Altitude

[passedpawn]

You could just check the specs.
They say: Global Navigation Satellite System (GNSS): GPS+GLONASS+GALILEO

Lol ok. You win.

I did have a look, The uBlox module that DJI uses can receive GPS/BeiDou/GLONASS/Galileo. Oddly, you have to choose 3 out of the four (only 3 simultaneous GNSS).

 

[Crusader14]

There is often confusion about measuring the vertical distance above a reference point. “Altitude” is the vertical distance above sea level. Height is the vertical distance above the ground. The drone will measure this using the take off point as reference.
In the U.K. at least the max height allowed is 400ft above ground level (agl). This means if you enter a valley the drone must maintain 400ft agl which may mean descending blow the take off point. Also, if you encounter a building/mast you cannot fly 400ft over that but you can pass over it so long as you maintain 150ft separation distance.

 

[Dave Maine]

It is incorrect to state that GPS does not know altitude. It does. However, it is referenced to sea level, not ground below the drone. To calculate height above ground requires calculating the ground height by subtracting the height of the terrain under the drone from the sea level altitude of that point. This requires a loaded map in the controller that has accurate height altitude.

The drone sends it’s barometric pressure information to the controller, and the controller calculates the height above ground and sends it back to the drone. All this relies on good fast communication back and forth between the controller and drone.

In practice using WiFi this method is too slow and unreliable for use in real time control of the drone flight system.

 

[hiflyer201]

the 400 ft rule is to avoid other aircraft ...use common sense. Like Meta says, aircraft use an altimeter/barometer also..in aircraft you have a setting knob. Altitude is measured in inches of mercury as in 29.92 (standard altitude at sea level) You are required to always have the current altimeter setting. If you take off from an airport where that is not available you use the field elevation which is what the DJI drone does. For the sake of our hobby/business..please don't bust the altitude regulations.

 

[Vic Moss]

I know this might be a stupid question as I am sure of the answer already, but just want to check. Dose that mean if you take off from high ground and fly over a valley, you can fly over the 120m/400ft limit ? if the valley floor is more then 120m/400ft below ?

Did not even think about this before I read this post, so just making sure 100% that I am sure before flying

No. The 400' rule is "above ground level (AGL)", not from take off.

The drone measures the AGL from the take off point. If you take off from a mountain top and then fly out over the valley (something I do often here in Colorado), you have to maintain the 400' AGL. That means if you fly into a valley that is 1000' below your take off point, you telemetry should be showing about -600'AGL if you're following the 400' rule.

Obviously that's pretty hard to be accurate with. Basically you need to maintain safety as best you can.

Remember, helicopters fly though valleys. We have many Flight For Life helicopters flying valleys if they're transporting a patient from the mountains to a trauma center in Denver.

Basically fly safe.

 

[Dbez1]

The altitude always will be calculated base in the home point location or (take off location). If you set for example max altitude 400FT down the hill. You are going to fly to the hill and assuming is a 600FT toll hill, you are going to crash (hypothetically) since you set the maximum altitude at 400FT. Will be nice if DJI include sensors to measure the ground level.

To the OP... I agree with the above and you’ve gotten some good information. I’d like to add that as a new pilot, you will find that if you’re flying up a gradual grade and not flying too fast, the downward sensors will slowly increase the altitude of the drone. This probably won’t work if you’re into the throttle too much or if the grade is too steep. Best wishes.

 

[Newguy126]

Having read all the altitude comments I am left with the question of how do we know how far above the ground out a.c. Is? We would need a radio altimeter to know that. But on the other hand, the authorities wouldn't know either, would they?

 

[zeusfl]

Litchi calculate the ground level using google maps, but you need to check the box in order to doit. If you set the altitude with the ground level checked in litchi will maintain the desired altitude all the time even up hill, of course there is a marge of error always, like buildings, antennas and how old is the map that litchi is using plus some meters or ft of precision.

 

[Capt KO]

To be sure of legality, jut stay within 400ft of land. Little tricky on the side of a mountain or valley so a good guess is necessary. ?? Not a stupid question. If you use the search function, you’ll see the same one from me, couple years ago. Oops, was Phantom forum.

 

[DanMan32]

The barometric sensors are accurate to a few inches.
They even work where GPS doesn't.
Real airplanes also use barometric sensors to provide altitude data.

DJI height once past VPS range is not accurate within inches. It doesn't take long in flight before my P3 and M2Z are as much as 10ft off.
Height based on VPS is determined by IR and in Go app is shown separately from the barometer based height.

 

[Meta4]

DJI height once past VPS range is not accurate within inches. It doesn't take long in flight before my P3 and M2Z are as much as 10ft off.
Height based on VPS is determined by IR and in Go app is shown separately from the barometer based height.

Your barometric sensor responds to changes of a few inches.
When beyond 10 ft, it does not show on the display, but the reported flight data shows the height from the sensor to 0.1 ft.

 

[DanMan32]

Interesting, but it does drift by feet within minutes and I wouldn't attribute that to atmospheric pressure changes.

The drift could be attributed to air in the AC near the sensor warming up, but I've even landed touch and go to reset to zero and it still drifted.

I guess you could say the barometer has a resolution in inches, but not accuracy.

 

[dronerdave]

Having read all the altitude comments I am left with the question of how do we know how far above the ground out a.c. Is? We would need a radio altimeter to know that. But on the other hand, the authorities wouldn't know either, would they?

In the US you are responsible for your actions and must follow FAA rules. If an incident occurs and they find you were flying under rec rules and out of compliance you might be in big trouble. The fact is you must be able to see and avoid manned aircraft at any altitude. 400' AGL is our operating zone but it doesn't mean we have some sort of right of way below 400' AGL. The way I tell if I'm too high is by visual sight and by using the camera to make sure I'm below the 400' ceiling. I can't visually tell what altitude a peak of a mountain is but I can see from my quads camera point of view if I'm too high and possibly encroaching the AGL limit. Surely if you are skipping about 100 feet above the average treeline top heights, it's obvious you're not close to 400' AGL. A shear cliff (or other tall structure) may pose a problem for obvious reasons but a little common sense goes a long way. Fly safe.

 

[Blad the Inhaler]

On Sunday I was out in the Moelwyns flying around after a bit of climbing. On top of this old slate mine I set off and began flying over the valley. Of course my altitude shot up so I returned quickly. I knew I was supposed to keep to the contours and below 400ft so I tried again but closer to the ground. However, at times when I could see the Mavic was clearly within the 400ft limit, my altimeter was 1000ft+. Am I going to have the FBI, MI5 and CAA coming to arrest me? To my understanding my flights are recorded and DJI get the data. What happens when I break the law, albeit momentarily? From reading past threads I'm seeing that altitude accuracy is an issue so surely that must be taken into account.

 

[Dbez1]

I hope you get an answer to your question. I’d only be guessing but I would try it again just to make sure it wasn’t just a one time thing. Best wishes.

 

[Crusader14]

What do u mean your altimeter was 1000ft+. What altimeter? As long as you remained within 400ft of the ground you were not breaking the law.

 

[Dbez1]

It sounded like he was actually within the 400’ zone but his screen was telling him that he was at 1,000’+.

 

[Meta4]

However, at times when I could see the Mavic was clearly within the 400ft limit, my altimeter was 1000ft+. Am I going to have the FBI, MI5 and CAA coming to arrest me?

Why should they?
There's no rule anywhere based on your height above the launching point.
They are all based on height above the ground below the drone.

To my understanding my flights are recorded and DJI get the data. What happens when I break the law, albeit momentarily?

Your understanding is sketchy.
Your app records your flight data and it stays on your phone or tablet.
The only way DJI sees it is if you choose to upload to their servers.

From reading past threads I'm seeing that altitude accuracy is an issue so surely that must be taken into account.

The altitude shown on your screen is quite accurate.
That the barometric sensor might drift a few metres during the duration of a flight isn't important.

 

[Crusader14]

The “altitude” shown on the screen is actually “height” above the launch point.

 

[Blad the Inhaler]

I hope you get an answer to your question. I’d only be guessing but I would try it again just to make sure it wasn’t just a one time thing. Best wishes.

I’ll keep an eye on it and search related post… see if anyone else has similar experiences. Cheers?