cm accuracy as the one that used GCPs. You can do the same test yourself 
If you are on a limited budget, you might be better off sticking to the 'traditional' workflow using cm-accuracy ground control points.
Regards,
SAV
I do not doubt the fact that you got the same results, as you explained that you re-ran the project from scratch,
but I would argue that the only reason why you did get these results was because you already had the GCP values and were able to initialise the bundle adjustment with reliable approximate values in order for it to happily converge within a few iterations.
At some stage, however, it becomes a 'philosophical' question, IMO. The Earth is a quite dynamic system that changes over time. For example Australia is moving NE by about 7cm each year (!!!). One should be aware of such 'natural error' that needs to be considered as well.
I think in the end it boils down to the project requirements. For some, the absolute accuracy is not crucial. For example, if I simply want to calculate a volume of an object (e.g, stockpile), I don't really care if the whole survey is off to the E/N/S or W by a few meters. As long as it is properly scaled and internally consistent, it will deliver the correct measurements/results.
ad 2) Note that your before/after point clouds will only be accurately aligned if you surveyed your ground control points at high accuracy (i.e., using and RTK GPS). If that's not the case, you could align your point clouds in CloudCompare using ICP before computing the cut/fill volumes. Details here: http://www.cloudcompare.org/doc/wiki/index.php?title=ICP
The "philosophical" question that you describe is taken care of by the realisation of the terrestrial reference frames, unless of course the drone continuously records images all year long! :-)
Based on your most recent post, I understand that you appreciate the importance of GCPs and that the ICP is the last resort, especially in the dynamically changing environment of a quarry.
Unfortunately, ground control and check points for almost normal blocks of almost flat areas are not a "tradition" but a necessity, if metric accuracy is required.
regards
