Beginner’s question here, my first project in WebODM generates results that are about 50m lower than the geodetic survey that was made earlier. I started the process on default settings. I noticed that there is mention on forums of a problem with DJI drones (DJI Mini 2 in my case) not reading/storing the correct altitude. Is this problem common with DJI drones and is it possible to get accurate values?
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Did you use GCP’s?
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No, just photos from drone.
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GPS is in WGS84 that have an ellipseoid and your geodetic survey probably have another geodetic system (projection) with a geoid.
So the planar coordinates and the hight system is probably different.
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Thank you for your prompt reply, my survey is in EPSG 3765. Is there a way to run the calculation in this projection in WebODM?
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if you look at the exif/xmp data from your images, DJI normally uses the above sea level altitude as referenced by the - GPS altitude reference tag. also in the metadata is -absolute altitude , -gps altitude and -relative altitude. relative altitude will be your altitude above ground gps and absolute are above sea level. you can use exiftool to apply an offset (offset being the difference of survey and your absolute altitude tags) taking into account odm will subtract relative altitude from gps altitude to determine the ground elevation. with the proper offsets applied you can run the images again in odm. However this will never be as accurate as using GCP’s. hope this helps.
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I checked the EXIF data and the heights were just way off. I corrected the altitude with exif tool and managed to get usable results. Fortunately, the other data except the heights are completely correct.
I have to take into account that GPS height does not provide reliable values. Thank you for your help, this really was a quick solution
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