Thank you for taking the time to help Andrew. You made some excellent suggestions and gave me food for thought to consider for the next mission. Please see my responses below to your questions.
The DJI X4S is a pretty good mapping camera, but everything about the mission must be perfect in order to get the kind of accuracy you’re looking for. I have essentially the same camera in my Phantom 4 Pro. What % overlap did you fly the mission? It should be at least 75/75. What capture software did you use?
I agree with your comment about set up being key. I was using a lab calibrated X4S from Klau Geomatics at 80% front and 65% side overlaps with UgCS mission planning software. I used Klau PPK software to resolve my baselines to my local Trimble R8-M3 Base Station position. Prior to PPK processing though I did baseline processing to 3 nearby CORS stations and 2 concurrent static Trimble R6 M4 GNSS set ups on NGS monuments to determine my base station position. I confirmed the Trimble Business Center final computed position against Trimble RTX, OPUS, and SMARTNET computed positions as well.
I have to use Drone Deploy to capture the images, as Pix4DCapture overrides the manual camera settings in DJI Go. Manually setting the focus to infinity and shutter speed to at least 1/800s is mandatory to prevent blurry images. The slightest out of focus images will add error.
For the same reason, I use UgCS to allow for proper manual exposure settings.
With your GSD of 0.086’, vertical accuracy between 0.172-0.258’ (2-3xGSD) is acceptable for elevations derived from photogrammetry. It’s possible to get 1-2x GSD vertical accuracy if all the surveying and image capture procedures are very precise. Your vertical RMSE of 0.113’ is actually very good (1.3x GSD). If you need higher accuracy, you have to fly lower or get a different camera. For example, using a 42MP Sony RX1RII camera, I get more accurate results at 400 feet than I do with my 20MP P4Pro camera at 200 feet.
Yes, I totally agree. I recently bought a RX1RII camera and custom gimble for my Dji M200. Amazing camera with crisp images and low distortion at 400’ (1.55cm GSD). However, recently the internal focus motor has caused image size to be cut in half and soft images (despite having all the manual exposure settings dialed in). This could be caused by too much vibration from the gimble rig and the M200. I have sent the camera off for repair so I hope they can fix it b/c I prefer that camera too. My X4S is my backup work horse. I usually fly at 1.5cm GSD (~180-200ft AGL) with that one, so I may have been pushing the limits with this mission at 2.7cm GSD.
Furthermore, I weighted down the aerial targets with rocks. Please see image below.
This is the problem point shown in images further up the thread. I suspect that the rocks caused the ATP to determine high points in relation to the center GCP point. However, when the DSM is subsequently created with Noise Filtering and surface smoothing checked ON, it blends the four corners, which would account for the ~+0.2’ vertical error I’m seeing. At the GCP it is fine, but the DSM output seems to have a smooth-factoring on the top of these small rocks, which disregard the GCP at the center of target. It is just a guess though. I just thought the GCP point was the anchor and regardless, the outputs should at a minimum should have that elevation close to the RTK-GNSS observations as the Quality Report indicated.
To generate the “cleanest” and most accurate point cloud possible, set the minimum number of matches to 6 instead of 4. This will result in fewer overall points in the point cloud, but the resulting DSM will be more accurate.
Ok I was always taught 4 to ensure somewhat efficient processing times. I will try with 6 as a test.
How was the RTK GCP/Checkpoint data collected? Using VRS or a local base with radio link to the rover?
I used a Trimble RTK (base/rover) system + 2 continuously logging GNSS setups on NGS monuments.
I’ve seen VRS shots vary by nearly 0.1’. When at all possible, I use a total station. For your project I would use a base-rover and not VRS. Your survey methodology has to result in points that are at least as accurate as your GSD.
I check my accuracy in ArcGIS and calculate RMSE values using the full resolution DSM. In ArcGIS I import my checkpoints then run a geoprocessing tool to extract the DSM values at each checkpoint. Then calculate my errors. I wouldn’t try to calculate errors from a resampled DSM.
Only difference is I use Virtual Surveyor so I can efficiently view the full resolution DSM and Ortho in 3D and do other analyses. I use ArcMap too to reduce the ortho size for Civil 3D consumption only.
Regardless, my checks on the decimated surface from Virtual Surveyor in Civil 3D could be some of the problem here, which is why I went back to the original raw output files generated from Pix4D and did the same checks. I see the same error.
I guess this is where I think the GCP placement relative to the surroundings is key. Meaning don’t place them near objects that could interfere with ATP and point cloud/dsm generation. Also I’m not going to use rocks to weight down the targets anymore. I will drive anchors in the ground and make target as smooth as possible on level ground.
I hope my comments are helpful.
They were thank you very much!