After checking generated DTM points and contours over my known survey points, I found a difference by 17.5 meters in elevation for ALL checked points which are well spread out within the study area. The entire DTM point fabric over 2 km2 has the same difference in elevation (less by 17.5m than reality).
DSM points and contours generated from DSM look good (reflect the reality of elevation) for same open areas without trees or man-made objects.
I don’t have a Pix4D processing window screenshots, but I used generic options such as DSM and DTM both x5 GSD, Contours 0.5m with resolution 1m and min line 10 vertices.
There is a screenshot from ArcMap window showing DTM contours and known survey points (red dots):
I would like to hear if anybody has experienced the same problem or can tell me what could cause that error which looks like an algorithm error by the same value.