Negative Altitude Data

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Getting Negative Altitude data on Phantom4 Pro & Mavic Platinum when mapping. Anyone no if Dani has issued a fix?
 

RodPad

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Getting Negative Altitude data on Phantom4 Pro & Mavic Platinum when mapping. Anyone no if Dani has issued a fix?
Who is Dani?

Rod
 

RodPad

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OK,
Sorry I have never done any mapping with DJI.
I do missions with Litchi, I'm used to -Altitudes.


Rod
 

sar104

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Getting Negative Altitude data on Phantom4 Pro & Mavic Platinum when mapping. Anyone no if Dani has issued a fix?

Altitude is relative to the take off point. Are you seeing negative altitudes when the aircraft is above the takeoff point, or is the aircraft descending over variable terrain?
 

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Yes, we need more info...

Rod
 

Meta4

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Altitude data shown in EXif
This gets asked about frequently.
It's very common for Exif altitudes to be out by +/- 200 feet.
Although the data field is labelled GPS data, it does not really come from GPS.
It is derived from the atmospheric pressure measured by the barometric sensor but DJI's calculation method doesn't allow for normal changes in air pressure due to weather.
i-MqRGc68-L.jpg
 
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Yep, areas below takeoff point. If you are not referencing you maps to real world elevations you will get + and - of where you took off. Some softwares use the ellipsoid of the GPS takeoff point though so it depends on what you are using. ???
 
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It's not related to GPS, GCPs, software or the altitude shown in the app.
That's the raw Exif info applied to the images by the camera.

Sorry to disagree, but it is.

GPS. Different makes of drones can report exif data differently. IE Yuneec vs DJI and if the data is not recorded in the correct altitude parameter, or if they write a new altitude parameter then some softwares will not read it at all.
1577629561886.png


GCPs. Working along with the GPS, if it is not reporting correctly then GCPs will fix it. From the OP's reference to mapping.

Software, see above and the fact that if GCP's are not used then they can report +/-.

Mapping
Phantom4 Pro & Mavic Platinum when mapping.
 

sar104

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Sorry to disagree, but it is.

GPS. Different makes of drones can report exif data differently. IE Yuneec vs DJI and if the data is not recorded in the correct altitude parameter, or if they write a new altitude parameter then some softwares will not read it at all.
View attachment 116448

GCPs. Working along with the GPS, if it is not reporting correctly then GCPs will fix it. From the OP's reference to mapping.

Software, see above and the fact that if GCP's are not used then they can report +/-.

Mapping

On DJI aircraft the altitude fields in the EXIF data have been mislabeled for years, and only recently corrected in some firmware. The DJI GPS altitude data field is actually barometric AMSL based on a standard atmosphere - hence the obvious discrepancies.


 
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So it's happening only at the see or near to it? Otherwise it wouldn't be logical because the takeoff point is measured by the same method as during the flight and as I understand only different air pressure can make that difference.
 
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All pilots of real aircraft reset the barometric altitude to their current height on the airfield just before takeoff to compensate for the current air pressure which continuously varies. DJI doesn't provide this capability, hence the reported barometric altitude varies considerably in direct proportion tho the current air pressure.
 
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On DJI aircraft the altitude fields in the EXIF data have been mislabeled for years, and only recently corrected in some firmware. The DJI GPS altitude data field is actually barometric AMSL based on a standard atmosphere - hence the obvious discrepancies.



Correct, but the values written to the exif are not derived from the barometer and vision algorithm. That is just what is used to position the aircraft over the ground including terrain following. This tells you that it is using both (maybe in just terrain following?). One to know the elevation of the drone in comparison to the SRTM that was loaded and the other to increase the accuracy of the drone's distance from the terrain.
 

sar104

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Correct, but the values written to the exif are not derived from the barometer and vision algorithm. That is just what is used to position the aircraft over the ground including terrain following. This tells you that it is using both (maybe in just terrain following?). One to know the elevation of the drone in comparison to the SRTM that was loaded and the other to increase the accuracy of the drone's distance from the terrain.

No - the values written to the EXIF are the absolute barometric altitude. The VPS system has nothing to do with it. Did you read the explanations that I linked to?
 

sar104

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So it's happening only at the see or near to it? Otherwise it wouldn't be logical because the takeoff point is measured by the same method as during the flight and as I understand only different air pressure can make that difference.

No - this affects all EXIF absolute altitude data that are computed from barometric pressure.
 

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