Altitude Reading

[EpicFlight]

The average accuracy of the elevation data used by Google Earth/Maps is 8.7m (over 28ft). Whilst that might help to give you a rough height AGL, it certainly not something you could rely on.

I disagree. For purposes of maintaining a much larger AGL elevation, it's very useful. There are all sorts of flying I can think of doing where I'm just fine if my actual AGL is 150 +/–30.

 

[maelstrom]

I disagree. For purposes of maintaining a much larger AGL elevation, it's very useful. There are all sorts of flying I can think of doing where I'm just fine if my actual AGL is 150 +/–30.

Perhaps I should have said “not something I would rely on”. Remember also that the 8.7m is an average and could be significantly more than this without you knowing about it. This is worth reading:

Understanding the Accuracy of the GPS Elevation Reading | Garmin Support

Garmin Support Center United Kingdom is where you will find answers to frequently asked questions and resources to help with all of your Garmin products.

support.garmin.com

+/- 400ft is definitely a problem!

 

[EpicFlight]

It's not a random number, it's a calculated value based on measurement with a quantifiable error.

Such data is used, and relied upon all the time in science and engineering for reliable outcomes all the time.

Quantifiable error is nothing to be scared of. It is something to be worked with.

 

[maelstrom]

Such data is used, and relied upon all the time in science and engineering for reliable outcomes all the time.

I agree, but scientists and engineers usually require much more accurate GPS data than is normally available to the general public. They would use things like “precise-point-positioning“ systems or special survey procedures that allow increased accuracy of the vertical GPS component. These systems are way outside the capability of the onboard GPS installed in recreational drones.

 

[EpicFlight]

I agree, but scientists and engineers usually require much more accurate GPS data than is normally available to the general public. They would use things like “precise-point-positioning“ systems or special survey procedures that allow increased accuracy of the vertical GPS component. These systems are way outside the capability of the onboard GPS installed in recreational drones.

Again, I disagree. Scientists and engineers – especially – will use the accuracy necessary for the application.

Our drone's GPS positioning could be made much more accurate, but at considerable cost. Why doesn't DJI?

Well, in fact they do... sub-foot accuracy IIRC, at a cost of tens of thousands of $$.

I'm simply arguing there are valid, serious applications for which even 100' error margins are just fine. for example where being within 100' of 1000' flight separation is just fine.

If I could have 100' accuracy AGL information while flying it would be incredibly useful to me. I suspect there are lots of others too.

 

[maelstrom]

But if Garmin (a major manufacturer of commercial and recreational GPS equipment) is to be believed, the error could potentially be in the hundreds of feet and you would have no way of knowing. They have nothing to gain by exaggerating the possible error - quite the opposite, in fact.

 

[EpicFlight]

But if Garmin (a major manufacturer of commercial and recreational GPS equipment) is to be believed, the error could potentially be in the hundreds of feet and you would have no way of knowing. They have nothing to gain by exaggerating the possible error - quite the opposite, in fact.

Of course I would know... I'm not instrument flying! I can also see how far away from the ground.

That's my primary data – my visual feed. The information I get from the AGL system (were it to exist) is helping refine that.

 

[maelstrom]

I can see we’ll have to agree to disagree on this. Enjoy your flying!

 

[Chaosrider]

Yes, in an emergency (and when flying VFR), but not for routine operations. There’s no way for ATC to deconflict aircraft vertically if one is using GPS altitude and another barometric.

That's why, when I encountered that situation after a major equipment failure, I informed ATC of the fact. It wasn't a big deal. Of course, I was sort of in the boonies, which is where I prefer to fly anyhow.

I learned to fly in the LA basin, and flew there for over a decade, when I moved to San Jose, and flew there for a few years, before I moved to Lake Tahoe.

Anyone who hasn't flown in that level of high traffic may have an exaggerated notion of how likely mid-airs are.

In my 40 years of flying, I've always been vastly more concerned about bird strikes, and I was only concerned about them in areas of high bird density, like near coastlines. They move around suddenly at low altitude, and can reach very high altitudes. I think the added risk of collision to manned aircraft by drones, should be viewed in the context of the drone population relative to bird + drone population.

I'm not disagreeing with your main point. I'm saying that knowing my AGL elevation +/- 28 ft for the purpose of staying below 400 ft AGL, is probably better than my eyeball estimates. Where and how I fly, ATO elevation is essentially irrelevant for AGL estimation.

Thx,

TCS

 

[maelstrom]

I'm not disagreeing with your main point. I'm saying that knowing my AGL elevation +/- 28 ft for the purpose of staying below 400 ft AGL, is probably better than my eyeball estimates.

But, as Garmin pointed out in the link I posted earlier, the height error could be as much as 400ft. It’s a problem with no easy solution.

 

[Meta4]

If I could have 100' accuracy AGL information while flying it would be incredibly useful to me.

So estimate?

 

[Scrapion]

The height it shows is the height Above Takeoff Location, not Above Ground Level.
You are doing nothing wrong, it is how it is for now. I would also like the function to see the real AGL height of the drone, for now it is not possible, at least with DJI Fly yet. Maybe it will be added in the future, but as the sensors of the drone can't measure heights higher than 20 meters (60 feet) it would need to use Google Maps terrain height data to do so. It's possible for DJI to implement it in the future, though!

So in effect it does not have the ability to read altitude at all. It can only calculate the overall distance it has traveled vertically from the point of take off based on its lower sensors. So I could take my drone to the top floor of a high rise flat, approx 380 ft and launch it standing at the foot of the flats and be able to fly to 780 feet legally. Am I correct?

 

[Yaros]

It can only calculate the overall distance it has traveled vertically from the point of take off based on its lower sensors.

Lower sensors too, but mainly barometer. It measures the pressure changes, which directly relate to altitude. The pressure during takeoff is recorded as 0 (meters or feet) height.

Am I correct?

No! This is not correct. Let me explain.
You are allowed to fly 120 meters (400 feet) Above Ground Level, not Above Takeoff Level. So, if you taken off from 380 ft AGL, your drone will still see it as 0 ft, but then you could only fly legally to 400 ft AGL, which is 20 ft Above Takeoff Level.

 

[EpicFlight]

So, if you taken off from 380 ft AGL, your drone will still see it as 0 ft, but then you could only fly legally to 400 ft AGL, which is 20 ft Above Takeoff Level.

How does one take off 380 feet above the ground?

 

[sar104]

How does one take off 380 feet above the ground?

From a tall building, for example.

 

[EpicFlight]

From a tall building, for example.

 

[Serenity Valley]

Ok, so we've established that the altitude reading on the screen is based on your takeoff/home point. If you reset your home point during flight, does it recalculate AGL based on that location?

 

[sar104]

Ok, so we've established that the altitude reading on the screen is based on your takeoff/home point. If you reset your home point during flight, does it recalculate AGL based on that location?

No. It's height above take off location, not height above home point. Changing the home point has no effect unless you do that by landing, then re-arming the motors and taking off again - that will reset the height reference to the new take off location.

 

[Suzygs1000]

Newbie to Drones, and just got my Air 2 S. Love this little machine! I was however a bit disappointed on how it measures altitude. If I am flying over a relatively flat surface, no problem. If flying in a hilly terrain, the altitude reading reflects the height from where I took off, not how far I am off the ground. So, if my drone is headed down a hill or up a mountain, I can’t get an accurate reading of my actual altitude. Am I doing something wrong? Appreciate any feedback.

If a plane crashes into a mountain top, I am pretty sure it's altitude doesn't show on the instruments as zero ...