Strange hight with rtk

WebODM
1

Hello

Got a strange message while processing images from a Phantom 4 RTK. It said there was negative hights.

It feels strange because I had rtk with ntrip.

But I also had GCP so it turned out okay anyway.

Just want to get what happened.

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2

Can you dump the elevations from the images using QGIS? Does the P4 RTK do the strange “height above takeoff” that some (most?) other DJI platforms do?

3

I also got a lot of distortion in the DSM/ortophoto that I haven seen with my Mavic.

Can it be the rtk??

4

You can consider whether the area where you use RTK has a lot of obstructions, such as buildings that are too high, or trees.
in the fact when rtk runs under occluders, they block the rtk signal, causing data anomalies, horizontal deviations, elevation deviations. In fact, this kind of problem is very common in practical engineering measurement. What you need to do is ignore this bad data.

5

Under normal circumstances, if you encounter similar problems, you can eliminate the machine failure. It is a signal problem. As long as there is not a lot of bad data, it is acceptable.

6

Could it be a timing issue due to flying at speed, or were you stopped for each photo?
Positions updated at 5Hz when flying at 10m/s could perhaps lead to 2m errors in recorded positions.

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7

2m/s and shutter at 1/1250

I calculated it would mean one mm

8

RTK positions updated at 2 kHz?

Image motion during exposure and position recorded are separate issues.

9

GPS hight is positive about 96, there’s no take off hight. I’m thinking that WebODM gets it wrong.

I also wonder if I should choose distortion correction when flying.

10

I did another run with the drone today, I checked the box for corrected photos.

And the result was great.

RTK problem with the hight is still there but I had GCP’s so it worked out.

3 Likes
11

so just out of curiosity, in your image metadata is there a relative altitude, absolute altitude and gps altitude entries?

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12

I can just se gps-altitude, I tried two different exif data programs and there’s less data than I expected to se.

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13

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