Aloha,

     Recently, I have been trying Litchi, especially to get negative altitudes.  There are almost ten missions in my folder and all of them appear to have their altitude increased by 100 feet or so.  My entry stays the same, but the "Ground Level" altitude is off by 100 feet.  Now, I will say that our weather changed to much more humid since then but now is going dryer (this swing happens all the time) so if the altitude is barometric, that may be part of the problem.  But it has not changed in Litchi so far.

     Anyone know what the "Ground Level" in Litchi is based on.  The satellite maps are Google Earth.  In DJI GO and DJI GO 4 it is based on the remote controller or recorded "home point" barometric altitude and you cannot record negative altitudes, or at least I have not been able to in the past.  If I use my current entered altitudes, it looks like I may fly right into the ground with Litchi.  The first time I tested the negative altitudes, the missions were near perfect, all waypoints at negative altitudes.

     Any ideas?

Aloha and Drone On!

Cetacean,
I wish I had a good electronics answer for you on this but I don't.  However, I can provide a tiny bit of insight on altitude.
As you seem to be aware you have pressure altitude and absolute altitude.  Pressure altitude is the height above MSL (Mean Sea Level) and is based on the standard deviation of pressure from MSL factor in standard day temperature.  However, as you correctly noted, due to changing weather systems the atmospheric pressure fluctuates, going from high to low pressure systems and back again.  In order to compensate for those changes a pressure altimeter must be frequently adjusted, in a manned aircraft this is done by setting the pressure as noted in inches of mercury reading for the area in use in what is called the Kollsman Window (i.e. setting 29.92 which by the way is the mean setting for sea level standard day & temp, and the setting all aircraft use once in the PCA FL180 and above).  Since I have yet to see a way to set the pressure altimeter in a UAV from DJI, I doubt their altitude is determined solely by pressure.  I believe that pressure only relates to VVI calculations at most.  
That leaves absolute altitude commonly known as AGL, the height above the ground, typically measure in manned aircraft by a radar altimeter and is similar in nature to the downward VPS system on a Phantom.  However, that does not answer the issue of what is determined to be the initial altitude for a phantom drone.  It might be pressure, as total flight time is less than 30 minutes and it is very unlikely that pressure will change dramatically enough in that time to matter greatly.  It could be mapping altitude, which depending on the mapping data used may or may not be accurate and it would not account for atmospheric pressure change either therefore introducing an error at the start, most likely.  Since I'm not 100% sure, my guess is it uses a combination of pressure and map, although as you seem to indicate that is how you can end up with errors from true like a negative altitude.  I don't think that a phantom can measure AGL past about 6 feet, accurately anyway, so that input can be eliminated directly.
Hope this helps, if not sorry for wasting your time.

Genghis9 Posted at 2017-9-16 14:26
Cetacean,
I wish I had a good electronics answer for you on this but I don't.  However, I can provide a tiny bit of insight on altitude.
As you seem to be aware you have pressure altitude and absolute altitude.  Pressure altitude is the height above MSL (Mean Sea Level) and is based on the standard deviation of pressure from MSL factor in standard day temperature.  However, as you correctly noted, due to changing weather systems the atmospheric pressure fluctuates, going from high to low pressure systems and back again.  In order to compensate for those changes a pressure altimeter must be frequently adjusted, in a manned aircraft this is done by setting the pressure as noted in inches of mercury reading for the area in use in what is called the Kollsman Window (i.e. setting 29.92 which by the way is the mean setting for sea level standard day & temp, and the setting all aircraft use once in the PCA FL180 and above).  Since I have yet to see a way to set the pressure altimeter in a UAV from DJI, I doubt their altitude is determined solely by pressure.  I believe that pressure only relates to VVI calculations at most.  

Aloha Genghis,

    Your comments are never a waste of time.  At least for me!  DJI does claim that their altimeters are barometric.  In fact the whole ATTI and Sport mode flying risk is due to their only sensor in play being the barometric altimeter and they mention the risk of that measurement.

   I wonder if the Litchi altitude problem is somehow related to the Google Earth maps.  Those maps have altitudes regularly listed and when the maps change, the altitude may also change.

     One thing is for sure.  If you are flying in tight altitudes, double-check and test the altitudes you are dealing with.

     Mahalo for the help!

Aloha and Drone On!

Cetacean Posted at 2017-9-16 17:06
Aloha Genghis,

    Your comments are never a waste of time.  At least for me!  DJI does claim that their altimeters are barometric.  In fact the whole ATTI and Sport mode flying risk is due to their only sensor in play being the barometric altimeter and they mention the risk of that measurement.

Oh yes, I completely agree, I don't think I would depend on UAV altitude for anything demanding tight tolerances.  
In my limited experience to date I have found when flying with other Phantoms that it was useful to coordinate our altitudes by at least 50 feet separation and assuming each UAV had a similar read on the altitude

If you plan a mission using another application, it may set the altitude at a given level, and then when you load that mission into Litchi, it will use that altitude as set in the mission.

But the only measurement used by the aircraft is the altitude measured by the barometric pressure transducer in the aircraft. And it only knows one thing, when it was initialised on startup of the aircraft (when you get Bitchin Betty telling you the Home Point has been stored), the altitude is zero. So if your imported mission says to start at 100 feet, then it will start at 100 feet above the take-off point.

Pressue changes due to changing weather patterns should not have any effect on the altitude, they are not usually rapid changes, so it is unlikely there will be any significant pressure change from when you took off to when you fly the mission.

Geebax Posted at 2017-9-16 20:20
If you plan a mission using another application, it may set the altitude at a given level, and then when you load that mission into Litchi, it will use that altitude as set in the mission.

But the only measurement used by the aircraft is the altitude measured by the barometric pressure transducer in the aircraft. And it only knows one thing, when it was initialised on startup of the aircraft (when you get Bitchin Betty telling you the Home Point has been stored), the altitude is zero. So if your imported mission says to start at 100 feet, then it will start at 100 feet above the take-off point.

Aloha Geebax,

     Mahalo for the input.  Now I am trying to remember if I created a waypoints mission in DJI GO 4 between my first tests and now.  I have done both DJI and Litchi missions with the current rig.

     Yesterday, I went up on my roof to fly my Litchi mission because I would have better video to see if I was going to hit anything and better line of sight.  I was no more than 20 feet higher than the normal takeoff point and control seat and indeed, I did use my normal takeoff point.  But I was higher up with my remote controller by as much as 20 feet.  The mission flew without any odd altitudes, so the altitude issue did not happen until after that flight when I came back down.  Do you think that climb to the roof could have made a difference?  I have been wondering.

     Any assistance would be greatly appreciated!

Aloha and Drone On!

Cetacean Posted at 2017-9-16 22:52
Aloha Geebax,

     Mahalo for the input.  Now I am trying to remember if I created a waypoints mission in DJI GO 4 between my first tests and now.  I have done both DJI and Litchi missions with the current rig.

The height of the controller should have nothing to do with the mission, as the controller does not have any method of measuring altitude, although your tablet might. But I seriously doubt DJI would use the data from the tablet.

Geebax Posted at 2017-9-16 22:58
The height of the controller should have nothing to do with the mission, as the controller does not have any method of measuring altitude, although your tablet might. But I seriously doubt DJI would use the data from the tablet.

Aloha Geebax,

     Mahalo for the input!  If you come up with anything - do not be a stranger!

Aloha and Drone On!

I have no Idea, but im gonna start using Litchi.

Hi Cetacean
Don’t know if this will help or cause more confusion.

As far as I know Litchi missions gathers its information from a number of different means google maps government surveys aerial photography, this is very helpful but also can be misleading , for instance buildings come, hillsides get bulldozed away rivers dry up tides also ebb and flow wax and wane.
Not 100% sure about this or if this has to do with your problem.

* The Phantom concept measures altitude from GPS at the takeoff point, and we call that 0 ATO (zero units above takeoff). From then on, everything in the software of the controller and the aircraft is measured relative to that 0 ATO reference point.

BTW off topic CS is now allowing app downloads, up to 20.

hallmark007 Posted at 2017-11-9 09:49
Hi Cetacean
Don’t know if this will help or cause more confusion.

Aloha 007,

     One of my problems is the ground measurement on the Litchi waypoints screen in the computer.  As you noted, these can vary considerably.   I set an altitude and one time it adds that to the take-off altitude.  Another time, it adds that to the Phantom altitude when I start the mission.  Drives me crazy.

    There was one mission when I started a very long mission and my Phantom did not get over the first tree.  It just stopped.  Then I realized it was the obstacle avoidance stopping the Phantom.  Sure enough, the Phantom was trying to clear the tree, and the altitude had been set in the computer to clear that tree by 50 feet.  Agrivating!

    Good to hear about the CS apps.  So can you run Litchi on it?

Aloha and Drone On!