What is with the very erroneous altitude readings?

[45er]

I can fly my MP Platinum on level ground and the altitude readings are way off. I can see the drone 7-8 feet off the ground in front of me, right where I took off, and the reading one the screen will be 20 feet! Is there a calibration I'm unaware of that will fix this significant altitude error?

 

[Meta4]

I can fly my MP Platinum on level ground and the altitude readings are way off. I can see the drone 7-8 feet off the ground in front of me, right where I took off, and the reading one the screen will be 20 feet! Is there a calibration I'm unaware of that will fix this significant altitude error?

A few feet difference is normal.
If you are seeing large differences, the IMU calibration is what you need to try.

 

[FoxhallGH]

Are you giving the MPP time to get itself set up on sensors & GPS etc before you lift-off?? The Mavic uses a barometer to guage it's height and set the zero for take-off. If it's reading high, then the air within the body of the Mavic (where the sensor is) is slightly warmer (less dense) than the surroundings. If you have just taken it out of a car or something, into cold air, that might be enough to show that amount of altitude difference until it cools to match the surroundings ...??

 

[sar104]

Are you giving the MPP time to get itself set up on sensors & GPS etc before you lift-off?? The Mavic uses a barometer to guage it's height and set the zero for take-off. If it's reading high, then the air within the body of the Mavic (where the sensor is) is slightly warmer (less dense) than the surroundings. If you have just taken it out of a car or something, into cold air, that might be enough to show that amount of altitude difference until it cools to match the surroundings ...??

The barometer is reading pressure, not density, and so it does not require temperature equilibrium with the surroundings.

 

[FoxhallGH]

The barometer is reading pressure, not density, and so it does not require temperature equilibrium with the surroundings.

Genuine 'Hmmm?' here, and wanting to learn ... Why is it then that we see Low pressure and High pressure zones on weather maps and associate those with temperature e.g. High pressure air moving in is a cold front ???

 

[Kilrah]

Those regions are hundreds if not thousands of kms apart. It's a large scale effect, in a "local" environment everything that is not separated by a sealed container is at the same pressure.

Sensors do get affected by temperature itself but that should be pretty well compensated.

 

[FoxhallGH]

OK ... thanks You can feel a cold front go through, and see the barometer rise - but I get your point about that 'interface' being a product of major pieces of atmosphere on both sides of the front ...

 

[Kilrah]

Yes, the whole reason for you being able to see it is that it's millions of tons of air that have to be moved over hundreds of kms, and that takes some time. But at the scale of what happens around your aircraft any pressure difference would be equalized in milliseconds.

 

[sar104]

Genuine 'Hmmm?' here, and wanting to learn ... Why is it then that we see Low pressure and High pressure zones on weather maps and associate those with temperature e.g. High pressure air moving in is a cold front ???

As mentioned above, you are confusing large-scale dynamic (non-equilibrium) systems with stationary gas locally in mechanical equilibrium. At any local point in free, stationary air, the earth's atmosphere obeys the ideal gas law, p = ρRT/m, and pressure varies only with elevation, where pressure, p, decreases as density, ρ, and generally temperature, T, decrease with elevation.

That has to be the case because if two adjacent, small volumes of gas (for example inside and outside the aircraft) were at different pressures then there would be a net force across the interface, and the higher pressure volume would expand, compressing the lower pressure volume, until their pressures were equal (mechanical equilibrium).

 

[45er]

I recalibrated the IMU. Seems to have done the trick. I hope the calibration holds for more than a couple of flights! Thanks for all the help.

 

[FoxhallGH]

As mentioned above, you are confusing large-scale dynamic (non-equilibrium) systems with stationary gas locally in mechanical equilibrium. At any local point in free, stationary air, the earth's atmosphere obeys the ideal gas law, p = ρRT/m, and pressure varies only with elevation, where pressure, p, decreases as density, ρ, and generally temperature, T, decrease with elevation.

That has to be the case because if two adjacent, small volumes of gas (for example inside and outside the aircraft) were at different pressures then there would be a net force across the interface, and the higher pressure volume would expand, compressing the lower pressure volume, until their pressures were equal (mechanical equilibrium).

Thanks @sar104 ... When you put it like that ... yes - I was being dumb Thanks for taking the time to explain.