I have made many terrain models, with good results, using a Phantom 3. I was testing some new parameters for image collection that I wanted to try, and the result produced an extremely curved model. The curve in the model, however, was caused by a curve in the photos. I flew a grid that had 15 or so back and forth lines, and I had enough overlap so that 1 automatic tie point was “seen” by 4 lines of photos. The height of the phantom was consistent throughout the flight. The software placed all the photos in the correct location, but the photos as a collective whole looked as if they were pressed over a ball: the center was high and each successive photo sloped down towards the edges. The curvature was so extreme that I immediately noticed it after the initial processing was complete. The curve is there with or without using the geolocation information. I haven’t tried GCP’s, but I’m more interested in why the curve is there. I went back and look at my earlier “successful” models, and noticed the same curve in almost all of them, only to a lesser degree.