How to determine height off ground in hilly terrain

[silverstreak]

Very new to P4 and somewhat confused when flying in hilly terrain. Flew from a mountain top into a canyon and it gives me a minus height, which I understand it is telling me it is flying lower than where I took off from. Instead of a negative height figure, does it tell me how far off the ground I am flying? Or is there something set wrong on my system?

 

[alokbhargava]

No it will not give you about your true elevation from the ground.

I refer google earth to plan out my missions.


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[BigAl07]

With the Phantom series of aircraft everything is relative to the point of take-off. While it does have a barometer which tells (with some slight degree of accuracy) how his it is flying it has no way to determine how high it is from the ground directly under it. That technology is either yet to be miniaturized, too expensive, or too heavy for our toy grade aircraft.

 

[Erised]

I don't know why DJI did not include altitude in the telemetry data, when I'm tracking 17 satelites I could get a decent altitude reading. Better than 3.5 meters accuracy.

 

[RPP]

The altitude readin in the EXIF data is also incorrect.

 

[With The Birds]

I don't know why DJI did not include altitude in the telemetry data, when I'm tracking 17 satelites I could get a decent altitude reading. Better than 3.5 meters accuracy.

For most consumer GPS altitude accuracy is +/- 15m 95% of the time. 5% of the time it could be significantly less accurate. Far from reliable.

 

[RPP]

For most consumer GPS altitude accuracy is +/- 15m 95% of the time. 5% of the time it could be significantly less accurate. Far from reliable.

It is off by much more than that and nearly 100% of the time. I am talking of the EXIF data recorded on photo files.

 

[Erised]

I have a samsung tab 4 with GPS and GLONAS,at this moment it is actually placing me exactly where I am sitting in my house! outside it is usually within 10 feet,(wifi off). This is a cheap tablet and the technology of the GPS is real good. P4 uses both also (gps, glonas), having two different signals on different frequencies would make calculating ionopheric corrections that reduces radio degradation caused by the Earth's atmosphere more accurate. We could be getting some altitude data that could help us when going up a mountain slope for example. I think dji may not want us to depend on data that may be ball park at times. And yes altidude is never as precise as location.

 

[Kevin70]

Someone correct me if I'm wrong. The DJI Go app displays height from the ground using sonar up to 25 ft high. This is not relative to the take off elevation. Above 25 ft it uses GPS data.
The sonar measurement is fairly accurate. To the extent that I've flown close to roof tops of my buildings at 2.5 ft above without concern.
To test, power up your P4 and connect with the controller and display. Do not start the motors.
Watch the elevation in the lower left and simply lift the P4 from the surface. Move it to a area where the surface is a different elevation than the original. It should display a fairly accurate distance to the surface.


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[Erised]

Elevation is by the birds barometer only, always relative to your take off point. Not to the ground or surface.

 

[Kevin70]

Page 54 of the P4 instruction manual states improvement to the P4 positioning uses four sonar sensors that function up to 10 meters above ground level. And goes on to claim a fairly tight accuracy of .1 m vertical accuracy and horizontal hover accuracy of .3 m

You aren't going to consistently achieve this with barometric pressure. To rely on barometric pressure as a sole indicator would by risky as it can quickly change while fronts pass through.

As I recommend above. Test it to fully understand how it works.

If I'm wrong, I'm all ears and will only learn more moving forward.



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[Erised]

The vps is made to hold a stable elevation and position when you are low and or landing. Designed mainly for indoors like a gym to maintain elevation (sonar)and position(cameras) without GPS. Your elevation readings will change as the temperature and pressure changes even during a flight. Therfore you must not depend on the readings being perfect if you are flying real close to the ground and that is another reason we are suppose to be in VLOS all the time.