Variance between Barometer (h) height and VPS height

[Cosmos 5]

I normally see a difference between the barometer height and the VPS height across a flight, but currently I am seeing this value be as great as 11m under the following conditions: at sea level, max flight height 30m, duration of flight 24min., air temp ~ 18deg Celsius, stable low wind conditions .....

Across a flight of this type I am seeing an increase in the difference between the two reported heights that can be up to 11m.
Previously I have seen maybe 1-2m difference between the two readings but to see the VPS reading of 5m and the h = -6m is kind of perturbing.

Some questions:
1.) Is this degree of height reading variance normal or is it indicative of a calibration or other error with the barometer (the VPS is accurate)
2.) The flights are directly by the ocean with quite a bit of sea mist... any possibility that this might lead to drift or change in the barometer reading.... the difference between the two readings increases over a flight but is reset if the AC is landed....

Cheers

 

[old man mavic]

I normally see a difference between the barometer height and the VPS height across a flight, but currently I am seeing this value be as great as 11m under the following conditions: at sea level, max flight height 30m, duration of flight 24min., air temp ~ 18deg Celsius, stable low wind conditions .....

Across a flight of this type I am seeing an increase in the difference between the two reported heights that can be up to 11m.
Previously I have seen maybe 1-2m difference between the two readings but to see the VPS reading of 5m and the h = -6m is kind of perturbing.

Some questions:
1.) Is this degree of height reading variance normal or is it indicative of a calibration or other error with the barometer (the VPS is accurate)
2.) The flights are directly by the ocean with quite a bit of sea mist... any possibility that this might lead to drift or change in the barometer reading.... the difference between the two readings increases over a flight but is reset if the AC is landed....

Cheers

the barometric pressure reading will change according to the conditions at the time of the flight ,and the VPS height is only accurate up to around 30 ft from the surface ,also flying over water can confuse the sensors, these readings are not exact, there are to many variables in the mix

 

[Deleted member 103366]

As above the normal indicated height is apparently via barometer ("apparently" because I am relying on what others have 'said' etc.) and subject to changes in air pressure, normally I am not bothered by its fluctuations but of late one thing has been causing me concern.
Is this used to control RTH height??

With regards to Barometic height vs VPS, over land if VPS data is displayed and I was height watching I think I would be inclined to go with the VPS data.

 

[Deleted member 103366]

I didn't like the use of "apparently" in the above post so.......,
I switched everything on, put my Mini in a plastic bag with a hose pipe leading into it, sealed the bag and blew into the hosepipe.
With the mini sat on the table it plummeted to 60m+ below switch on height. I also got a navigation error warning.
I was going to try sending it 'up' to ridiculous heights but it was too awkward to get an airtight enough seal to get partial vacuum inside the bag. I have used a plastic bag and a vacuum cleaner for such purposes before (squashing duvet etc. flat) but am worried it might be too severe for the mini's equipment and I would have much less control when compare to using my lungs etc.

Yes I am cuckoo

 

[Deleted member 103366]

If over water I would generally not fly below 10m above the surface.
If I do go below that then I watch the drone like a hawk, either through direct line of sight or the screen, and am ready to shove the throttle to the up stop if it shows any sign of sinking.
My flying tends to be over the sea with nothing near me or the drone so whilst using the screen to 'watch the height' is incorrect there is no danger to anyone or anything other than my drone. When using line of sight and coming down over water if I lose sight of the drone against the background it is sent up to get back into clear sight, even if coming down using the screen there are frequent glances at the drone to check, by line of sight, for birds etc. (the field of view on the screen is rather narrow) and if I can not see it against the back ground, up it goes.

 

[SkyeHigh]

I didn't like the use of "apparently" in the above post so.......,
I switched everything on, put my Mini in a plastic bag with a hose pipe leading into it, sealed the bag and blew into the hosepipe.
With the mini sat on the table it plummeted to 60m+ below switch on height. I also got a navigation error warning.
I was going to try sending it 'up' to ridiculous heights but it was too awkward to get an airtight enough seal to get partial vacuum inside the bag. I have used a plastic bag and a vacuum cleaner for such purposes before (squashing duvet etc. flat) but am worried it might be too severe for the mini's equipment and I would have much less control when compare to using my lungs etc.

Yes I am cuckoo

The barometers have the capability of being accurate to within 10-20cm but it doesn’t appear that DJI are using them in precision mode so a metre or so can be expected. The reference obtained at sea level is 1013mBar, or 101.32kPa, which is the equivalent of 14.7PSI. Increase in altitude results in lower pressure, which reduces to approximately 998.7mBar at 400ft.

Referencing a barometer reading at power-up will give the AC it’s approximate height above sea level, which is its zero height above absolute ground level. In flight upto around 30ft, the downward facing sensor will give a more accurate height above AGL figure but beyond that the barometer is acting as an altitude data source but data from the IMU is also taken into account as part of the sensor fusion processing.

 

[Cosmos 5]

Thanks everyone for your thoughts/advice. Yes I am aware that the VPS is only valid until ~ 9m and it appears to be working correctly on my drone....
I am wondering if anyone else is seeing up to 11m difference in the readings between the VPS and the barometer (H) height value over the course of a flight...
i.e VPS =5m H= -6m
If not, I am concerned that for whatever reason my barometer reading has a major drift in calibration over the duration of a flight....

 

[boblui]

The maximum difference I have seen on my M2P is 3 meters.