I have taken a number of photos of a 400m long bridge with a UAV and have had problems getting it to calibrate. On inspection it seems that most of the keypoints for the images are coming from the surrounding vegetation with hardly any coming from the bridge itself (see images). The bridge is wrought iron with an iron deck with gravel infill (see close up). Is it that the photo was taken to far away…too close or is it just that this is not a suitable subject for photogrammetric modelling? .
Many thanks in advance for any help.