I conduct monthly volumetric surveys with a phantom 4. In the last month’s flight, my results have been skewed vertically. I.e ground levels are correct, however all stockpiles and buildings are near twice the height that they truly are. Horizontally my results are consistent. Any ideas on what could be causing this?
Bad altitude EXIF data by DJI. Check the data in the pictures and I bet you find a huge discrepancy thanks to DJI rubbish…
If there is a big terrain change, e.g. stockpile, we recommend adding some GCPs in the surroundings in order to also get a good vertical representation of the area. It might have happened that all the GCPs at the ground have a similar height and are therefore not optimal for defining the vertical axis.
You can also try to add scale constraints, but keep in mind that you will probably need to remove the Image geolocation information in order that the constraint will be taken into account. More here: How to scale a project
Also, double check the camera optimization in the quality report, especially the focal length optimization. In case that the computed camera parameters are far from initial you can try to use All-Prior calibration option in Step 1. Initial Processing (Advanced) -> Calibration -> Camera Optimization -> Internal Parameters Optimization -> All Prior.