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How does the Mavic measure it’s height?

Huddy

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#1
I’ve heard it said in here that the Mavic uses a barometer to measure its height. Or at least it uses air pressure. I’ve always wondered if and why this would be. It seems air pressure can vary during a flight as weather systems move about. It just doesn’t seem a robust and reliable method.

I’ve always thought they used GPS to measure height. GPS is a 3D deal, not 2D. My current car shows my height above sea level, and there’s no barometer on board. So how is it done?
 

Dronason

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#2
It is as you expect, a barometer measuring the air pressure that gives altitude as many airplane since many many years. GPS altitude is not that reliable and accurate due to the distribution of satellites. Yes the air pressure can vary on its own, but not that fast and it is relative to the first measure at take off.
On return flight, it is always better to have a few meters margin as there could be a small change usually more when the weather is changing.
 
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Huddy

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#3
But surely the accuracy of GPS is the same in both horizontal and vertical axis? It certainly seems accurate horizontally.
 

Mossiback

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#4
In the best conditions you have less than 25 minutes of flight time. The barometric pressure will not change enough to effect the height reading during that time.
 

Thunderdrones

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#5
But surely the accuracy of GPS is the same in both horizontal and vertical axis? It certainly seems accurate horizontally.
No its not. Its more accurate in horizontal. GPS uses triangulation. Your barometric altimeter and downward positioning sensors are what determine your distance from the ground.
 

Thwyllo

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#6
From Garmin:

"GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, these are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating."
 

Thunderdrones

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#7
From Garmin:

"GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, these are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating."
Zactly
 

msinger

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#8
I’ve always thought they used GPS to measure height.
The Mavic keeps track of two altitudes. The altitude displayed at the bottom of DJI GO is calculated using the Mavic's internal barometer. That value is always the altitude above the takeoff point (not the Mavic's current location). The VPS altitude is calculated using the sensors on the bottom of the Mavic. It's the distance between the Mavic's current location and the closest object/obstacle below it (the ground, a tree, etc.). You'll only see that value at the bottom of DJI GO if the Mavic is close enough for the downward sensors to detect any objects/obstacles below it.
 
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#9
My current car shows my height above sea level, and there’s no barometer on board. So how is it done?
Possibly the height information is stored in the maps of your car navigation system. This way by knowing where you are, your car also knows how high you are above sea level.
 

Huddy

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#10
Possibly the height information is stored in the maps of your car navigation system. This way by knowing where you are, your car also knows how high you are above sea level.
It’s possible. But it seems to me the information would also be available more easily from GPS measurement. I suppose I could check it out in a multi-storey car park.
 
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gnirtS

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#12
I’ve heard it said in here that the Mavic uses a barometer to measure its height. Or at least it uses air pressure. I’ve always wondered if and why this would be. It seems air pressure can vary during a flight as weather systems move about. It just doesn’t seem a robust and reliable method.
Barometric pressure which is extremely accurate for the first 15 miles or so. Its also how every aircraft measures altitude.
Yes air pressure can vary but in the short space of time the mavic flies combined with the short distances it travels not enough to make a difference. Its not the most accurate though, out by a few metres some times once its warmed up.

I’ve always thought they used GPS to measure height. GPS is a 3D deal, not 2D. My current car shows my height above sea level, and there’s no barometer on board. So how is it done?
GPS altitude can be out by 30m or more. Its not accurate to use for any sot of precision.
Understanding the accuracy of the GPS elevation reading | Garmin Support for more detail.
 
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#15
I have no idea of the accuracy of the dji GPS receiver, but I have a marine GPS receiver that routinely displays an accuracy of 2 to 3 feet horizontally and supposedly is almost as accurate vertically. But it cost four to five times as much as ordinary GPS receivers and is no longer available. (No one bought it- too expensive.) GPS receivers that employ the WAAS (Wide Area Augmentation System) routinely achieve accuracies of 3 feet or 1 meter horizontally and 5 feet or 1.5 meters vertically throughout North America. See the Wikipedia article: Wide Area Augmentation System - Wikipedia. Perhaps the GPS receiver is more accurate than you think, especially if used in conjunction with barometric and the ultrasonic sensors on the bottom of the Mavic.
 
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Chirp

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#16
It is very accurate in tandem the Mavics downward sensors.

... a Mavic wont crash on it's own. It takes a pilot to make that happen... :)
How True! As opposed to an Inspire 1V2 that will crash nicely if you don't slow its decent and lower the gear so you don't touch down camera first.
 

BradBowers

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#17
A barometer doesn't measure height/elevation - at any given moment, it measures the the downward force (weight) of a column of air above it.
 

MaxHam

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#20
The Mavic keeps track of two altitudes. The altitude displayed at the bottom of DJI GO is calculated using the Mavic's internal barometer. That value is always the altitude above the takeoff point (not the Mavic's current location). The VPS altitude is calculated using the sensors on the bottom of the Mavic. It's the distance between the Mavic's current location and the closest object/obstacle below it (the ground, a tree, etc.). You'll only see that value at the bottom of DJI GO if the Mavic is close enough for the downward sensors to detect any objects/obstacles below it.
One question:
The Phantoms altitude calculation is apparently based on GPS. Any idea why it's implemeneted like this?
If Mavic uses Barometer that's good to hear.
EXIF issues / Altitude confusion?