Altitude Reading

[sdfenton]

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.

 

[db4476]

You are doing nothing wrong. The Air 2s has downward proximity sensors, but they do not display the altitude above ground during the flight.

Cheers!

 

[Yaros]

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!

 

[old man mavic]

@sdfenton no you are doing nothing wrong ,the altitude shown on your screen device is indeed above the home point ,the drone its self does not know its exact height above the surface ,when it is close to the surface ,then the downwards facing sensors take over and give a reasonably good indication of height above the surface ,but again those sensors can be fooled by very reflective surfaces such as very calm water

 

[Deleted member 84685]

Man that's crazy ,how's it know 400ft ,,guestimate,,,a drone that cant know its altitude dont seem very clever or have I missed something here or is that how the air 2 is ??
I'd want my money back if that's the case,almost sounds dangerous

 

[Timbo141]

Man that's crazy ,how's it know 400ft ,,guestimate,,,a drone that cant know its altitude dont seem very clever or have I missed something here or is that how the air 2 is ??
I'd want my money back if that's the case,almost sounds dangerous

Drones get their altitude from GPS satellites, they measure from the homepoint at takeoff for the starting point.
The same way GPS navigation gets their elevation, which is locked at sea level for their starting point.
One reason why vlos is important.

 

[Meta4]

Am I doing something wrong?

No .. that's how it works ... just like a real light aircraft.

 

[eEridani]

Man that's crazy ,how's it know 400ft ,,guestimate,,,a drone that cant know its altitude dont seem very clever or have I missed something here or is that how the air 2 is ??
I'd want my money back if that's the case,almost sounds dangerous

Almost all DJI commercial drones use atmospheric pressure to measure and maintain altitude. They also log GPS data, but don't display it. Even my non-DJI drone uses pressure instead of GPS for altitude. And it is always launch point relative, not AGL.

 

[sdfenton]

Thanks all for the feedback! Perhaps a future offering will provide real altitude readings.

 

[Meta4]

Thanks all for the feedback! Perhaps a future offering will provide real altitude readings.

That's not likely in the near future.
There are no sensors that can provide that information in a small enough and cheap enough package.
Most small aircraft don't that facility as radar altimeters are expensive and planes have managed without the ability to tell height above the ground below them.

It's really not necessary for drone flying either.
You should be able to estimate near enough for most purposes.

 

[Deleted member 93449]

Drones get their altitude from GPS satellites, they measure from the homepoint at takeoff for the starting point.
The same way GPS navigation gets their elevation, which is locked at sea level for their starting point.
One reason why vlos is important.

Someone correct me if I'm wrong but I believe DJI drones uses barometric pressure to determine altitude above the takeoff point.

 

[Meta4]

Drones get their altitude from GPS satellites, they measure from the homepoint at takeoff for the starting point.
The same way GPS navigation gets their elevation, which is locked at sea level for their starting point.
One reason why vlos is important.

Your drone uses a barometric sensor to measure altitude, not GPS.

Someone correct me if I'm wrong but I believe DJI drones uses barometric pressure to determine altitude above the takeoff point.

You are correct, the height shown on your screen comes from a sensor that measures air pressure, not from GPS.

 

[Faster]

Someone correct me if I'm wrong but I believe DJI drones uses barometric pressure to determine altitude above the takeoff point.

I think you are correct Sr.

 

[Timbo141]

Well thank you for the correction, I just learned something today.
Vertical measurements are from a drones altimeter and horizontal measurements are from GPS.

Sorry for my previous misinformation.

A good article: How Accurate Are Drone Altimeters? - Pilot Institute.

 

[Chaosrider]

You are doing nothing wrong. The Air 2s has downward proximity sensors, but they do not display the altitude above ground during the flight.

Cheers!

I don't have an Air 2S, but IIRC, the downward proximity sensors only work up to a relatively small AGL value.

Is that right?

Thx,

TCS

 

[Chaosrider]

No .. that's how it works ... just like a real light aircraft.

Unless the real light aircraft has a modern GPS. The Garmin 300 (IIRC) gives a continuous AGL reading during flight, by matching the GPS location to some mapping database, maybe Google but more likely Garmin proprietary.

I'd pay a fair amount more to get a true AGL read-out on my drone control screen, perhaps as much as $500 more.

Thx,

TCS

 

[maelstrom]

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.

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.

 

[maelstrom]

Unless the real light aircraft has a modern GPS. The Garmin 300 (IIRC) gives a continuous AGL reading during flight, by matching the GPS location to some mapping database, maybe Google but more likely Garmin proprietary.

Garmin uses data based on global average mean sea level to calculate altitude and warns against relying on it. This from their user manual:

“The altitude calculated by the GNC 300 is geometric height above mean sea level and could vary significantly from altitude displayed by pressure altimeters in aircraft. Never use GPS altitude for vertical navigation.”

 

[Chaosrider]

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.

The only thing I'd use it for is to make sure I stay under 400 ft AGL, without even having to think about it. If I could set an alarm that would go off when I bumped into 400 ft AGL, it certainly do that, and almost invariably drop lower to stay below 400.

For that purpose, an accuracy of +/- 28 ft is just fine!

Garmin uses data based on global average mean sea level to calculate altitude and warns against relying on it. This from their user manual:

“The altitude calculated by the GNC 300 is geometric height above mean sea level and could vary significantly from altitude displayed by pressure altimeters in aircraft. Never use GPS altitude for vertical navigation.”

True in general.

One time about 10 years ago I was flying a C-162, and I had a catastrophic instrumentation failure. The only thing I had left was the GPS, including the only altitude data that I had. And there was no transponder signal to ATC.

It was a hard-VFR day, and I generally use flight following when it's available.
I called up ATC, told them my situation, and they just asked that I let them know when I changed altitude or course, which I did.

The flight successfully concluded without further incident.

Sometimes good enough, is good enough!



Thx,

TCS

 

[maelstrom]

Sometimes good enough, is good enough!

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.