New warning - flight altitude accuracy below expected threshold.

[JoshBerna]

Hey yall, I have a question regarding a new warning.

I do a good bit of flights for a university I work for. I've been helping monitor construction progress in a specific area, obtaining new imagery ~once a week. The drone is a Mavic3E, and I use flighthub 2 to setup and run all the flights, if that's relevant. I then process these images in Drone2Map. I have flown this particular area twice before and successfully produced orthophotos for those flights.

I recently updated Drone2Map to 2024.1. Upon attempting to process my newly collected imagery, I am met with the error: "flight altitude accuracy below expected threshold," when I attempt to start processing.

Upon checking my images, the altitudes collected are all as they should be, with the maximum variation in range coming out at ~2.0 meters or less. I believe this to be reasonable for my project. Images were all successfully taken with RTK, connected to the university's NTRIP profile.

I believe this to be a new aspect of Drone2Map 2024.10, but I was unable to locate the relevant changes in Esri's release notes, found here: https://doc.arcgis.com/en/drone2map/latest/get-started/whats-new.htm

I also see a new option under Options--Adjust Images to create an elevation surface, which I don't see mentioned in the release notes either, though this could be from a previous update I had not yet installed. I have this disabled for this project.

I am curious as to why I am receiving this error, when I would personally deem my obtained altitude values as both precise and accurate, relevant to my theoretical value. Is there a specific range that Drone2Map considers excessive for variance in altitude between images, or another reason unbeknownst to me that results in this error? I am only seeking to produce an orthophoto, so this shouldn't even be a big deal as far as I know, but I am aware that I have much to learn.

Thanks in advance for the help.

[JoshBerna]

Found an explanation for the error here: https://doc.arcgis.com/en/drone2map/latest/help/vertical-coordinate-systems.htm

From the Page (their math is wrong):

"Note:

If your image altitude is below the expected threshold based off the coordinate systems in use, then you will receive a warning recommending the use of ground control points or the adjust image altitude tool to apply a correction. The threshold is determined by comparing the relative altitude from the images (if available) to an adjusted relative altitude that is calculated automatically.

For example:

An image has an absolute altitude of 192.09m, a relative altitude of 62.74m, and the ground elevation at that point is 142.84m.

An adjusted relative altitude is calculated (absolute altitude - ground elevation): 192.09m - 142.84m = 49.25m

If the difference between the original relative altitude and the adjusted relative altitude is greater than 10% then the image is flagged for adjustment.

Difference: ABS(62.74m - 49.25m) = 21.54m

10% of 62.74m = 6.274m

21.54m > 6.274m so the image is flagged."

As to why this error occurred for this imagery and not for those obtained in previous flights of the same project, I am still investigating.

[MarkBarker1]

Hi @JoshBerna,

You are correct, we had a typo in the calculation and it will be fixed in the next documentation release with 2024.2. Thank you for bringing that to our attention. The example should read:

Difference: ABS(62.74m - 49.25m) = 13.49m

10% of 62.74m = 6.274m

13.49m > 6.274m so the image is flagged.

What you are seeing is a warning, not an error. So you could continue to process, but your vertical accuracy may be affected and products may not appear at the correct altitude when displayed on a basemap. We implemented this warning because many drone models have difficulty with obtaining accurate altitude readings. Drone2Map will query 100 random images from your flight and run the above calculation to determine if the image altitudes should be adjusted. If you have 100 or less images then all the images will be evaluated. 

As to why you are receiving the warning with one particular flight, you may want to examine the metadata and make sure the RTK accuracy flags are returning similar readings to the other flights. RTK flights don't always guarantee accuracy so it would be good to check to make sure the signal was reliable. Did this particular flight have a significant difference in altitude compared to the others?

Regards,
Mark

[JoshBerna]

Hi @MarkBarker1,

Thank you for the in-depth response. I have processed two 3D models with Terra, since we haven't obtained the advanced licenses yet and I have a free trial. They are i3s models, brought in as an .slpk. When brought into pro, the models both sit about 10.8 meters below the surface. I had attributed this to differences in world elevation data with the basemap, but I now see it could very well be my drone with incorrect altitude readings.

Upon examining both images tables from the flights in Drone2Map, I can see that the maximum range in altitude between either set sits around 2.5 meters. In comparison with another flight performed in a slightly distant, lower elevation area, with the same set altitude (244.3 ft AGL) the altitudes accounted for this elevation change (about 5 meters).

I am lead to believe the altitudes were collected correctly. The values are consistent with the elevation in the area, to the extent that no striking difference is identifiable. I can not distinguish any substantial differences in the lat/long values recorded either. Interestingly, when I go to process the project on another computer running Drone2Map 2023.2.2 rather than 2024.1.0, I do not receive the warning. I updated our processing workstation to 2024.1.0, but not my laptop yet. On this workstation, I have processed two other data sets, both of which returned the processing warning. My laptop and a colleagues, both still on 2023.2.2, do not receive this warning however for the same datasets.

Is this warning itself a new feature brought in 2024.1.0?

In any case, I'll have to look more into the altitude accuracy for the Mavic 3E. As it stands, I can remedy this issue with the cartographic offset option in pro. I intend to obtain some elevation readings at ground level with my Trimble to check the values from the drone.

Thanks again for your interest and the information.

Best,

Josh

[CodyBenkelman]

Josh

Yes, the altitude warning is a new feature at v2024.1.  Incorrect altitudes are one of the most common problems for our users - This warning was added seeking to avoid confusion and problems.  I've seen drones report altitudes with errors more than 100 m - although this should not happen with RTK. 

Users with RTK drones should have highly accurate (X,Y,Z) values but note RTK is almost always referenced to ellipsoidal height, not orthometric height - so with RTK or PPK you must set the proper vertical datum.   

Lacking RTK/PPK, I recommend *always* using control points.  (With RTK, I recommend using check points).

If you have surveyed points, be sure you know the proper Horizontal and Vertical coordinate systems for the points.  If you do not have surveyed points, you can extract points from the Esri Imagery Basemap - those points will not have any documented accuracy but *will* ensure your 3D products don't float above or sink below the basemap in a 3D scene.

Cody B.