Question answered today . . .

[Squerly]

I've often wondered if my drone calculated its altitude from the drone to the earth, or from the drone to the place where it took off from. Today, I answered that question.

I live on a mountain where the elevation is approx 4,000'. About 1/2 mile down the mountain is a field that is elevation 3,500'. I lifted off from the field and ascended to 400', then headed off towards my house. When I arrived a couple minutes later, I found that the obstacle avoidance would not let me get any closer. The altitude on the controller still said I was still at 400' but the video later showed that at the house, I was about 50' off the ground and being challenged by a huge oak.

So yes, the drone was still 400' above the controller, but only 50' off the ground at the drones real position. If the obstacle avoidance had been turned off, I would have flown straight into the old oak.

As a further test, I thought about taking off from the house and ascending to 400', and then flying to the field, which would actually put the drone at 750' above the ground. I fairly confident that the controller would still tell me I was 400' above the ground, even though it was actually 750'.

 

[herein2021]

I've often wondered if my drone calculated its altitude from the drone to the earth, or from the drone to the place where it took off from. Today, I answered that question.

I live on a mountain where the elevation is approx 4,000'. About 1/2 mile down the mountain is a field that is elevation 3,500'. I lifted off from the field and ascended to 400', then headed off towards my house. When I arrived a couple minutes later, I found that the obstacle avoidance would not let me get any closer. The altitude on the controller still said I was still at 400' but the video later showed that at the house, I was about 50' off the ground and being challenged by a huge oak.

So yes, the drone was still 400' above the controller, but only 50' off the ground at the drones real position. If the obstacle avoidance had been turned off, I would have flown straight into the old oak.

As a further test, I thought about taking off from the house and ascending to 400', and then flying to the field, which would actually put the drone at 750' above the ground. I fairly confident that the controller would still tell me I was 400' above the ground, even though it was actually 750'.

All drones that I have flown to date (since 2014) calculate altitude based on the takeoff point. This is one of the reasons why I don't think drones should have an altitude limit at all. When you are flying up a mountainside you could be only a few feet above the mountain but when you hit the altitude limit it is because it is measuring the distance from the takeoff point vs. the ground beneath it.

When you think about it though, there's really no way for the drone to know how high it is off the ground so the only reference point it has is the takeoff point.

 

[kenautelevo2pro]

I understand everything that has been said so far, not a problem. can't expect the drone to follow the terrain below. where I live it is flat so for sure, it was never a question in my mind...because it was never put to the test. however, this past week in the autel mission planner, I started dealing with MSL instead of AGL and I don't think I understand it as well as I should or know why I really need it. as I said, everything is flat so if I want to set AGL to 150 - 200 then whenever I create a mission, I really don't have to worry about anything. why would I need to use MSL and how will it help me if my entire mission is above 150?

 

[herein2021]

I understand everything that has been said so far, not a problem. can't expect the drone to follow the terrain below. where I live it is flat so for sure, it was never a question in my mind...because it was never put to the test. however, this past week in the autel mission planner, I started dealing with MSL instead of AGL and I don't think I understand it as well as I should or know why I really need it. as I said, everything is flat so if I want to set AGL to 150 - 200 then whenever I create a mission, I really don't have to worry about anything. why would I need to use MSL and how will it help me if my entire mission is above 150?

One scenario that comes to mind is if you were somewhere that wasn't flat. The software was written to accommodate many different types of terrain. If you were somewhere that wasn't flat you would probably want to use a topographical chart to figure out the height of obstacles and those heights could be listed as MSL vs AGL.

The EVO II does know its MSL and it knows AGL but its AGL reference point is the takeoff point. A more precise measurement is MSL but unless you know the exact MSL of the takeoff point the two can get complex to calculate. For something like a mission in a location that is not flat, MSL would be more accurate. For somewhere that is perfectly flat; MSL and AGL would probably be equally accurate.

Another example is if you took off from the top of a building that was 100' tall but needed precise flying 150' off the ground. The MSL would not be affected by the takeoff height of the building but the AGL would be since you started out 100' AGL.

 

[kenautelevo2pro]

One scenario that comes to mind is if you were somewhere that wasn't flat. The software was written to accommodate many different types of terrain. If you were somewhere that wasn't flat you would probably want to use a topographical chart to figure out the height of obstacles and those heights could be listed as MSL vs AGL.

The EVO II does know its MSL and it knows AGL but its AGL reference point is the takeoff point. A more precise measurement is MSL but unless you know the exact MSL of the takeoff point the two can get complex to calculate. For something like a mission in a location that is not flat, MSL would be more accurate. For somewhere that is perfectly flat; MSL and AGL would probably be equally accurate.

Another example is if you took off from the top of a building that was 100' tall but needed precise flying 150' off the ground. The MSL would not be affected by the takeoff height of the building but the AGL would be since you started out 100' AGL.

thanks for the explanation. makes sense. although I would love to know how and learn to use MSL just in case I need it one day. I try it on one of my mission. I see that I can capture this information using the Mission Recording function and yeah, it's "really flat" around here.

so here's another question. if you are flying in controlled airspace and you have, let's say, a limit of 200'
AGL then isn't it possible for you to exceed that limit if you calculate in MSL or is it the other way around, you are actually taking advantage of the true height. I feel like it's the latter where you could be 175' AGL but 300' according to your controller.

 

[UasDriver]

When you think about it though, there's really no way for the drone to know how high it is off the ground so the only reference point it has is the takeoff point.

my understanding, altitude is "calculated" based off barometric pressure. so "sample at start" at sea level "0" = 14psi
go up 400ft = something < 14. Launch from a mountain "0" = the pressure at launch point, max altitude is calc'd off the new "0"
all of which isn't very accurate since pressure changes over time even at same height. I could be all wet though, still learn'n

 

[herein2021]

my understanding, altitude is "calculated" based off barometric pressure. so "sample at start" at sea level "0" = 14psi
go up 400ft = something < 14. Launch from a mountain "0" = the pressure at launch point, max altitude is calc'd off the new "0"
all of which isn't very accurate since pressure changes over time even at same height. I could be all wet though, still learn'n

The problem though is when the drone flies over non flat terrain...so if the drone takes off and flies 100' up in the air, it knows it is 100' above its takeoff point, if it then flies in a perfectly straight line without gaining altitude but it is flying towards a 50' hill......it would have no way of knowing it is only 50' AGL as it flies over the hill and would continue to think it is 100' AGL. So in that scenario, the drone has no way of knowing how high it is off of the ground.

 

[Sjs]

I've often wondered if my drone calculated its altitude from the drone to the earth, or from the drone to the place where it took off from. Today, I answered that question.

I live on a mountain where the elevation is approx 4,000'. About 1/2 mile down the mountain is a field that is elevation 3,500'. I lifted off from the field and ascended to 400', then headed off towards my house. When I arrived a couple minutes later, I found that the obstacle avoidance would not let me get any closer. The altitude on the controller still said I was still at 400' but the video later showed that at the house, I was about 50' off the ground and being challenged by a huge oak.

So yes, the drone was still 400' above the controller, but only 50' off the ground at the drones real position. If the obstacle avoidance had been turned off, I would have flown straight into the old oak.

As a further test, I thought about taking off from the house and ascending to 400', and then flying to the field, which would actually put the drone at 750' above the ground. I fairly confident that the controller would still tell me I was 400' above the ground, even though it was actually 750'.

Very eye opening.

 

[UasDriver]

the drone has no way of knowing how high it is off of the ground.

that's why I leave OA on, it's saved me more times than I can count. It does know, but crudely, +/- 50' in barometric pressure is a small value. I just did a flight where I had to clear 100+ ft trees to "drop into" a clearing, no possibility of VLOS, I simply flew FPV and trusted the OA, AND stayed >10ft off the deck. zero problems.

 

[frosty beach]

If you pull a sectional chart of your flight area you'll find that It lists the altitude of the highest obstruction in each section in MSL (Mean Sea Level) altimeters read in MSL but you need a current barometer setting. However things like towers and other obstructions have their altitude listed in MSL and AGL. You should note that the AGL is from the base of the obstruction.

You're limited to 400' AGL so if you lift off from the top of a 200' cliff and ascend to 400' AGL and fly over the edge you're now at 600' AGL. However the MSL would not change.

 

[Squerly]

Very eye opening.

Well, yes it was!

lol, anyway, here is a link to the flight. When watching the drone real-time on a small iPhone screen, it was difficult to tell where the obstacles were. But when i played back the video, it was obvious just how close to the tree tops i was. And all the time, the software is telling me I'm 400' above ground.

So yes, I learned a bunch yesterday.

Solo 2.mp4

drive.google.com

 

[Isky]

Drones don’t have altimeters. When they get them you’ll be able to dial in your pressure and fly like planes do utilizing MSL altitudes but for now it’s just math you need with AGL. As far as I know, when you use msl on a drone it uses published topo info when making its height adjustments like when using litchi for instance. Since the FAA uses AGL when it comes to the rules, I’m using AGL.

 

[quinn karter]

All drones that I have flown to date (since 2014) calculate altitude based on the takeoff point. This is one of the reasons why I don't think drones should have an altitude limit at all. When you are flying up a mountainside you could be only a few feet above the mountain but when you hit the altitude limit it is because it is measuring the distance from the takeoff point vs. the ground beneath it.

When you think about it though, there's really no way for the drone to know how high it is off the ground so the only reference point it has is the takeoff point.

At least Autel gives us an extra 1000 ft of altitude compared to DJI. Flying up a mountain with a DJI would be pretty limiting, if you hit their 1600ft limit.

 

[herein2021]

At least Autel gives us an extra 1000 ft of altitude compared to DJI. Flying up a mountain with a DJI would be pretty limiting, if you hit their 1600ft limit.

Very true, that's definitely another plus for the Autel. I noted that too, in my DJI vs EVO comparison. There is actually a hack to get rid of the limit altogether, but the times I will be filming a mountain and run into the limit are so rare that I don't think it's worth the risk to the drone to use 3rd party hacks like that.

 

[quinn karter]

my understanding, altitude is "calculated" based off barometric pressure. so "sample at start" at sea level "0" = 14psi
go up 400ft = something < 14. Launch from a mountain "0" = the pressure at launch point, max altitude is calc'd off the new "0"
all of which isn't very accurate since pressure changes over time even at same height. I could be all wet though, still learn'n

I suppose, if you took off at a low level and were able to land at a higher level, you would get a new AGL take off point and be able to keep going higher and higher every time you landed. But I doubt there are any places to land on the side of a mountain and still keep your RC connection as well.

 

[quinn karter]

Very true, that's definitely another plus for the Autel. I noted that too, in my DJI vs EVO comparison. There is actually a hack to get rid of the limit altogether, but the times I will be filming a mountain and run into the limit are so rare that I don't think it's worth the risk to the drone to use 3rd party hacks like that.

I heard No LIMIT DRONES actually modifies the main board of DJI drones creating a hardware hack. This lets you do almost any mod to the limits of the drone. Expensive though.

 

[herein2021]

I heard No LIMIT DRONES actually modifies the main board of DJI drones creating a hardware hack. This lets you do almost any mod to the limits of the drone. Expensive though.

Yes they do have that option....but the fine print is that you have to remove your own mainboard and send it to them then reassemble the drone when you get it back. They also sell pre-modded boards but its expensive and you still have to put the drone back together yourself; no small feat in and of itself. I researched all of those options before discovering the EVO II.

I suppose, if you took off at a low level and were able to land at a higher level, you would get a new AGL take off point and be able to keep going higher and higher every time you landed. But I doubt there are any places to land on the side of a mountain and still keep your RC connection as well.

I think you have to restart the drone to reset the take off point but I've never tried this so I could be wrong.

 

[Quad808]

Drone-Hacks has a modded Autel Explorer app...2km limit. Altitude limits of Autel drones is set by the app. The altitude limit is the only thing changed in the app, completely safe to use.

 

[herein2021]

Drone-Hacks has a modded Autel Explorer app...2km limit. Altitude limits of Autel drones is set by the app. The altitude limit is the only thing changed in the app, completely safe to use.

How much you want to bet the limit is stored in some simple config file somewhere on the tablet? I might look through the files one day to see if I see something obvious that can be easily changed in an editor.

 

[Quad808]

Trust me when I tell you it's not. You have to reverse engineer the android .apk, then look hard, use some tools and find the right bits to flip.