Flight record shows incorrect elevation

[Hermey]

I checked over the flight record from one of my recent flights, and noticed it shows the wrong elevation. I did the flight replay, and noticed that for at least the first half of the flight, the elevation was not being recorded correctly. Elevation showed as 10-11 feet for the first 7 minutes of the 16 minute flight, which is not correct. Actual elevation was over 30 ft most of the time, and for a good while was probably between 50 and 150 ft. Does anyone know what might have caused this error? Oddly, the elevation began showing accurately on the second part of the flight. I then checked previous flight records and found the same problem had occurred on both 2 earlier flights for this new drone, where for instance, on a flight where the drone was at least 50 ft high, the max altitude of the whole flight was stated as 1 meter or 3 ft. Other flight data seems to be correct, eg total distance covered, distance from home, route shown, etc.

 

[GFields]

I checked over the flight record from one of my recent flights, and noticed it shows the wrong elevation. I did the flight replay, and noticed that for at least the first half of the flight, the elevation was not being recorded correctly. Elevation showed as 10-11 feet for the first 7 minutes of the 16 minute flight, which is not correct. Actual elevation was over 30 ft most of the time, and for a good while was probably between 50 and 150 ft. Does anyone know what might have caused this error? Oddly, the elevation began showing accurately on the second part of the flight. I then checked previous flight records and found the same problem had occurred on both 2 earlier flights for this new drone, where for instance, on a flight where the drone was at least 50 ft high, the max altitude of the whole flight was stated as 1 meter or 3 ft. Other flight data seems to be correct, eg total distance covered, distance from home, route shown, etc.

I didn't see which drone you are refferring to. Here's an article that might help you

 

[Yorkshire_Pud]

Which of the following do you think of as "elevation"?
1) The height above the ground directly beneath the drone.
2) The height of the drone above sea level.
3) The height of the drone in relation to the take off point.
4) Something else, please explain what that something is.

You might like to have a look at the webpage DJI Flight Log Viewer | Phantom Help , read the instructions there, up load the flight log to that page and post the resulting URL here.

 

[Hermey]

The way I'm referring to "elevation" I am talking about #1, the height of the drone above the ground directly beneath it.

Thanks...it's a DJI Air 2S...
I checked out the article you referred to...it's good to know that me re-calibrating would not likely fix this problem.
As with the person who posted of a similar problem in that other thread, I have never had this problem on my DJI Mini 2. But with the new Air 2S (which I have flown very little thus far) it seems that this problem has occurred in every one of the 4 flights I've done. I do think it was a "hopeful" sign that midway through the last flight, the elevation of the drone above ground began to show correctly...whereas it had not done this at any point in the previous 3 flights .

 

[Yorkshire_Pud]

In general DJI drones measure and display height in terms of 3). They use a barometer to measure this so there can be some 'drift' as the ambient air pressure changes. It is also the basis of all height limits that you can set.

If you want height in terms of 1) then look at the VPS height ....... if the Air 2S measures and displays that.

The Mini 2 measures and displays height in terms of 3) I do not remember if it displays VPS height in the replay of the flight in the App.

 

[Hermey]

Oh I didnt' realize the drone measures height in relation to take off point. VERY good to know, important info!!

That would make a lot of sense then about what I've been seeing, because in each of the 4 flights I've done so far, I was at a ridge or promontory, and flew the drone out away from the promontory, over terrain that was lower in elevation. So, it could well be the case then that the drone's height in relation to the take off point, remained at 10 or 11 ft, for half the flight, even though the drone was actually mostly 40 to 150 ft above ground during this time, because the terrain sloped downhill from the take off point.

What is VPS height and how does one access that info?

 

[Yorkshire_Pud]

VPS height is measured by the sensors on the bottom of the drone, they have a limited sensing range 30m. The range is detailed in the manual, page 17 https://dl.djicdn.com/downloads/DJI_Air_2S/DJI_Air_2S_User_Manual_v1.0_enIV.pdf

I do not know if the Air 2s can display the VPS height whilst flying, nor do I know if it displays it whilst replaying the flight in the App but I am pretty sure VPS height is a column is the csv for Phantomhelp and might be in the Airdata csv or on the Airdata webpages.

Ditto the Mini 2 but change "do not know" to "do not remember", the Mini 2 does, from memory, display the VPS height in the Phantomhelp csv,

 

[Hermey]

Thanks! I went into the CSV files of a flight record for the Air2s and the Mini 2, and there is a column ready to show height above ground, but the data is not shown, and instead there is this statement;

"Available with any HD 360 subscription"

So that leads me to think that you would have to pay for a subscription in order to be able to obtain this data of height of the drone above the ground, at any point in the flight.
When flying over terrain that is either level or higher than the starting point, this would not be an issue, but the lack of info over height above ground when flying out over LOWER elevation terrain than the starting point, is unfortunate.

 

[Yorkshire_Pud]

Height above the ground directly beneath the ground is referred to as AGL. AGL could only be displayed LIVE for all AGL if
a) the drone's VPS worked at/for all AGL in which case the transmitter would likely have to be a lot more powerful than it is....... or
b) the drone or App carried a Lat/Long ground altitude database within itself and then calculated the AGL by reference to that.

Unfortunately neither are available thus maintaining AGL compliance remains the responsibility of the pilots, beside basing it on VPS could lead to illegal flights as VPS measures the height to a reflecting 'surface' e.g. a roof, tree tops, mist.

 

[Hermey]

Okay so I am happy that I found the solution to my dilemma, thanks to some of your contributions both on this and my other post related to flight records.

So as Yorkshire pointed out, the height shown in the DJI Fly App is based on drone launch point or home point.
However, based on info shared on my other post relating to Flight Records and how to access and read them, I was directed to upload them to the AirData site. There, I discovered something else, based on the suggestion by someone replying to that thread about how you get more info if you upgrade and pay for an AirData subscription. I paid for a subscription, and now am able to see both the wind speed and wind gust info, as well as obtain the data for drone height in relation to ground, at every measured point along the flight. The latter doesn't show up in the flight report itself on the AirData site, but is viewable in the csv file you can export and save on your computer and open in Excel. There's a column in the spreadsheet that is for "height above ground at drone location". If you are using a free AirData acct you won't get this info, and the column will be blank. But if you pay even for just the least expensive subscription, the data in this column will be filled in. This is exactly what I was looking for. So it does allow me to see that.

I also realized a problem related to the drone pilot not being able to see height of the drone above the ground directly beneath it, on their app as they fly. Which is that if you can't see this info, you can't actually tell if you are staying within legal limits and under 400 ft elevation. For instance if you are on a mountaintop or ridge and raise the drone to 10 ft and then fly your drone directly forward from the launch point, not changing its vertical height, the drone height as shown on the app will stay at 10ft, but the drone could be well over 400 ft above the ground below if the dropoff from your location is significant.

Anyway, I'm happy to have discovered a way to obtain the info I sought, even if it does come after the flight and can't be found during the flight itself.

 

[Yorkshire_Pud]

Kicking myself, one of the log gurus did some WONDERFUL grahics by downloading I think the KML file from Phantomhelp and then uploading it to Google Earth. Sorry it didn't come to mind until just now.

 

[MS Coast]

Okay so I am happy that I found the solution to my dilemma, thanks to some of your contributions both on this and my other post related to flight records.

So as Yorkshire pointed out, the height shown in the DJI Fly App is based on drone launch point or home point.
However, based on info shared on my other post relating to Flight Records and how to access and read them, I was directed to upload them to the AirData site. There, I discovered something else, based on the suggestion by someone replying to that thread about how you get more info if you upgrade and pay for an AirData subscription. I paid for a subscription, and now am able to see both the wind speed and wind gust info, as well as obtain the data for drone height in relation to ground, at every measured point along the flight. The latter doesn't show up in the flight report itself on the AirData site, but is viewable in the csv file you can export and save on your computer and open in Excel. There's a column in the spreadsheet that is for "height above ground at drone location". If you are using a free AirData acct you won't get this info, and the column will be blank. But if you pay even for just the least expensive subscription, the data in this column will be filled in. This is exactly what I was looking for. So it does allow me to see that.

I also realized a problem related to the drone pilot not being able to see height of the drone above the ground directly beneath it, on their app as they fly. Which is that if you can't see this info, you can't actually tell if you are staying within legal limits and under 400 ft elevation. For instance if you are on a mountaintop or ridge and raise the drone to 10 ft and then fly your drone directly forward from the launch point, not changing its vertical height, the drone height as shown on the app will stay at 10ft, but the drone could be well over 400 ft above the ground below if the dropoff from your location is significant.

Anyway, I'm happy to have discovered a way to obtain the info I sought, even if it does come after the flight and can't be found during the flight itself.

DJI drones and similar drones are unable to measure altitude above ground level, AGL. Most general aviation airplanes also lack that ability.

The information shown as altitude above ground level in Airdata is an estimate calculated using an estimate of the elevation of the ground at the position of the drone. The figure provided is only an approximation.

Fly more in 2019 with these 12 new features | Airdata UAV Drone Blog

Fly more 2019! We're taking off this year these 12 new features:... Read more!

airdata.com

"You can now monitor the true AGL (Above Ground Level) for safer flights."

I find this statement from Airdata to be misleading. You can see the estimated AGL altitude only after the flight is uploaded to Airdata. Saying that you can monitor true AGL altitude for safer flights suggests that the information is available during the flight.

 

[Hermey]

Interesting...I wouldn't know how to use a KML file --- haven't done that before....but I suppose one could look into that and learn how to do it...

 

[MS Coast]

Interesting...I wouldn't know how to use a KML file --- haven't done that before....but I suppose one could look into that and learn how to do it...

It's pretty straightforward. You import the .kml file into Google Earth, a free download that's well worth having.

 

[Yorkshire_Pud]

Not the best example I have seen but the best I have found so far, post #13 of Mavic Air never returned


Ahhh here's a beauty post #7 of Mavic Air 2 lost

 

[Hermey]

Thanks...I just did it! It worked and put the flight map onto Google Earth. The only disadvantage of this is that as you expand the map eg increase magnification, the map view tilts and you're no longer directly overhead but at an angle, so the flight route shifts, so you can't see exactly what part of the terrain you were flying over.

It's pretty straightforward. You import the .kml file into Google Earth, a free download that's well worth having.

 

[Yorkshire_Pud]

lol you come here for an answer and get an education too, ain't it great?

It's what I love about this place and occasionally the DJI forum from both sides of the 'coin'.

 

[MS Coast]

Thanks...I just did it! It worked and put the flight map onto Google Earth. The only disadvantage of this is that as you expand the map eg increase magnification, the map view tilts and you're no longer directly overhead but at an angle, so the flight route shifts, so you can't see exactly what part of the terrain you were flying over.

That's an annoying "feature" of Google Earth. It tilts away from overhead as you move the view around. But, you can reset the view to overhead by pressing "r" on the keyboard. You can also switch that behavior off in the program settings.

 

[Hermey]

Thanks MS Coast! That was really helpful. I just did that and the view reset to overhead as you mentioned. Because if one can do that, reset the view to overhead, and then magnify the view, it's then possible to get a good "guestimate" of the ground elevation below the drone at various points along the flight path, which can help to guest-imate the drone's height over the ground directly beneath it.

 

[lookout]

After playing back a flight record where I did nothing but hover in the wind, the movement of the flight seemed to be all over the place. If I don't have the DJI stability function and trying to fly a regular quad by the seat of pants, I would actually be all over the place for sure. So this doesn't seem to indicate actual drone movement. It may have more to do with the GPS signal moving around where it thinks it is which can vary several feet especially when sitting still. Looking at the flight records and paying attention to the actual movement indications may be difficult to follow. However it does record the compass direction of the quad itself instead of going by the GPS which while at rest is assumed to be north regardless of which direction you are facing. I happened to noticed that I started at 0 ft and when landing at the same place I was at -3ft. It would be nice to compare GPS altitude readings versus the barometric altitude readings.