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Error in height AMSL
853 1 2020-11-29
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nikolaospaschalisgmail.com
Second Officer
Flight distance : 2372808 ft
Greece
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Hello all
after working in the field with RTK I noticed something strange.
As I was started for the flight I checked my altitude before take-off and both RTK and Matrice 300 were showing around 50m AMSL. But the altitude at that point was around 20m-this was measured by a precise civil engineer GNSS rover.
Why is that difference? Did anyone notice that also?
2020-11-29
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robert300
lvl.1

Belgium
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Hi,

I am researching the capabilities of the DJI M300 for a project with strict RTK requirements.
I found this in the OSDK API reference:
https://developer.dji.com/onboard-api-reference/structDJI_1_1OSDK_1_1Telemetry_1_1PositionData.html#aa81ed7a6ac0ff27d399f42182f86dd7d
https://developer.dji.com/onboard-api-reference/structDJI_1_1OSDK_1_1Telemetry_1_1GlobalPosition.html

So according to me, the altitude uses the pressure altitude based on the pressure altitude.
The API states this for the barometric altitude:
The ICAO model gives an MSL altitude of 1013.25mBar at 15° C and a temperature lapse rate of -6.5° C per 1000m. In your case, it may be possible that the take off altitude of the aircraft is recording a higher pressure than 1013.25mBar. Let's take an example - a weather station shows that SFO (San Francisco International Airport) had recently recorded a pressure of 1027.1mBar. SFO is 4m above MSL, yet, if you calculate the Pressure Altitude using the ICAO model, it relates to -114m. You can use an online calculator to similarly calculate the Pressure Altitude in your area.


Another factor that may affect your altitude reading is manufacturing differences in the barometer - it is not uncommon to have a variation of ±30m readings at the same physical location with two different aircraft. For a given aircraft, these readings will be consistent, so you will need to calibrate the offset of your system if your code relies on the accuracy of the absolute value of altitude.

So in your case, I suppose the GNSS rover uses the altitude measured from the GNSS system (GPS/GALILEO/GLONASS) with for example WGS84 as reference ellipsoid while the DJI M300 with RTK uses the pressure altitude - which changes based with changing air pressure according to the ICAO model described above.

Hope this helps.
If you have found a solution in the mean time, can you let me know please?

Kind regards,
Robert
2-25 01:15
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