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Ground to bird height not accurate

ColoradoSwede

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This may be self explanatory but I noticed that even though I relocated my MP2 to a new home location several times, when I’m flying just above ground level the bird-to-ground distance is off my about ½ meter or so. Is this because of the error built into the GPS. Fortunately it is ABOVE ground level ....LOL. When it indicates it’s “0” feet above the ground it is observed to be about 1.5 feet to 2 feet above actual ground level in my field. At about 1 foot it registers a “-“ altitude. I was learning to fly my bird and was messing around with the horses in my field.
 

tcope

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Altitude is measured from the level of the ground at the take off point, not what is currently under the Mavic.
 

ColoradoSwede

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Thank you. I understand that. My takeoff point is exactly the same elevation as it point I’m asking. Why is there a difference when the bird is just 100 get away from the takeoff point?
 

sar104

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The aircraft determines altitude relative to the home point by a combination of differential inertial and absolute barometric measurements. GPS altitude is not used. Barometric altitude has a resolution of better than one meter or so, but an accuracy worse than that. What you are seeing is well within the expected bounds.
 

Thomas B

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Thank you. I understand that. My takeoff point is exactly the same elevation as it point I’m asking. Why is there a difference when the bird is just 100 get away from the takeoff point?
As I understand it there are 2 point discussed and they are 100’ apart. I’d suggest that visual judgement of the “same elevation” could easily be misjudged as a pilot by 18-24” at distance especially if there is a constant gentle slope.
That’s why the drone does it’s own calculation.
 

Meta4

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This may be self explanatory but I noticed that even though I relocated my MP2 to a new home location several times, when I’m flying just above ground level the bird-to-ground distance is off my about ½ meter or so. Is this because of the error built into the GPS. Fortunately it is ABOVE ground level ....LOL. When it indicates it’s “0” feet above the ground it is observed to be about 1.5 feet to 2 feet above actual ground level in my field. At about 1 foot it registers a “-“ altitude. I was learning to fly my bird and was messing around with the horses in my field.
If you are flying close to the ground you have two different altitude measuring systems.
If close to the ground, the VPS is the one to use.
Have you looked into what your VPS is indicating?
 

gnirtS

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This may be self explanatory but I noticed that even though I relocated my MP2 to a new home location several times, when I’m flying just above ground level the bird-to-ground distance is off my about ½ meter or so. Is this because of the error built into the GPS. Fortunately it is ABOVE ground level ....LOL. When it indicates it’s “0” feet above the ground it is observed to be about 1.5 feet to 2 feet above actual ground level in my field. At about 1 foot it registers a “-“ altitude. I was learning to fly my bird and was messing around with the horses in my field.
Its not GPS - its the barometer and when low enough. The drone doesnt use GPS for altitude.
And its a cheap barometer that doesnt appear to be temperature compensated. It'll drift. Its quite common to see -15ft on landing for example.

If youre referring to the VPS that cuts in when low, again its not a precision sensor. Don't worry about it.
 

Rangerider

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I had no idea they used a barometer on these things. I thought it was gps altitude. Learned something today. I quite often see negative altitudes landing after flying around especially after flying well below take off point altitude. Maybe I should turn the landing protection back on?
 
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Thomas B

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I had no idea they used a barometer on these things. I thought it was gps altitude. Learned something today. I quite often see negative altitudes landing after flying around especially after flying well below take off point altitude. Maybe I should turn the landing protection back on?
I would have the landing protection and downward sensors on in almost all circumstances.
 

Rangerider

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Yeah you are right. I just don't like the hesitation when landing that the ground protection provides. It requires more energy and time to land than it should. I guess if I sneezed and accidentally jerked the control stick downward it could cause a crash and the protection would be nice to have then. It can also be nice for that lo-level nap of the earth flying as well.o_O Better safe than sorry?
 

GadgetGuy

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Yeah you are right. I just don't like the hesitation when landing that the ground protection provides. It requires more energy and time to land than it should. I guess if I sneezed and accidentally jerked the control stick downward it could cause a crash and the protection would be nice to have then. It can also be nice for that lo-level nap of the earth flying as well.o_O Better safe than sorry?
Just hand catch it and pull down on the left stick! No hesitation and no delay.
 
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gnirtS

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Call me a wuss. Those whirling blades of death bother me being that close to me face.
Thats why when hand catching you grab it from underneath with a thumb and fingers and dont let it autoland/descend on its own. You grab it securly then hold left stick to kill the props *while its hovering safely just above head and face height*
 

Rangerider

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Thats why when hand catching you grab it from underneath with a thumb and fingers and dont let it autoland/descend on its own. You grab it securly then hold left stick to kill the props *while its hovering safely just above head and face height*
I'll leave the hand catching to you young guys for now. I'm attached to my fingers and face and want to keep them the way the are. What happens if you sneeze when you're just about to grab the four whirling bladed bird or a horse fly bites you on the back of the neck? How about your big dog rams into you when your hand is just an inch from the eggbeater? Do you feel lucky? Do you know of any accidents by those using the hand catch method?
 

Rangerider

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This may be self explanatory but I noticed that even though I relocated my MP2 to a new home location several times, when I’m flying just above ground level the bird-to-ground distance is off my about ½ meter or so. Is this because of the error built into the GPS. Fortunately it is ABOVE ground level ....LOL. When it indicates it’s “0” feet above the ground it is observed to be about 1.5 feet to 2 feet above actual ground level in my field. At about 1 foot it registers a “-“ altitude. I was learning to fly my bird and was messing around with the horses in my field.
I am still having the same issue with the altitude being 5-20 ft off from the beginning to end of each flight. I contacted DJI tech support and they advised me to do a calibration of the IMU. Even though mine shows no indication before flight of any irregularity I did another calibration of the IMU as suggested. The next flight, the very same problem returned with altitude being -10ft off. The direction of altitude deviation is always negative for me. DJI did mention that the barometer sensor does have some drift and that DJI is coming out with a firmware update addressing this issue so evidently it is a wide spread problem that is occurring frequently. The only other alternative offered by DJI is to send the drone in for repair! No thanks. As long as the error is in the negative direction I shouldn't get into trouble if the dreaded auto RTH kicks in.
 

DanMan32

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I've noticed altitude drift also, lately more than I've recalled seeing before.

I was checking it out by landing on the other side of the field to "reset" ground to zero, forgetting that also reset home point. Imagine the surprise when I hit RTH and it went in the opposite direction I was expecting it to. I was able to quickly cancel before it could crash.

The Tello also seems to have altitude drift but I rather expect that for its cost.
 

GadgetGuy

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I am still having the same issue with the altitude being 5-20 ft off from the beginning to end of each flight. I contacted DJI tech support and they advised me to do a calibration of the IMU. Even though mine shows no indication before flight of any irregularity I did another calibration of the IMU as suggested. The next flight, the very same problem returned with altitude being -10ft off. The direction of altitude deviation is always negative for me. DJI did mention that the barometer sensor does have some drift and that DJI is coming out with a firmware update addressing this issue so evidently it is a wide spread problem that is occurring frequently. The only other alternative offered by DJI is to send the drone in for repair! No thanks. As long as the error is in the negative direction I shouldn't get into trouble if the dreaded auto RTH kicks in.
In the past, it was recommended to let the aircraft warm up for two to three minutes to get a more accurate barometric reading before launch, for greatest accuracy. You might want to give that a try, and see if it still helps, if the barometer accuracy is important to you.
 
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Rangerider

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In the past, it was recommended to let the aircraft warm up for two to three minutes to get a more accurate barometric reading before launch, for greatest accuracy. You might want to give that a try, and see if it still helps, if the barometer accuracy is important to you.
Well it's not that important to me but it would be nice if DJI integrated gps altitude into the barometric for improved accuracy. Maybe the gps altitude info is not as accurate as baro? It is 'detuned' for civilian use. Farmers use a gps system that is very accurate but requires multiple receivers that compare to each other. I'm not sure what impact this error has on RTH as I haven't tried it yet. Maybe I will later today? It seems to be enough of a problem that DJI is putting out an update to help with this issue. Warming up the engine before flying sounds like a good idea. I'll try it. I think the problem will still be there though as I have notice it even with succeeding flights with little delay between battery swaps.
 
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