Altitude Question

B_Squared · Feb 22, 2016

I live above a rock quarry, and wasn't too sure how the altitude is treated with the P3A. Since it is a large drop off, does it ever adjust or stay to the level it was at the home point? What happens if I drop below the edge of the cliff, I am assuming I will lose signal, and it will RTH. If it does the RTH, I am guessing it will run into the wall because it would keep flying at the current altitude. I have attached a video showing the quarry and drop off. I searched the forums and from what I can tell it uses the barometer to calculate altitude, and that this isn't very accurate. I’ve only done 3 flights, and when I did the one in the video I didn’t notice any change in altitude, but I may have missed it.

alokbhargava · Feb 22, 2016

Altitude reference is the home point. If you go down the quarry for a home point set on the above ground, you might see negative altitudes. In case of RTH, it will gain height above the home point as set in your app and return.

make sure you are in LOS with the P3 when you go down. I am not sure how the signals will be affected by the quarry, no experience but if there are lots of steel around, your P3 might get disturbed in directional computation.

joe21 · Feb 22, 2016

This has been asked and answered many times. Not judging... just pointing it out.

The P3 calculates altitude based on your home point. Home point is 0 feet. Every reading after takeoff is relative to that.

If you take off at the edge of the quarry and ascend to 50ft, your altitude reading will be 50 feet. If you maintain constant altitude and fly in the straight line out of the quarry, your Phantom will still read 50 ft, even though the "ground" at the bottom of the quarry may be 100, 200 or 500 feet below.

If you fly towards a hill or structure that is 100 ft high, your P3 will continue to read 50 feet and fly into the side of the hill. If you ascend to 110 feet (above home point) your P3 will be 10 feet above the peak of that hill, but your reading will be 110 feet.

If you descend into the quarry and go 75 feet down, your altitude reading will be -75 ft because you are 75 feet below the homepoint.

I have NOT tested RTH, however, this is how it is supposed to work:

Lets say you take off at the edge of the quarry and RTH is set at 100 feet. You now fly into the quarry to -75 feet. Activate RTH. The P3 -should- ascend above the edge of the quarry to +100 feet (above home point) and return home (a total ascent of 175 feet from its position in the quarry).

Hope this helps.

Meta4 · Feb 22, 2016

B_Squared said:
I live above a rock quarry, and wasn't too sure how the altitude is treated with the P3A.
Since it is a large drop off, does it ever adjust or stay to the level it was at the home point?
What happens if I drop below the edge of the cliff, I am assuming I will lose signal, and it will RTH. If it does the RTH.
I am guessing it will run into the wall because it would keep flying at the current altitude.

If you fly over a cliff edge, your Phantom has no sensors that tell it this.
It can't tell whether the ground is 10 feet below or 1000 ft so it will happily maintain altitude.
Why would you assume you'd lose signal just because the Phantom is below you?
It won't run into the wall unless you fly it into the wall.
DJI's design engineers are pretty smart.
They've considered things like this and have it all worked out.

ps ... the barometer is more accurate than you imagine.

Meta4 · Feb 22, 2016

joe21 said:
I have NOT tested RTH, however, this is how it is supposed to work:

Every Phantom owner should read and understand RTH but they should also go out and experiment with it.
That way, when it happens or when they need to initiate RTH, they know what's happening or how to make it happen or sometimes, how to stop it happening..
The time you need RTH isn't the time to start learning.

B_Squared · Feb 22, 2016

Thank you guys. I was reading a bunch of the altitude threads, but just wanted to clarify. Once the weather warms up, I'll be experimenting more. I thought that I might lose signal due it not being in my line of sight and being below.

Meta4 · Feb 22, 2016

B_Squared said:
I thought that I might lose signal due it not being in my line of sight and being below.

If you fly behind an obstacle and lose connection anywhere your Phantom will initiate RTH but it will not happen just because you are flying below launch point.

sonof40 · Feb 22, 2016

If you don't mind a small hijack,

What happens when flying to a point over 400 feet high? There's a hill about a half mile away that is 600 feet above my home point. Will it crash into the hill at 400 if I don't stop it?

tcope · Feb 22, 2016

sonof40 said:
There's a hill about a half mile away that is 600 feet above my home point. Will it crash into the hill at 400 if I don't stop it?

Let me see if I have this correct... you fly up to 400' above your home point and then fly it into a hill that is 600' and you want to know if it will crash?

Meta4 · Feb 22, 2016

sonof40 said:
If you don't mind a small hijack,
What happens when flying to a point over 400 feet high? There's a hill about a half mile away that is 600 feet above my home point. Will it crash into the hill at 400 if I don't stop it?

Your Phantom has no way of knowing what is under it and how high it is above the terrain.
There's no terrain following radar or collision avoidance.
If you fly straight into a hill, that's what will happen.

Air Ontario · Feb 22, 2016

Yes. If you leave the home point and ascend up to 400', fly at that altitude horizontally to an obstruction 600' you will crash.

tcope · Feb 22, 2016

B_Squared said:
I’ve only done 3 flights, and when I did the one in the video I didn’t notice any change in altitude, but I may have missed it.

I agree with everyone else. I'll only add a slightly unrelated item to your post... when you are at less then 10' and slowly climb up a hill the VPS will compensate for the _slight_ gain and keep the P3 at that same height. So if you slowly go up a _slight_ hill and are less then 10' above ground, the P3 will climb slightly in order to maintain that same height. This does not apply to large differences or when the P3 is higher then 10' at any point.

sonof40 · Feb 22, 2016

I'm trying to recall if the bird will, in fact go higher than 400.

tcope · Feb 22, 2016

sonof40 said:
I'm trying to recall if the bird will, in fact go higher than 400.

It will go to 1640 feet.

joe21 · Feb 23, 2016

Meta4 said:
Every Phantom owner should read and understand RTH but they should also go out and experiment with it.
That way, when it happens or when they need to initiate RTH, they know what's happening or how to make it happen or sometimes, how to stop it happening..
The time you need RTH isn't the time to start learning.

You are absolutely correct.

To clarify my comment, I meant to write that I had not tested RTH in the scenario described (I have tried it in typical positive altitude scenarios). I do not fly in area where negative altitude readings are possible. Lacking specific experience with a negative altitude scenario, I didn't want to mislead with information that was gleaned from the manual.

joe21 · Feb 23, 2016

sonof40 said:
I'm trying to recall if the bird will, in fact go higher than 400.

Yes, it definitely will.

The US "guideline" is to stay below 400ft AGL... above ground level. If you are flying over a hill that is 600 ft over your home point, you would be within the guidelines to be flying at up to 1000 ft over that hill (1000 feet as shown on the DJI GO app = 400+600) because you are still within the 400 ft guideline.

Of course, common sense and individual circumstances may dictate otherwise.