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Question on how Altitude is measured
13659 25 2017-1-2
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Incident
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Hi all,A couple of questions on how flight altitude is calculated. Here is an example and I am trying to understand how the drone will behave.

Say I am launching the Mavic from a flat, wide open area at the top of a cliff near the ocean. The cliff is 200 feet high, so is my home point set at an altitide of 200 feet above sea level, or is the home point considered 0 feet (obviously both me and my Mavic are 200 feet above sea level, but what does the Mavic think)?

Also, if I want to fly the Mavic over the side of the cliff and descend half way (100 feet above sea level, but -100 feet from the set home point) down towards the ocean. Is my Mavic now believeing that it is at an altitude of 100 feet, or -100 feet?

In short, I guess I am trying to understand if the Mavic considers it's starting altitude as 0 based on the home point, or is it always based on the true altitude above sea level. Hope my question is clear in what I am asking.

Thanks in advance!

2017-1-2
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hallmark007
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Your height is taken from your take off position which is 0 , if you fly upwards it will be plus if you fly downwards i.e. Dow the cliff it will show a minus figure
2017-1-2
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Harbourside
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Altitude is set to zero at the takeoff point and is measured by a barometric sensor.
You are correct in your example, it will show minus altitude.
2017-1-2
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Incident
lvl.2

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Thanks for the quick replies!
2017-1-2
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DJI-Ken
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As the others have said, it is not based on MSL, it's AGL from when the home point is recorded.
2017-1-2
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Incident
lvl.2

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DJI-Ken Posted at 2017-1-2 10:08
As the others have said, it is not based on MSL, it's AGL from when the home point is recorded.

So one more question based on this....

Using my same example, I set my RTH altitude to 98 feet (30m). I fly the Mavic over the cliff and descend 150 feet, so now the Mavic records it's altitude as -150 feet.

If it enters RTH or I initiate it at that -150 feet, will the Mavic ascend to it's starting altitude, then an additional 98 feet, before RTH, or will it ascend to 98 feet from the point it goes into RTH mode?

I think it is the first way, otherwise if it is the second way, the Mavic would be -52 feet below it's starting altitude and it would eat the side of the cliff.

Thanks again...just trying to understand some of the nuances.
2017-1-2
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DJI-Ken
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Incident Posted at 2017-1-2 10:25
So one more question based on this....

Using my same example, I set my RTH altitude to 98 feet (30m). I fly the Mavic over the cliff and descend 150 feet, so now the Mavic records it's altitude as -150 feet.

Yes, that's correct. It will ascend to takeoff altitude and continue rise to the RTH altitude
2017-1-2
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Incident
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DJI-Ken Posted at 2017-1-2 10:55
Yes, that's correct. It will ascend to takeoff altitude and continue rise to the RTH altitude

Thanks Ken!
2017-1-2
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DJI-Ken
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No problem, have a great week and Happy New Year.
2017-1-2
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helo2
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DJI-Ken Posted at 2017-1-2 10:08
As the others have said, it is not based on MSL, it's AGL from when the home point is recorded.

Why not call it AHL, what's one more acronym?
2017-1-2
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DJI-Ken
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helo2 Posted at 2017-1-2 15:24
Why not call it AHL, what's one more acronym?

I'm sorry, what is AHL?
I used MSL and AGL as they are common aeronautical terms.
2017-1-2
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Jason Lane
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DJI-Ken Posted at 2017-1-2 19:22
I'm sorry, what is AHL?
I used MSL and AGL as they are common aeronautical terms.

I think helo2 was (sarcastically?) suggesting Above Home Level.

I personally think DJI should be a little more precise in the terminology used. Even though most people think altitude and height are the same thing, those of us with any aeronautical background know there's a difference, and since our Mavics etc are actually aircraft, we really ought to be using correct terminology. I was a little surprised to see "altitude" being used throughout the app and documentation, when in fact "height" would be more correct. And it's technically not AGL, as it's referenced from the home point. AGL would be the height above ground wherever the aircraft is at the time (the downward sensors give us AGL when close enough to the ground).

Height Above Aerodrome Elevation (AAE) or Height Above Field Elevation (AFE) would probably be closest to the correct terms, but at the very least I think DJI should be using the term "height" instead of "altitude."
2017-1-3
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hariedo
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helo2 Posted at 2017-1-2 15:24
Why not call it AHL, what's one more acronym?


The technical term in aviation is ATO - altitude measured above take off.

The Mavic system measures all altitudes relative to the takeoff point.

The default setting for maximum altitude in the controller/aircraft may be something like 120 meters ATO; this means it won't accept a command to rise higher than 120 meters (400 ft) above the takeoff point. You can change this limit however you like, up to a maximum of 500 meters ATO.

Some jurisdictions' laws may specify a ceiling of 400 ft AGL (above ground level) as the US currently does. This may include a reduced ceiling across canyons or a raised ceiling around buildings and mountains. The Mavic has no idea where the ground is, unless it happens to be hovering within about 2 meters of some object. It does not understand geographical dropoffs or hills at all. Your compliance with the 400 ft AGL rule is entirely on your own mental calculations.  If standing on a bridge or cliff, you can fly below the take off altitude, for example to negative numbers like -50 ft ATO.

The Mavic actually has an internal barometric altitude device, and the GPS/GLONASS receivers, both of which measures altitude relative to the average sea level. Thus your hometown may be at 2000 ft AMSL (above mean sea level) and the Mavic would use that measurement at the time of takeoff. However, NOWHERE on the controller screen nor the DJI Go app screen will you see altitude described this way. This is only used for internal calculation.

By the way, people have flown Mavic above 14000 ft AMSL (by taking off from points high in the mountains). Aerodynamically, propeller thrust gets weaker as the air gets thinner, so beware of underpowered flight if you try to go much higher than this.

If you stand on the top of a skyscraper, or in the bottom of a canyon, all of these measuring systems may clash. Your country or the country you visit may have different rules that include buildings, airports, population density or other hazards to avoid. It's up to you as a pilot to be aware of these and fly safely.
2017-1-3
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dana5
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Someone above suggested that the take off altitude is recorded at takeoff, and then after that, it's a barometric pressure sensor. Is there really a barometric sensor?

Seems to me that it is more likely to work like this: The home point is GPS based, which is not just an X, Y, but also a Z (altitude). It then makes this altitude the offset for the height at takeoff. At low altitude, I would assume it uses the sonic sensors, but overall, the height would more likely be GPS based, taking the elevation above sea level, and subtracting the elevation recorded at the takeoff point. This would give GPS level accuracy, which I would think would be more accurate than barometric pressure, if that sensor even exists.

Please correct me if I am wrong... I am just curious how it really works.
2017-1-3
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DJI-Ken
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dana5 Posted at 2017-1-3 07:02
Someone above suggested that the take off altitude is recorded at takeoff, and then after that, it's a barometric pressure sensor. Is there really a barometric sensor?

Seems to me that it is more likely to work like this: The home point is GPS based, which is not just an X, Y, but also a Z (altitude). It then makes this altitude the offset for the height at takeoff. At low altitude, I would assume it uses the sonic sensors, but overall, the height would more likely be GPS based, taking the elevation above sea level, and subtracting the elevation recorded at the takeoff point. This would give GPS level accuracy, which I would think would be more accurate than barometric pressure, if that sensor even exists.

Yes, DJI aircraft have an onboard barometer. That is how the aircraft hold it's altitude position.
The attached image is from page 11 in the manual.
Screen Shot 2017-01-03 at 8.18.37 AM.png
2017-1-3
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DJI-Ken
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Jason Lane Posted at 2017-1-3 03:35
I think helo2 was (sarcastically?) suggesting Above Home Level.

I personally think DJI should be a little more precise in the terminology used. Even though most people think altitude and height are the same thing, those of us with any aeronautical background know there's a difference, and since our Mavics etc are actually aircraft, we really ought to be using correct terminology. I was a little surprised to see "altitude" being used throughout the app and documentation, when in fact "height" would be more correct. And it's technically not AGL, as it's referenced from the home point. AGL would be the height above ground wherever the aircraft is at the time (the downward sensors give us AGL when close enough to the ground).

Altitude can mean the height above sea level or ground level.
I've been a private pilot for over 20 years and I'm used to AGL and MSL as that's what we use in the US.
2017-1-3
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Jason Lane
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DJI-Ken Posted at 2017-1-3 07:23
Altitude can mean the height above sea level or ground level.
I've been a private pilot for over 20 years and I'm used to AGL and MSL as that's what we use in the US.

But when we set something like "RTH Altitude" in the app, it's referring neither to AGL or MSL. It's referring to the height from the home point.
2017-1-3
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DJI-Ken
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Jason Lane Posted at 2017-1-3 08:58
But when we set something like "RTH Altitude" in the app, it's referring neither to AGL or MSL. It's referring to the height from the home point.

You could look at it that way. Or AGL from home point (not current aircraft location).
2017-1-3
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Jason Lane
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DJI-Ken Posted at 2017-1-3 07:23
Altitude can mean the height above sea level or ground level.
I've been a private pilot for over 20 years and I'm used to AGL and MSL as that's what we use in the US.

Also, I don't know if it's a country-related difference, but during my training in Europe, I was taught to use "height" when referenced to ground, terrain, buildings, etc, while "altitude" is generally used when referencing MSL. That's why I personally feel "RTH Height" would be more accurate than "RTH Altitude."
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DJI-Ken
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Jason Lane Posted at 2017-1-3 10:00
Also, I don't know if it's a country-related difference, but during my training in Europe, I was taught to use "height" when referenced to ground, terrain, buildings, etc, while "altitude" is generally used when referencing MSL. That's why I personally feel "RTH Height" would be more accurate than "RTH Altitude."

I totally understand,
2017-1-3
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kit_nz
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Sorry to restart an old thread, but it seems most relevant to my question. I've only tested a few times, but it seems features like waypoints and active track won't work if the Mavic is below a certain height (2m seems to be the height in the manual). But that height is relative to the take off/home point. I fly my drone from our house which elevated and looks over a bay, meaning the drone is flying at negative height all the time. If I try to use active track along the beach, it won't work as the drone is below the take off point. Is there anyway to work around this?
2017-4-24
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fans766366c5
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I had an unusual (normal?) experience today.  I'm flying at about 7000' near Taos, NM.  My intent was to fly 460' down into the Rio Grande Gorge--I was on the edge of the Gorge 10 mi west of Taos.  It was a very hot day near noon time and warming up.  Frequently a popup window said the battery was too warm to charge. Going down was no problem.  Got some excellent images of the river at about 50'.  I was out about 1600' and did not have line of sight to the drone.  As I brought it back, planning to gain line-of-sight, I could not see the drone.  It was supposed to be about 50' above the launch site.  At one point, I thought I could hear it faintly but I never did see it.  It apparently was moving tangentially to me about 300' away--my hearing is not great otherwise I might have been able to sight it.  The plateau around me did have some higher points but I doubt more than 30' higher.  If my DJI quad-copter was 50' above launch, it should not have crashed.  In any case, it did crash from my FPV, but I did not know where...last distance indicated 298'.  After hiking a couple hills, I went home dejected,.  At home, I pulled up my flight log for the day, and it was obvious exactly where my drone had crashed.  I used Google Earth to get still a better view of its location, rushed back, and immediately located the drone.  It seems undamaged. Happy ending and quite amazing.
In analyzing what happened, (1) I should have brought it back at 100' rather than 50', and (2) I think the extremely hot and variable conditions caused an error in the height reading.  I believe the drone was flying closer to 20-30' above launch rather than the indicated 50'.

Keith/Chekika

2017-7-9
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fans12d74da1
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I live in the Adirondack mountains on the side of the mountain at an MSL of 253 feet and the mountain height goes to 1500 feet.  If I set my return to home elevation at its max which is 50m, and I fly to the top of the mountain which is 1500 feet and I lose connectivity with my controller will the drone drop down  and hit the mountain or will it stay 150 feet above the ground as it returns home.
2017-12-30
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A CW
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fans12d74da1 Posted at 2017-12-30 08:06
I live in the Adirondack mountains on the side of the mountain at an MSL of 253 feet and the mountain height goes to 1500 feet.  If I set my return to home elevation at its max which is 50m, and I fly to the top of the mountain which is 1500 feet and I lose connectivity with my controller will the drone drop down  and hit the mountain or will it stay 150 feet above the ground as it returns home.

Picture this scenario: -
You pack your drone in the case and walk up a hill to it's peak say 2000 feet up. You set the drone up, turn it on and it records the home point on the map. The home point is the position where the drone is when it's recorded (zero altitude at the top of the hill). You take off and fly away from the top of the hill. The gradient of the terrain drops significantly as the drone flies away from the hill. You fly the drone down in line with the gradient of the hill. The barometer will therefore record the altitude as a minus figure as the drones altitude descends below the home point. If you lose signal, it will fly up to the altitude you set at the home point. So if you set 65M (as I do) it will fly to zero altitude (the peak of the hill) then an additional 65M in line with the preset RTH altitude you set then fly back and land where it took off (in front of you). The key in these situations is not to change the home point/your location during the flight. Now, if you reverse this scenario and take off from the bottom of the hill, fly around the hill and lose signal - your RTH needs to be set higher than the peak of the hill or it will crash into the hill side on it's autonomous return to you. Not setting RTH high enough to cover obstacles in the drones flight path is one of the main reasons drones crash. The P4P uses flight autonomy whereby it sets 3D images of the environment as it flies so if you lose signal/activate RTH it will auto fly back the same route it flew to the point of signal loss rather than adopt the more basic straight line RTH that the Mavic does. So tech is already out there to overcome these situations but not on the Mavic so you need to envisage how the drone 'thinks' and 'reacts' to your environment.
2017-12-30
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DroneFlying
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fans12d74da1 Posted at 2017-12-30 08:06
I live in the Adirondack mountains on the side of the mountain at an MSL of 253 feet and the mountain height goes to 1500 feet.  If I set my return to home elevation at its max which is 50m, and I fly to the top of the mountain which is 1500 feet and I lose connectivity with my controller will the drone drop down  and hit the mountain or will it stay 150 feet above the ground as it returns home.

If I set my return to home elevation at its max which is 50m, and I fly to the top of the mountain which is 1500 feet and I lose connectivity with my controller will the drone drop down  and hit the mountain

As described in the manual, the Mavic never descends during failsafe RTH until / unless it's over the home point and you confirm that you want it to land.
2017-12-30
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Mike’s Dad
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Thank you
2018-1-8
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