Weird elevation discrepency

I mapped an area with WebODM (default options) based on images taken from a phantom 3 pro 2.0. The dsm shows elevations that are about 80m lower than correct values according to survey maps/google maps.

Based on searching the forum for similar problems, there is consensus that standard GPS altitude values are not accurate. However, that doesn’t seem to be the issue. I checked the exif data of the images and the drone’s recorded GPS altitude has near perfect accuracy. Also the drone took the photos from 80m above ground. It is as if the drone records the elevation at ground level while webodm assumes the drone records the camera’s elevation. This is my first use of both the drone and the software so my understanding of this issue is probably lacking. Hoping to hear from those with better knowledge.


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I had problems just getting a correct model with the rtk data so I deleted the rtk in exif and just ran with the GCP.

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thanks for replying but I don’t understand the sentence. What are GCP and RTK?

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RTK: gnss(”gps”) with corrections from a base station. When talking about drones it’s normally a drone with an rtk receiver built in.

GCP: Ground Control Point. A marking on the ground measured with RTK or better.

Bonus fact: GPS is just the US satellite navigation system. GNSS is the umbrella name for all systems. An GNSS receiver can use several of the systems at the same time.


Thank you for the explanations!

I tried adding GCPs and ODM choked on them. But the problem for me is I get excellent RTK data, almost perfect on all axes, and the resulting map is accurate except the elevation that seems to be a quite simple misinterpretation by ODM. I was hoping that this is something that can be easily fixed.

Is it DSM or a cloud?

it is DSM

You can adjust the hight in qgis if it helps. There’s a lot of raster tools there

The drone must record the camera positions. It is a common issue for phantom drones that record a consistently wrong altitude, between 70 and 100 meters below the real one. I don’t know why.

If you can’t use control points, I don’t know of any other way to solve it than by changing the altitude exif tag on all images.

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Thanks for all youe replies!

Every time I’ve seen this, it has been a malformed GCP file. If you are able to share your GCP file (and preferably also your images), we can help troubleshoot.

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I’ve seen that rtk and GCP at the same time can give strange results

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Mmm, good point. Probably need to override reported GPS values.

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