RTH altitude

[Kent Lewis]

How does the drone determine altitude. Is it based on GPS or distance to the ground?

For instance, assume my home point is on top of a 100 foot cliff at the beach and my RTH altitude is 90 feet. If I fly the drone down almost to the beach and, without landing, activate the RTH. Will it will it rise up 90 feet and then run into the cliff 10 feet below me or will it rise 190 feet and return home?

 

[THE CYBORG]

RTH is from the take off point, it you flew lower than the take off point that would be minus altitude.

 

[scro]

The altitude displayed in the Fly app and what is used for RTH etc is always relative to your take off point, and is derived from the atmospheric pressure sensor inside the drone. The higher you go the lower the pressure.

In your example when you fly from cliff top down to the beach you'll see a negative altitude readout (ie -90ft)

 

[Deleted member 103366]

It will climb to an altitude 90ft above the take off point so, if RTH is triggered 99ft below the takeoff point the total climb will be 189ft.
That said excessive RTH height is a waste of power both in the climb and the descent. Obviously you need to be sure to clear any obstacles but going up to for example 200ft to clear a 50ft obstacle is a waste and descent times can be quite long if you are high. Not good on the nerves if the battery is getting low.
I have done the reverse and flown from the bottom of a cliff to 300ft over its top, the descent worryingly slow

 

[joeke3el]

It would be great if some day DJI could give us both height relative to the home point and from the done's present position. The free app, Virtual Litchi Mission, which works along with Litchi Waypoint, does this by linking to Google Earth's topographical data. When creating a waypoint mission with it, you get to see both heights and I see no reason why this data could not some day be brought on board the DJI apps. I live in a mountainous area and would appreciate that feature as most times I'm taking a guess by sight as to how high over a given terrain I may be.

 

[Kent Lewis]

Thank you all. Good info and exactly the answers I was hoping for.

PhiliusFoggg -- Point taken. Where I usually fly my RTH is set to 60 feet.

 

[BokiFly]

RTH etc is always relative to your take off point, and is derived from the atmospheric pressure sensor inside the drone

That's one very sensitive sensor

 

[scro]

That's one very sensitive sensor

Sure is! Its generally more sensitive than GPS for altitude, though needs a datum to work from, as atmospheric pressure changes slowly all the time. My smart watch has a similar barometric sensor for altitude, and my old galaxy s4 did too. The S4 could distinguish down to about 30cm altitude difference. Just moving it from floor to ceiling you could see the altitude change by about 2.5m. The Tello drone uses a similar sensor too for height. It has no GPS

 

[Dave Maine]

Using Litchi‘s calculation for above terrain flight takes a little care. You need first to make sure the Litchi mission is set up to enable height above ground. For this to work well the takeoff point must be defined the same for both the waypoint mission and the actual takeoff point of the drone.

Also know that the GE ground reference is to ground. Trees, buildings, towers are not referenced, and spell disaster if the mapped flight path attempts to fly through them.

 

[Capt KO]

It would be great if some day DJI could give us both height relative to the home point and from the done's present position. The free app, Virtual Litchi Mission, which works along with Litchi Waypoint, does this by linking to Google Earth's topographical data. When creating a waypoint mission with it, you get to see both heights and I see no reason why this data could not some day be brought on board the DJI apps. I live in a mountainous area and would appreciate that feature as most times I'm taking a guess by sight as to how high over a given terrain I may be.

Wouldn’t that be sweet. And make it much easier to stay legal. Someday.....maybe

 

[Deleted member 103366]

@ Kent, just a point, if you are changing the limits, before you close the menu window just check that you have not accidentally moved one of the other limits. I ended up with an RTH climb of over 800ft because the RTH got set to 875ft as I increased the distance limit and I didn't notice. There is absolutely no way I would set that height deliberately.
I had either forgotten or missed the fact that
"If the aircraft is at an altitude of 65 ft (20 m) or higher and has not yet reached the RTH altitude, the throttle stick can be moved to stop the aircraft from ascending. The aircraft will fly directly to the Home Point at its current altitude "
page 14 of the manual.
It was a long few minutes as it was a failsafe triggered RTH and I was concerned about interupting it before the drone was well inside control range. Hence my concern about excess height. It also turns out that once it climbed back into a solid connection I would have been perfectly safe cancelling the rth and resuming manual control,

 

[Capt KO]

@PhiliusFoggg , good lessons. ?

 

[joeke3el]

Using Litchi‘s calculation for above terrain flight takes a little care.

Hi Dave,

Have you built a mission using Virtual Litchi Mission? It's a program for the PC. That, along with Google Earth Pro installed, will give you two numbers for height at each WayPoint. One is height from home while the other is height in current location. You just have to have "Use Online Elevation" checked in settings. Both heights appear automatically as you create each waypoint.

Joe

 

[alopeznj]

RTH is from the take off point, it you flew lower than the take off point that would be minus altitude.

So, if I take off from ground ZERO, fly down into the mouth of a volcano (big drop 200ft) hit RTH it will rise back to ground ZERO ?
Not a Joke, plan to do this in EL Salvador. Using Mini, cant risk my Mavic pro in that Country.

 

[EyesWideShut]

So, if I take off from ground ZERO, fly down into the mouth of a volcano (big drop 200ft) hit RTH it will rise back to ground ZERO ?
Not a Joke, plan to do this in EL Salvador. Using Mini, cant risk my Mavic pro in that Country.

The ground zero is the point where the mini was started. If you descend lower than ground zero the altitude will show as minus. If RTH is initiated and you are further than 60 feet from home position, the mini will rise up past zero to the altitude you just set and return and land near the recorded Home Position. IMPORTANT: If the mini is less than 60 feet distance from HP ** IT WILL LAND ** no matter what you have the RTH settings at.

To clarify: You first start the mini then go into the settings and set the RTH altitude to a height that will easily clear any objects in the area and also set "Loss Of Signal RTH" to RTH, not Hover or Descend. Make sure enough satellites are recorded for proper GPS Home Point and do "Check it on the map".

If you are not in complete visual line of sight with the drone as it goes over the edge and down into the volcano and lose the signal, after about 10 seconds the mini will go into "Loss Of Signal RTH" so make sure you have it set to RTH.

I don't see this going well so I suggest you also bring your Mavic Pro so you have something to fly after you sacrifice your mini to the Gods.

 

[RogerDH]

It would be great if some day DJI could give us both height relative to the home point and from the done's present position. The free app, Virtual Litchi Mission, which works along with Litchi Waypoint, does this by linking to Google Earth's topographical data. When creating a waypoint mission with it, you get to see both heights and I see no reason why this data could not some day be brought on board the DJI apps. I live in a mountainous area and would appreciate that feature as most times I'm taking a guess by sight as to how high over a given terrain I may be.

I'm really looking forward to using Litchi on the Mini. Another thing you can do is define a waypoint mission by saving viewpoints in Google Earth, including camera angles, so using the 3D buildings and topography, you can frame your shots very accurately instead of guessing.

 

[alopeznj]

The ground zero is the point where the mini was started. If you descend lower than ground zero the altitude will show as minus. If RTH is initiated and you are further than 60 feet from home position, the mini will rise up past zero to the altitude you just set and return and land near the recorded Home Position. IMPORTANT: If the mini is less than 60 feet distance from HP ** IT WILL LAND ** no matter what you have the RTH settings at.

To clarify: You first start the mini then go into the settings and set the RTH altitude to a height that will easily clear any objects in the area and also set "Loss Of Signal RTH" to RTH, not Hover or Descend. Make sure enough satellites are recorded for proper GPS Home Point and do "Check it on the map".

If you are not in complete visual line of sight with the drone as it goes over the edge and down into the volcano and lose the signal, after about 10 seconds the mini will go into "Loss Of Signal RTH" so make sure you have it set to RTH.

I don't see this going well so I suggest you also bring your Mavic Pro so you have something to fly after you sacrifice your mini to the Gods.

Excellent clarification. Kudos.