Drone mapping in challenging terrain - cliffs, canopy, and elevation change
Our crew just finished a nightmare site - 200 acres of mixed terrain for a proposed residential development. Steep hillsides (some near vertical), dense tree canopy in the ravines, and about 350 feet of total elevation change. Figured I'd share what worked and what didn't.
Flight planning: We split the site into three zones by terrain type. Open hilltops got a standard grid at 300ft AGL with terrain following. Steep slopes got a cross-hatch pattern at lower altitude (200ft) with 85/80 overlap. The ravines we flew manually at 150ft because the terrain following couldn't handle the rapid elevation changes safely.
GCP placement: This was the hardest part. We placed 22 GCPs total, way more than a flat site would need. Focused on getting them at elevation extremes - ridge tops and valley bottoms. Access to some locations required rappelling gear which was a first for me on a survey job.
Processing: Pix4D choked on the first attempt with default settings. The steep terrain and heavy canopy meant lots of poor tie points in the ravines. We ended up processing in three blocks and merging, which gave much better results. Used the 3D Maps template with high keypoint density.
Results: Final checkpoint RMSE was about 4cm horizontal and 6cm vertical, which is decent given the terrain. The canopy areas are basically holes in the surface model though - we supplemented with ground shots from total station in the critical areas.
Biggest lesson - budget twice the field time you think you need for terrain like this. And bring someone comfortable on steep slopes.