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True height?
3149 28 2015-11-27
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Italian Dronema
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United States
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Does the PH 3 measure above sea level or Barometric pressure ht? I want  to do a flight around the river and it is down a hill by 100 ft do I have to raise my ceiling on the heigth? I am on top the hill.
2015-11-27
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labroides
Captain
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Australia
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Your Phantom doesn't know sea level.
The only height it knows is home = zero and it calculates everything else relative to home.
Nothing special required to fly down from a high home point.
2015-11-27
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Italian Dronema
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labroides@yahoo Posted at 2015-11-28 04:54
Your Phantom doesn't know sea level.
The only height it knows is home = zero and it calculates every ...

thanks so much friend
2015-11-29
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DJI-Patrick
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Hong Kong
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Hi ,
The drone will take where it took off as zero height position . All heights are calculated relative to where it took off .
2015-11-29
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steve_aldous
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United Kingdom
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DJI-Patrick Posted at 2015-11-30 06:39
Hi ,
The drone will take where it took off as zero height position . All heights are calculated rela ...

So if I am operating with a max altitude of 400ft presumably I need to subtract my home point altitude above sea level from 400 ft when setting my height limit. Or does the 400ft limit relate to above ground level?
2016-1-27
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rulppa
lvl.3
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Finland
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There is no such thing as 400ft max limit.
2016-1-27
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SHamers
Second Officer
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but when you upoload the flightplan.txt to Heathydrone.com it knows the sea level?
2016-1-27
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pantera989
lvl.4

New Zealand
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It's based from where you took off. Most laws/regulations are based of AGL, or Above ground level, rather than above sea level.

For example where I live, I can fly up to 140m (400ft) AGL. Because the way the DJI works, basing its height off starting point I do have to be careful, not to start the drone at a high point, and then fly at max height above somewhere lower than where I started.

For example, If I took off and flew right to 140m above starting point (140m AGL), the flew across somewhere that was 50m lower than my starting point, I would technically be at 210m AGL (140m+50m) which would be in breach of the 140m AGL limit where I am.
2016-1-27
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pantera989
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SHamers Posted at 2016-1-28 04:30
but when you upoload the flightplan.txt to Heathydrone.com it knows the sea level?

I don't know how the logs, or Healthy Drones works, but one or both of them may be using GPS log and/or Google maps to extrapolate Height above sea level.
2016-1-27
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dacofty
Second Officer
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United States
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Mine seems to be about 8 to 10 feet off of the accurate height if I fly in return and her Everett 6 foot it shows 10 to 12 foot I posted this here and was told that is an ongoing issue yet to be corrected in a firmware
2016-1-27
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dacofty
Second Officer
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Hover.  Gotta love auto correct
2016-1-27
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nigelw
First Officer
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United Kingdom
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dacofty Posted at 2016-1-27 20:56
Mine seems to be about 8 to 10 feet off of the accurate height if I fly in return and her Everett 6  ...

Mine's landed 24 metres higher than when in took off in the same place...obviously the earth moved a bit.  

More often than not it's 4 or 5 metres out (higher).  You can easily tell by checking your flight records at the end of each flight.  It should show zero.
If it's as much as mine you need to factor it in for RTH height.
2016-1-27
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Geebax
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Australia
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nigelw Posted at 2016-1-28 08:33
Mine's landed 24 metres higher than when in took off in the same place...obviously the earth moved  ...

The height is measured by the on-board barometer, it is not unusual for it to be different when you return from a flight. Temperature changes in the aircraft can change the barometer reading.
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nigelw
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Geebax Posted at 2016-1-27 22:03
The height is measured by the on-board barometer, it is not unusual for it to be different when yo ...

But 24 metres?  That's not good if your RTH height is 30 metres though is it.
2016-1-27
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Geebax
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Australia
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nigelw Posted at 2016-1-28 09:59
But 24 metres?  That's not good if your RTH height is 30 metres though is it.

No. But there is not much that can be done about it. GPS altitude is not accurate, so the barometer is all you have. It is not the sort of thing that gets fixed by firmware updates either. Just don't rely on it.
2016-1-27
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labroides
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nigelw Posted at 2016-1-28 09:59
But 24 metres?  That's not good if your RTH height is 30 metres though is it.

24 metres is too much.
Try recalibrating your IMU and see if that makes it better.
2016-1-27
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labroides
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SHamers Posted at 2016-1-28 07:30
but when you upoload the flightplan.txt to Heathydrone.com it knows the sea level?

HealthyDrones take their data from what your Phantom records and the Phantom has no idea where sea level is.
All heights are relative to where you launched.
Home = zero.

2016-1-27
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nigelw
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labroides@yahoo Posted at 2016-1-27 23:28
24 metres is too much.
Try recalibrating your IMU and see if that makes it better.

I plan to, but not expecting miracles.  These records cross a couple of FW updates, so at least 2 IMU calibrations.
2016-1-27
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pantera989
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New Zealand
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labroides@yahoo Posted at 2016-1-28 07:30
HealthyDrones take their data from what your Phantom records and the Phantom has no idea where sea ...

Does the Phantom include it's GPS cord's in the logs, because estimated height above sea level could gathered from GPS data.
2016-1-27
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labroides
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pantera989@gmai Posted at 2016-1-28 11:25
Does the Phantom include it's GPS cord's in the logs, because estimated height above sea level cou ...

But they don't.  The altitude information showing in HealthyDrones is all relative to homepoint.
When you are trying to work out why or where someone crashed, you don't care where sea level is, just where the Phantom was relative to home point.
It would be more than difficult if it did anything else.
2016-1-27
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Anderjon
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pantera989@gmai Posted at 2016-1-27 20:54
It's based from where you took off. Most laws/regulations are based of AGL, or Above ground level, r ...

400 feet = 121.95 meters (400 / 3.28).  If you're flying up to 140m AGL, you're flying at 459.2 feet. Personally I like to lock in 120 meters as my upper limit to be confident I'm legal (same rules here in UK :-)

On a general note, altitude means your vertical distance above mean sea level (MSL) and its calculated either by reference to the regional barometric pressure setting (which changes constantly) or, if you're a full sized aircraft flying high enough, a standard barometric pressure setting of 1013.25mb. In both cases you need to have the ability to manually adjust the barometric reference on your altimeter (plus a live source of barometric data).

Height on the other hand is your vertical distance measured in reference to a point on the ground. This can be done either by using a barometric reference, VPI or GPS.

Since there's no way to periodically adjust the barometric reference point in a Phantom,  we can't use altitude (and it probably wouldn't be very useful to in the vast majority of cases anyway), so instead the drone automatically references the pressure at our take off point (once) and it uses height instead. Lots of factors affect the starting barometric reading the phantom takes and after 20 minutes of flying, minor changes in the barometric pressure where you are located plus heat from the aircraft and its battery mean the height difference between take off and landing can be larger than you'd expect.

Happy flying all!
2016-1-27
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pantera989
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New Zealand
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Anderjon Posted at 2016-1-28 17:31
400 feet = 121.95 meters (400 / 3.28).  If you're flying up to 140m AGL, you're flying at 459.2 fe ...

Correct indeed, 120m is what I have set in the drone, not sure where I came up with 140m
2016-1-27
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Geebax
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Australia
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Anderjon Posted at 2016-1-28 15:31
400 feet = 121.95 meters (400 / 3.28).  If you're flying up to 140m AGL, you're flying at 459.2 fe ...

Agreed, but it is probably worth mentioning that GPS height is the least accurate parameter, and not reliable enough for quadcopter purposes.
2016-1-27
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SHamers
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I understand that GPS height is not so accurate but the follow example.

I put my P3p on the gras switch everything on and wait until GPS is "Ready to fly".
- I take off to 5 meters above take off place and land again.
- Upload the flight data to Healhty drone
- go to Healthy drone/my logs / choose my log / click on  "Details"  and....

Take Off location: 43.423423423 13.4234343 above Sea Level 458.8 m

(and this value is more or less correct :-)


2016-1-27
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labroides
Captain
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Australia
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SHamers Posted at 2016-1-28 18:43
I understand that GPS height is not so accurate but the follow example.

I put my P3p on the gras sw ...

That's most likely taken from Google Earth elevation for the start point.
HealthyDrones flight data is all height above homepoint - and you wouldn't be able to make sense of it if it was anything else.
2016-1-27
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SHamers
Second Officer
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Okay thanks
2016-1-27
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Drgnfli
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You would think with all the 'sensors' on the Mavic it would give you a "true" height/altitude reading - that is unreal!
2017-11-22
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Geebax
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Australia
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Drgnfli Posted at 2017-11-22 13:42
You would think with all the 'sensors' on the Mavic it would give you a "true" height/altitude reading - that is unreal!

What do you call true?
2017-11-22
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Labroides
Captain
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Australia
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Drgnfli Posted at 2017-11-22 13:42
You would think with all the 'sensors' on the Mavic it would give you a "true" height/altitude reading - that is unreal!

If you want height above sea level, that's irrelevant flying a small drone with limited range and flight time.
If you want height above ground, just look at how much that costs and how big the unit is if you want to fit that to a Cessna.
Here's one of the smallest and cheapest available: http://sarasotaavionics.com/avionics/gra55
3.5 lbs and $6000 + antenna + display unit.
2017-11-22
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