How does the drone determine the 0.0 m altitud? Does it set that point at whatever height it takes off (meaning that taking off from ground would be the same as taking off from your hand, held above your head, at actually ~2m above ground)? Or does it use it's sensors to measure how high it is from the nearest surface underneath it?

I was performing a rather teicky flight over a stream, densely covered by trees, so i was ultra careful as to how high i was at every second.... But at some point, the Dji Fly app was showing -4m height, despite the drone being well over 10m above the tallest gree, which was already close to 20m tall.

However, the home point was set at the top of a tall hill nearby, which was, indeed, higher than said stream. So.... How am i supposed to be able to fly when i dont have a line of visual to my drone? Negative altitudes make no sense and it's actually dangerous for the drone and pilot...

The aircraft uses a barometric altimeter, meaning it measures air pressure to determine height. When you start the drone, it sets the current altitude as zero. All altitude readings from then on are measured from the take-off point. If you take off from an elevated position then fly down into a valley, it will show negative altitude readings. The aircraft cannot sense the ground, so it does not show height above ground.

"So.... How am i supposed to be able to fly when i dont have a line of visual to my drone?"

In that case, do not let the aircraft fly out of sight. In almost every country of the world, the regulations state you must be able to keep sight of your aircraft.

"Negative altitudes make no sense and it's actually dangerous for the drone and pilot..."

What else do you expect the controller to show you? If you are flying lower than where you took off from then of course the altitude will be negative. As I said earlier, the aircraft has no method of determining where the ground is.

It is not a big deal to go below the HomePoints altitude which is set by the Barometer as Geebax explained.  Altitude from Ground is possible to calculate using the GPS elevation data but it has not been implement in the DJI apps.  

Some third party apps like Litchi have Altitude from Ground from what I understand.  But it still relies on the accuracy of the Barometer to calculate the drones height and make it relative to the GPS HomePoint elevation.  To get the Altitude from Ground it must add the drone's Barometer height, whether positive or negative, to the GPS HomePoint elevation then subtract the drone's built in GPS elevation from that height.

It is unwise to drop below the height of the HomePoint if you can not see the drone as the signal more than likely would be obstructed.  

It is not dangerous to fly below the hight of the HomePoint.  It is actually fairly common for shooting things from a location with clear visual line of sight that is below the HomePoint or dropping off the edge of cliff to give a cinematic shot.  Often flying from a higher elevation gives a better signal with less obstruction and interference.

Negative altitudes make no sense
OK .. what would you have the altitude show when you fly below the level of the home point?

and it's actually dangerous for the drone and pilot...
Not understanding how the drone works is dangerous

Labroides Posted at 1-11 21:55
Negative altitudes make no sense
OK .. what would you have the altitude show when you fly below the level of the home point?

1) well, i think you could get a report on the actual height from ground and, now that you mention it, it could be also useful to have the height with respect to the home point. The more info i get during my flught, the better

2) you can crash onto something (or even someone)! lets say i don't know if i am really  flying over a tree branch or somthing and i'm only readin "-4.0m" on my screen. Judging by the vide playback, i may think i'm good to go. But we only have 1 camera. depth of field isn't exactly perfect. I might actually be just at the same height of that branch and if what you are filming requires you to have the gimbal pointing down while passing over that tree, bum. That's what i mean.
And yes, i know you should be flying in a way you KNOW is safe. Believe me, i try to make sure it's safe. I'm no drone expert (been flying for 4 days) and this drone is gonna be used for work and it cost me 2/3 of my monthly income. I do not want to loose it). But let's face it. There are times when things don't work out as you expect. Accidents happen. And one way to prevent it would be to have more (and more representative and accurate) data to begin with.

I know this thread could sound like a rant (and it kinda is) but it shouldnt'. It's a very important thing to understand (and now i do) and keep in mind. And yes, there could (and, in my opinion, should) be better ways to measure the height even in the most basic drones from DJI. Safety first.

BORRERO.MD Posted at 1-12 08:32
1) well, i think you could get a report on the actual height from ground and, now that you mention it, it could be also useful to have the height with respect to the home point. The more info i get during my flught, the better

2) you can crash onto something (or even someone)! lets say i don't know if i am really  flying over a tree branch or somthing and i'm only readin "-4.0m" on my screen. Judging by the vide playback, i may think i'm good to go. But we only have 1 camera. depth of field isn't exactly perfect. I might actually be just at the same height of that branch and if what you are filming requires you to have the gimbal pointing down while passing over that tree, bum. That's what i mean.

One of the big detractors of using Altitude from Ground is that it does not account for the height of other objects like trees.  This can be very deceptive and I understand why many pilots have had catastrophic accidents using the Altitude from Ground measurement.  

Some other issues with GPS based elevation:

1) It is based on a tile system.  The elevation tiles can be 10-500 square meters and often do not accurately represent the terrain below giving a false sense of clearance.  

2) GPS elevation requires an internet connection or the maps to be cached in advanced.  If DJI were to implement offline map downloads for the DJI Fly app this would make a relative Altitude from Ground more viable for rural areas.

3) The downward vision sensor, a.k.a sonar height or VSP, is a built in safety feature that works in conjunction with obstacle avoidance to improve flight safety.  But the systems have difficulty sensing permeable objects like tree branches.  

Relying on an arbitrary measurement that does actually reflect the ground clearance is a recipe for disaster.  With your gimble panned to 0° you can check the horizon of impedance more accurately and set a height.  Then flying at that height you can change the gimble angle to get the desired shots.   

It's from wherever you lift off from. Like for example I've flown mine from the top of a mountain down into a valley, and the drone will register negative altitude. It'll still be far above the treetops, but it will be way lower than where it lifted off from.

BORRERO.MD Posted at 1-12 08:32
1) well, i think you could get a report on the actual height from ground and, now that you mention it, it could be also useful to have the height with respect to the home point. The more info i get during my flught, the better

2) you can crash onto something (or even someone)! lets say i don't know if i am really  flying over a tree branch or somthing and i'm only readin "-4.0m" on my screen. Judging by the vide playback, i may think i'm good to go. But we only have 1 camera. depth of field isn't exactly perfect. I might actually be just at the same height of that branch and if what you are filming requires you to have the gimbal pointing down while passing over that tree, bum. That's what i mean.

1) well, i think you could get a report on the actual height from ground
there could (and, in my opinion, should) be better ways to measure the height even in the most basic drones from DJI.That's great ... Now all you have to do is invent and make a sensor that can do that and is affordable and small enough to be carried by a drone.
The pilots of most general aviation planes don't have what you are wanting.

lets say i don't know if i am really  flying over a tree branch or somthing and i'm only readin "-4.0m" on my screen
Lets say that you need to have better situational awareness and know where your drone is if you want to always bring it home again.
Taking risks close to obstacles is probably the #1 cause of drone crashes

Ok I have to ask this. Why does DJI's apps report altitude as related to the home point? What possible information could that tell me about the aircraft's height above the ground? It should report MSL, if it uses a barometric altimeter. Ideally it would compare that with its GPS coordinates and calculate an elevation for me to quickly find my height AGL since I AM REQUIRED to operate below a certain height ABOVE GROUND LEVEL not "as compared to your home point"

Change it to MSL. The "altitude above home point" is useless. I don't want height above trees, I want height above ground. The law says I need to be 400' above the ground or less, not above the trees or less. I'm not looking for an altitude readout to tell me if I am about to hit a tree.
The simplest solution would be to allow for a setting to change the altitude to be "with regard to homepoint" or "MSL only" because I would choose MSL each and every time because that is WHAT MATTERS. Sure, if you could provide AGL in a simple and accurate manner, that'd be great. But I can do my homework and find the elevation of where I intend to fly and figure my own AGL. Relating that to homepoint just adds more figuring. Another simple solution would be just DISPLAY both AIRCRAFT ELEVATION in MSL and RC ELEVATION in MSL, separately.

How big a data base would the drone have to carry to work out its height above the ground, what would be the cost of that data base, how accurate are such data bases?
Without the data base MSL is of equal use as "height above the take off point".
Summary, if your fear you are breaking the 400ft rule, lower the drone, there is nothing to say you MUST fly at 400ft AGL.

djiuser_Ks5LZHST1kqX Posted at 10-21 06:28
Ok I have to ask this. Why does DJI's apps report altitude as related to the home point? What possible information could that tell me about the aircraft's height above the ground? It should report MSL, if it uses a barometric altimeter. Ideally it would compare that with its GPS coordinates and calculate an elevation for me to quickly find my height AGL since I AM REQUIRED to operate below a certain height ABOVE GROUND LEVEL not "as compared to your home point"

Change it to MSL. The "altitude above home point" is useless. I don't want height above trees, I want height above ground. The law says I need to be 400' above the ground or less, not above the trees or less. I'm not looking for an altitude readout to tell me if I am about to hit a tree.

Why does DJI's apps report altitude as related to the  home point? What possible information could that tell me about the  aircraft's height above the ground?
If you are standing somewhere close to where you launched, knowing the height of the drone relative to where you are should be all you need to know.

Change it to MSL. The "altitude above home point" is useless.
Useless?   It's the most sensible and practical way to display height.

I don't want height above trees, I want height above ground.
The law says I need to be 400' above the ground or less, not above the  trees or less. I'm not looking for an altitude readout to tell me if I  am about to hit a tree.
No-one is giving you height above trees, or mountains or anything except height relative to the launch point.

The simplest solution would be to allow for a setting to change the altitude to be "with regard to homepoint" or "MSL only" because
The simplest solution is for the app to show height relative to the launch point.
Unless you are flying at the beach, your height relative to sea level is irrelevant and confusing.

I would choose MSL each and every time because that is WHAT MATTERS.
Make your mind up, earlier in your post, you said that AGL is what matters ??

Sure, if you could provide AGL in a simple and accurate manner, that'd be great.
There is no practical way to do that.
Drone flyers have to figure out their own AGL height, just like the pilots of real small planes do.

But I can do my homework and find the elevation of where I intend to fly and figure my own AGL. Relating that to homepoint just adds more figuring. Another simple solution would be just DISPLAY both AIRCRAFT ELEVATION in MSL and RC ELEVATION in MSL, separately.
If that were possible, it would only unnecessarily complicate things.
If you are smart enough to calculate your elevation and figure your own AGL height, it should be no problem at all to work things out knowing how much higher or lower the drone is relative to where you launched.

Read my words as I type them then:

M S L
That's all I want then. If it's too much of a hassle for anything else, MSL is all I want. Just show the drone's altitude in MSL and I'll shut up.

Only showing the home point altitude is about useless to me. I'm glad you find it useful, but if I'm supposed to stay <400' AGL and I don't live on any flat surfaces. The ground OVER THERE is going to be a different elevation than the ground here.

djiuser_Ks5LZHST1kqX Posted at 10-21 18:24
Read my words as I type them then:

M S L

Only showing the home point altitude is about useless to me.
How much drone flying experience have you got?
Knowing the drone's height relative to sea level won't help you maintain <400 ft AGL.

I'm glad you find it useful, but if I'm supposed to stay <400' AGL and I don't live on any flat surfaces. The ground OVER THERE is going to be a different elevation than the ground here.
And if you have an IQ higher than room temperature and you know where your drone is and it's height relative to you, you should have no trouble estimating its height relative to the terrain below it.
The suggestion that knowing the drone's height relative to sea level is more useful is simply ridiculous.

I don't think it matters to you how much drone experience I have. I don't have much; that's why I'm asking questions and trying to customize my flights. I  have plenty of freestyle RC experience and actual real manned aircraft experience, so trust me when I tell you that MSL or AGL is what I want, compared to some number that I don't care about. I KNOW the elevation my RC is. What I would LIKE to see is my drone's altitude. SINCE IT CHANGES. I need to know my home point altitude exactly one time for the whole flight, unless I am dancing through the woods again.

I guess the main thing you need to take away from this is, other people want different data. All it takes is a simple change to the settings, to let people pick what outputs they want on the screen. Not everyone wants height above home point.

So that's the main take away. For me home point is not the information I want. I want drone height AGL. If I can't have that then I want altitude MSL, preferably both RC and aircraft MSL.

djiuser_Ks5LZHST1kqX Posted at 10-21 19:43
I don't think it matters to you how much drone experience I have. I don't have much; that's why I'm asking questions and trying to customize my flights. I  have plenty of freestyle RC experience and actual real manned aircraft experience, so trust me when I tell you that MSL or AGL is what I want, compared to some number that I don't care about. I KNOW the elevation my RC is. What I would LIKE to see is my drone's altitude. SINCE IT CHANGES. I need to know my home point altitude exactly one time for the whole flight, unless I am dancing through the woods again.

I guess the main thing you need to take away from this is, other people want different data. All it takes is a simple change to the settings, to let people pick what outputs they want on the screen. Not everyone wants height above home point.

There is no consumer drone that has dynamic AGL, or MSL. The beat you can do is a topographical map of your flight location, get the altitude ASL at your take off home point and figure your owm flight path AGL. SOmething else to consider, your flight logs only record AGL from home point zero.

djiuser_Ks5LZHST1kqX Posted at 10-21 19:43
I don't think it matters to you how much drone experience I have. I don't have much; that's why I'm asking questions and trying to customize my flights. I  have plenty of freestyle RC experience and actual real manned aircraft experience, so trust me when I tell you that MSL or AGL is what I want, compared to some number that I don't care about. I KNOW the elevation my RC is. What I would LIKE to see is my drone's altitude. SINCE IT CHANGES. I need to know my home point altitude exactly one time for the whole flight, unless I am dancing through the woods again.

I guess the main thing you need to take away from this is, other people want different data. All it takes is a simple change to the settings, to let people pick what outputs they want on the screen. Not everyone wants height above home point.

so trust me when I tell you that MSL or AGL is what I want
Having read your posts and taken it all in, I think you don't know what you want.
You have locked onto something that's not achievable and you are trying to solve a perceived problem that really isn't a problem at all.

I guess the main thing you need to take away from this is, other people want different data. All it takes is a simple change to the settings, to let people pick what outputs they want on the screen. Not everyone wants height above home point.
The things you need to take away are:
Some people have been flying a lot longer than you and understand how these things work and what's really important.
There is no practical or affordable sensor that can provide AGL height
DJI aren't going to change anything to give you the odball thing that you and no-one else wants.

So that's the main take away. For me home point is not the information I want. I want drone height AGL. If I can't have that then I want altitude MSL, preferably both RC and aircraft MSL.
And MSL isn't going to be any help to you despite your imagining it will.

>DJI aren't going to change anything to give you the thing...
Then I'm really just wasting my  time here.

Some people have more little drone icons next to their name so I guess they created the fundamentals of flight.

But since I am an idiot newbie, what POSSIBLE good is knowing my height above home point? No law anywhere states I have to be within a certain range of my home point. It's ALL everywhere about AGL. I even went so far as to mention that I had read that AGL doesn't seem feasible with current sensors, but that knowing both device MSL would be better. FOR ME.  YOU are the one who keeps coming back with YoU DoNT NeEd ThaT InFoRMaTiON even when, repeatedly, I have stated that I would prefer it.

But since DJI isn't changing anything anyway we're back to just wasting time.

djiuser_Ks5LZHST1kqX Posted at 10-21 22:32
>DJI aren't going to change anything to give you the thing...
Then I'm really just wasting my  time here.

But since I am an idiot newbie, what POSSIBLE good is knowing my height above home point?
If you have to ask, you can't be very smart.
You are likely to be standing close to the launch point with a good view of trees and other obstacles within flying range.
If you can't estimate how high the drone is relative to those obstacles and the ground below, already knowing how much higher the drone is than where you areyou shouldn't be flying.
It's what drone flyers have been doing for years and it's very simple.

No law anywhere states I have to be within a certain range of my home point. It's ALL everywhere about AGL.
So use your brain and estimate based on the information you have.
Drone flyers have been doing that since drones were available and haven't had a problem with it.
How do you think knowing the drone's height relative to the sea (which could be tens or hundreds of miles away)is going to help you?

I even went so far as to mention that I had read that AGL doesn't seem feasible with current sensors, but that knowing both device MSL would be better. FOR ME.  YOU are the one who keeps coming back with YoU DoNT NeEd ThaT InFoRMaTiON even when, repeatedly, I have stated that I would prefer it.
You really need to get over your obsession with your perceived problem and think outside the box you've restricted yourself to.
It's really no problem at all.

"what POSSIBLE good is knowing my height above home point?"
Like everyone else you could just think about whether or not the drone is likely to be over the AGL limit. It isn't difficult.
If too close to the AGL limit for your comfort then lower the drone, there is no rule that says YOU MUST fly near the AGL limit.